Caribbean - Central America Weather

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
428 AM AST Fri Aug 20 2021

.SYNOPSIS...A tropical wave will pass mainly south of the area
today. A drier air mass along with a Saharan Air Layer will result
in mainly fair weather conditions through Saturday. A tropical
wave will move over the area on Sunday, which could increase
shower activity somewhat. Thereafter, an extended period of fair
weather conditions due to a drier than normal air mass and a
Saharan Air Layer will prevail through much of next week.

&&

.SHORT TERM...Today through Sunday...

An upper level low between the Bahamas and Cuba, and another one in
the western central Atlantic, just east of Bermuda will continue
gradually moving south as they strengthen. By Saturday afternoon,
the Tropical Upper Tropospheric Trough (TUTT) in the Atlantic
would have strengthen considerably and positioned itself just to
the north northwest of the local islands. Based on the latest
guidance, there will be convection related to this TUTT on
Saturday, but it will be to the south of the local islands, but it
will also help cause thunderstorms across the local islands on
Sunday. There are also 2 tropical waves, one that will approach
the local area today and one on Sunday. For the tropical wave
expected today, most of the moisture will be to our south, while
the wave for Sunday is expected to bring moisture further north
and over the local islands. This means that the increase in
moisture on Sunday will combine with the local effects and the
TUTT, which could cause showers and thunderstorms over the local
area. That said, the global models have backed off on the amount
of rain, opting for more significant rain over the waters than
over land. Saturday is expected to be a rather dry day.

Another thing to note is that there Saharan dust over the local
area, which will continue through at least Saturday. This means that
hazy skies will be observed. For that reason, the shower activity is
expected to gradually diminish in the morning hours, leaving mainly
isolated showers, but local effects could produce scattered showers
and perhaps even an isolated thunderstorm across western PR, while
the rest of PR and the USVI will have fair weather with hazy skies.
Saturday will be a much drier day as the tropical wave would have
passed by and below normal moisture is expected. The local
guidance is suggesting little rain in the forecast and mainly hazy
skies. Then for Sunday, the Saharan dust should have moved away
mostly, allowing for the combination mentioned above, where the
tropical wave moisture combined with the TUTT to cause showers and
thunderstorms over the local area.

Daytime high temperatures are expected to be similar to yesterday,
around the upper 80s to lower 90s across the lower elevations and
the upper 70s to mid 80s in higher elevations.

.LONG TERM...Monday through Friday...

Not much change in the overall pattern during the long term
period. Model guidance continues to suggest mainly fair weather
conditions prevailing across the local area through much of next
week due to a drier than normal air mass along with a Saharan Air
Layer. This will result in a trade wind inversion and thus will
limit shower activity across most of the area. Nevertheless, the
typical afternoon convection will still be possible across
portions of western Puerto Rico due to local and diurnal effects.
A weak tropical wave may affect the area by the end of next week.
Overall, not much in the way of significant rainfall activity is
anticipated during the long term period.

&&

.AVIATION...VFR conds expected across the local terminals through
the forecast period with HZ due to Saharan dust. Winds will be up to
around 15KT gusting to 25KT from the E-ENE with sea breeze
variations at TJPS developing after 20/14Z. VIS is expected to
remain P6SM even with HZ, but will monitor for obstructions in VIS
reports. VCSH possible at TJBQ after 20/17Z. Winds will decrease
after 20/23Z.

&&

.MARINE...Moderate to fresh easterly winds will prevail across the
local waters during the next several days. Choppy seas of up to 6
feet are expected through Saturday. Seas are forecast to become
hazardous, up to 7 feet, on Sunday and Monday associated with a
wind surge. Therefore, Small Craft Advisories will most likely be
needed during this period. For beachgoers, there is a moderate
risk of rip currents for most of the local beaches of the islands
for today.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 79 88 79 / 30 20 20 30
STT 90 79 89 80 / 20 20 20 20

&&

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
417 AM AST Sat Aug 21 2021

.SYNOPSIS...
Mainly fair weather conditions are expected to prevail today as a
slightly drier airmass with Saharan dust continues to filter into
the region. However, scattered showers with a few isolated
thunderstorm are forecast over the northwestern quadrant of
Puerto Rico this afternoon. For tomorrow, a slight increase in
shower activity are expected, as a tropical wave moves into the
area and combines with a upper level trough, resulting in
afternoon convection over the western sections of Puerto Rico.
Choppy to hazardous marine conditions will prevail for the next
several days.

&&

.SHORT TERM...Today through Monday...

Mainly warm to hot temperatures are expected for the next few days,
with haze due to Saharan dust. The Saharan dust will be in higher
concentrations today compared to what is expected on Sunday and
Monday. Mainly fair weather will be expected today and Monday, but a
chance of showers and isolated thunderstorms is forecast for this
afternoon across western PR, as near normal moisture combines with a
strengthening TUTT to our north, local sea breeze convergence and
diurnal heating. However, Monday afternoon should remain dry through
the day as a very strong trade wind inversion could develop and the
mid levels are expected to be rather dry, on top of below normal
precipitable water. A higher chance of rain is forecast for Sunday,
making Sunday the wettest day of the short term period, though the
model guidance is backing off on the amounts of rain expected. The
increase in chances of rain for Sunday is due to a tropical wave
passing south of the local area, causing moisture to increase and
bringing scattered showers across eastern PR and the USVI, while
western PR could have thunderstorms in the afternoon hours.

Surface high pressure across the west central Atlantic will promote
easterly winds that could be between 10 and 20 mph across the local
islands, and winds up to 20 knots or slightly higher possible across
the local waters. Daytime high temperatures are expected to be
similar to yesterday, around the upper 80s to lower 90s across the
lower elevations and the upper 70s to mid 80s in higher elevations.
The heat index values could be in the 100 to 105 degree range across
coastal areas, especially across the north coastal areas.

.LONG TERM... Tuesday through Saturday...

A dry pattern is expected for the long term forecast period across
the local region. Model guidance continues to suggest a mid- upper
level ridge located just north of the forecast area, promoting
stable conditions. Meanwhile, at the surface, a drier air mass
with Saharan dust particles is expected to filter into the region
on Tuesday into Thursday. Given the expected conditions, a dry
weather pattern and hazy skies can be expected for most of the
upcoming week with minimal precipitation forecast over western
Puerto Rico, due to the local effects. On Friday, a slight
increase in moisture is expected over the local area as a weak
tropical wave passes over the area. The moisture increase with a
southeasterly wind flow will result in shower activity over the
northwestern quadrant of Puerto Rico on Friday afternoon. By
Saturday, fair weather conditions are anticipated once again as
precipitable water values drop below normal and a mid to upper
level ridge promotes stable conditions.

&&

.AVIATION...VFR conds expected across the local terminals through
the forecast period with HZ due to Saharan dust. Winds will be up to
around 15KT gusting to 25KT from the E with sea breeze variations at
TJPS developing after 21/14Z. VIS could be very close to 6SM due to
HZ, but will keep P6SM until observations confirm the obstruction in
VIS, which we will monitor closely. VCSH/VCTS possible at TJBQ after
21/17Z. Winds will decrease after 21/23Z.

&&

.MARINE...
A surface high pressure will continue to result in a moderate to
fresh easterly winds over the forecast area during the next
several days. Therefore, choppy seas up to 6 feet are expected
across the regional waters through Saturday. A tropical wave is
expected to move over the region on Sunday into Monday, which will
cause an increase in seas, becoming hazardous. While isolated to
scattered showers over the local waters are also expected with
this tropical wave. There is small craft advisory for the
Atlantic offshore waters, which will go into effect on Sunday
morning.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 80 87 79 / 20 30 30 20
STT 89 79 89 79 / 20 20 10 10

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
441 AM AST Sun Aug 22 2021

.SYNOPSIS...
Mainly fair weather conditions are expected in the morning hours
as a slightly drier airmass with Saharan dust continues to filter
into the region. However, a the combination of a tropical wave and
a Tropical Upper Tropospheric Trough (TUTT) could result in some
shower activity, mainly over western Puerto Rico. Choppy to
hazardous marine conditions will prevail at least until Monday
evening.

&&

.SHORT TERM...Today through Tuesday...

A drier than normal air mass due to an extensive Saharan Air Layer
will prevail across the area throughout the day to result in limited
shower activity once again. At upper levels, there is a Tropical
Upper Tropospheric Trough (TUTT) located north of the area.
Normally, this feature would bring unstable conditions with
enhanced shower and thunderstorm activity, however, the Saharan
Air Layer, which at this moment continues to be significant is
going to be too much for the TUTT to have any noticeable effect in
the local weather conditions. This Saharan Air Layer is producing
a strong capping inversion at around 800 mb, which will limit the
potential for any significant shower activity to develop. As a
result, the forecast is for very low precipitation chances with
most of the area remaining dry. The only area that may observe
some convection may be across portions of western Puerto Rico this
afternoon, where the combination of strong daytime heating and
sea breeze convergence may be sufficient to break the cap.
However, with the limited moisture in place, the convection that
manages to develop should be short- lived with no significant
rainfall accumulations expected.

Weather conditions for Monday and Tuesday look to remain very
similar with fair weather conditions prevailing as well as hazy
skies due to the continued presence of the Saharan Air Layer. Latest
forecast soundings continue to show a very pronounced capping
inversion at around the 850-800 mb layer, limiting vertical cloud
development and thus shower activity. The best chance to observe any
measurable rainfall activity will be across western portions of
Puerto Rico, where strong daytime heating and sea breeze convergence
may be sufficient to break the cap. Either way, not expecting
significant activity due to the limited moisture that will be
available.

In terms of temperatures, another hot and uncomfortable day is in
store with highs topping out in upper 80s to lower 90s. These
readings combined with the high levels of humidity will result in
heat indices in the lower to middle 100s across most of the lower
elevation areas.

.LONG TERM...Wednesday through Sunday...

For the long term, a dry pattern is expected across the local
region. Model guidance continues to suggest a mid to upper level
ridge located just north of the forecast area, promoting stable
conditions. Meanwhile, at the surface, a drier than normal air
mass will continue through the long term period. Saharan dust
particles are expected to continue across the region through this
period, though in lower concentrations on Wednesday and Thursday,
increasing briefly on Friday. As a result, a dry weather pattern
with hazy skies can be expected for the latter part of the workweek
with limited coverage of rainfall expected over western Puerto
Rico, which would be due to the local effects. However, a brief
surge in moisture is possible on Friday.

On Friday, both model model guidance suggests an Upper Level
Tropospheric Trough (TUTT) moving westward into the region
creating unstable conditions aloft. At the same time, a slight
increase in moisture is expected over the local area as a weak
tropical wave with precipitable water values near normal passes
over the area. The combination of the TUTT and Tropical Wave
could result in localized afternoon showers with a few isolated
thunderstorms mainly over western sections of Puerto Rico. One
thing to note is that an increase in Saharan dust is also expected
on Friday, but could be limited to the morning hours, with much
lower concentrations by Friday afternoon. For Saturday and
Sunday, fair weather conditions are expected, as precipitable
water values drop below normal and a mid to upper level ridge
promotes stable conditions.

&&

.AVIATION...
VFR conditions expected to prevail during the period across all
area terminals. Winds will be from the E at around 10 kts through
22/12z, increasing to 15 to 20 kts with gusts up to 25-30 kts and
sea breeze variations after 22/14z. HZ due to suspended Saharan
Dust Particulates could reduce the visibility down to 6SM at
times.

&&

.MARINE...
Across the regional waters, choppy marine conditions will
continue today, with seas up to 6 feet and winds up to 20 knots.
Therefore, small craft operators should exercise caution. A weak
tropical wave is expected to cause an increase in the local winds
and seas starting tonight. As a result, there is a small craft
advisory in effect from late tonight into Monday evening. Shower
activity is possible today and tonight over the local waters.
Isolated thunderstorms are possible this afternoon over the
waters to the west of Puerto Rico and across portions of the Mona
Passage, as the possible thunderstorms move offshore from western
Puerto Rico.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 78 88 79 / 10 10 10 20
STT 89 80 89 78 / 10 0 0 10

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
509 AM AST Tue Aug 24 2021

.SYNOPSIS...Mainly fair weather condition with limited shower
activity will prevail during the next few days. Saharan dust
particulate will continue to support hazy conditions, though
lower concentrations are expected through the end of the workweek.
A slight increase in shower activity may occur on Friday, but
uncertainties remain high.

Across the forecast area, there is a slight threat for heat,
winds, and waves, as well as a moderate threat for rip currents
today. Please refer to the Hazardous Weather Outlook (HWOSJU)
issued by the National Weather Service San Juan forecast office
for details.


&&

.SHORT TERM...Today through Thursday...

Relatively fair weather conditions are anticipated through the short
term period. Hazy skies due to Saharan dust particles will continue
to linger over the area during the morning hours. Satellite imagery
and aerosols dispersion models show less saharan dust cloud coverage
over the forecast area by this afternoon. Thus, visibilities are
expected to improve under mostly sunny to partly cloudy skies for the
rest of the short term period. At the upper levels, the TUTT low
will move away from the area today into tomorrow, bringing less
favorable conditions aloft for shower enhancement. The strong
inversion cap at mid levels is the most predominant feature in the
forecast. This feature, together with below normal PWAT values
will result in limited shower activity, if any, across the
forecast area. Warm daytime and night time temperatures will
persist during the next days and heat indices above 100 degrees
are also expected mainly for parts of the coastal and urban
sectors of the islands. Winds will be generally from the east
between 15 to 20 knots with higher gust and sea breeze variations.

.LONG TERM...Friday through Tuesday...

No significant changes in the long-term forecast period with
today`s update, with the most recent model guidance suggesting a
generally stable and dry weather pattern dominating the forecast
area. A slight increase in moisture can be expected by Friday due
to the passage of a weak tropical wave, with PWAT briefly
reaching above-normal values around 1.70 inches by Friday
afternoon. This increase, along with somewhat favorable conditions
aloft generated by a westward-moving Upper Level Tropospheric
Trough (TUTT) may generate favorable conditions to support
enhanced convective development across the region; following the
seasonal diurnal shower pattern.

By Saturday and continuing through the rest of forecast period,
model guidance do not suggest any other rainfall enhancer, with
mid to upper level ridging becoming the dominant feature. That
said, mainly fair weather conditions are expected, with below
normal PWAT values of 1.50 inches or below supporting limited
shower activity. Saharan dust particulate will also be present,
gradually filtering the area in the wake of the aforementioned
tropical wave and reaching a concentration maxima by Sunday into
Monday. Thus, expect hazy conditions and reduced visibilities.
Daytime highs are forecast to peak into the lower 90s across lower
elevations and urban areas, with heat index that could reach the
lower to mid 100s each day.


&&

.AVIATION...Mainly VFR conditions are expected across all terminals
through the 24 hour forecast period. HZ due to Saharan dust is
expected through the afternoon, but no impact to VIS is anticipated.
Winds will be up to 15KT gusting to 20-25KT from the E with sea
breeze variations. Winds will decrease after 24/23Z.


&&

.MARINE...Small craft operators should continue to exercise
caution due to choppy seas up to 6 feet and/or winds up to 20
feet expected across the Caribbean waters and both Anegada and
Mona Passages. Elsewhere across the regional waters, tranquil
marine conditions with seas up to 5 feet and winds up to 15 mph
will continue. A moderate risk of rip currents continues for most
beaches except for some of the protected beaches along the south
and west coast of Puerto Rico and the northern US Virgin Islands.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 81 90 81 / 20 10 10 30
STT 90 81 90 81 / 20 10 10 20

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
440 AM AST Thu Aug 26 2021

.SYNOPSIS...No significant changes to the expected weather pattern
during the next few days, with above normal temperatures, hazy
skies and limited shower activity. A weak tropical wave and
accompanying wind surge will move across the region by Friday,
followed by a dense cloud of Saharan dust by late Friday through
at least late Sunday. Without a rainfall-producing feature
expected through at least mid-week, further drying of soils and
fuels can be expected.


&&

.SHORT TERM...Today through Saturday...

Overall, the entrance of drier air will maintain generally fair
weather conditions today. Nevertheless, limited shower activity is
still expected to develop in the afternoon, clustering along the far
northwestern sections of Puerto Rico, as suggested by the High
Resolution model. There is a slight change of isolated thunderstorms
from the northwestern quadrant as well due to somewhat favorable
upper-level dynamics. Elsewhere, including the US Virgin Islands,
mostly sunny to partly cloudy skies will prevail. Weather conditions
are expected to change by Friday, as moisture from a passing tropical
wave begins to move into the local area. PWAT values are forecast to
reach 1.5 inches.

Satelite imagery and aerosol models indicates a Sahara dust cloud
reaching the local area by late Friday. Thus, hazy conditions are
expected for the rest of the short term period. Saturday is expected
to be the day with higher concentrations of Saharan dust particles.
Winds are shifting more to the east northeast on Saturday. Daytime
high temperatures are forecast to peak into the lower 90s across
coastal and urban areas, with a heat index that could reach the
lower to mid 100s each day.

.LONG TERM...Sunday through Thursday...

The reasoning behind the forecast and analysis for the long-term
period has not change much with today`s update. The most recent
model guidance continues to support a generally stable and dry
weather pattern, with mid to upper level ridging as the dominant
feature and no signs of a rainfall enhancer through at least mid-
week. Model-estimated precipitable water (PWAT) suggests below
normal values of 1.50 inches or lower through most of the period,
with driest conditions now delayed to Tuesday, when PWAT are
forecast to fall around an inch. Despite the expected conditions,
localized convective development remains possible, clustering
across the northwestern quadrant of Puerto Rico each afternoon. A
dense cloud of Saharan dust particulate will continue to affect
the area through at least late Sunday, though gradually lower
concentrations are anticipated thereafter. Daytime highs are
forecast to peak into the lower 90s across lower elevations and
urban areas, with heat index that could reach the lower to mid
100s each day. Thus, expect hot and hazy conditions with reduced
visibilities, particularly during the weekend into early next
week.


&&

.AVIATION...VFR conditions are expected for the next 24 hours at all
terminals. -SHRA enroute could cause VCSH conditions with possible
VCTS for TJBQ in afternoon hours. Winds will be from the east-
southeast, at 10 to 15 knots, with stronger gusts, after 26/14Z.


&&

.MARINE...Tranquil marine conditions with seas of 5 feet or below
and winds up to 15 knots are expected today. By tonight and
continuing through the weekend, small craft operators are urged to
exercise caution due to choppy conditions generated by higher
winds. There is a moderate risk of rip currents for some beaches
along the north coast of Puerto Rico, with a low risk expected
elsewhere.


&&

.FIRE WEATHER...Although mainly fair weather conditions with
limited shower activity are expected today, tropical moisture
embedded in east to east southeast trade winds should maintain
relative humidities above fire danger thresholds. Despite the
potential for local winds of 15 mph or higher and KBDI values
above 600 across portions of southern Puerto Rico, the overall
expected scenario should not support fire danger conditions today.
The entrance of a drier air mass and increased wind speeds could
support fire danger conditions during the upcoming weekend.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 78 91 81 / 20 20 20 20
STT 90 78 90 79 / 10 30 30 20

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
439 AM AST Sun Aug 29 2021

.SYNOPSIS...Saharan dust will gradually diminish during the next
several days even while stability increases and shower activity
lessens. Nevertheless scattered showers are expected each
afternoon over western Puerto Rico. On Friday a tropical wave will
approach with somewhat increase shower activity that will continue
through Saturday. Then drier air returns to the area Saturday
night.

&&

.SHORT TERM...Today through Tuesday...

An upper level trough remains over Hispaniola, providing some
divergence aloft. However, this should not have a big impact over
the local weather, since at the mid levels, high pressure dominates,
resulting in a trade wind cap inversion around 850 mb. At the
surface, a weak high pressure near the Bahamas will maintain
winds from the east-northeast, while the low level flow weakens to
near 5 knots or even less through Tuesday. The aerosol models are
still showing a Saharan Air Layer (SAL) over the local islands,
but with lower concentrations than what was observed on Saturday.
As a result, visibilities should improve in general, but skies
should remain hazy. This drier than normal air mass will continue
to result in mainly fair weather conditions. The GFS forecast
soundings also have precipitable water (PWAT) values decreasing
through the short- term forecast period, and mid-level relative
humidities values below 40%. Taking all these factors into
consideration, a mainly fair pattern is anticipated to continue.
Each afternoon, however, due to local effects and diurnal heating,
isolated to scattered showers will remain possible across western
Puerto Rico. In terms of temperatures, above normal readings will
continue across the islands, with lows in the 80s, highs in the
90s, and heat indices around 102 to 107 degrees.

.LONG TERM...Wednesday through Sunday...

High pressure aloft and at mid levels will completely dominate the
weather pattern Wednesday and Thursday and drier air will be in
place. On Friday moisture moves into the area as a tropical wave
approaches. This will likely be the day with the best rainfall,
although, due to the drier mid levels, exceptionally heavy
amounts are not expected then either. On Saturday night much drier
air moves into the area and most showers activity should
disappear. Currently an upper level low is expected to drop into
place northwest of Puerto Rico at that time. By Sunday moisture
will have returned to more seasonable levels and shower activity
will return . Dust from the Sahara will not completely disappear
but concentrations during the period are expected to be minimal.
Also at this time no tropical activity is expected to impact the
island save the weak tropical wave with some moisture on Friday
night.

&&

.AVIATION...Mainly VFR conditions expected across all terminals. HZ
due to Saharan dust will continue, although diminishing in
concentration, and VISs should remain at P6SM. Fair weather is
expected through the forecast period. Winds will be from the ENE at
5 to 15 knots, with sea breeze variations.

&&

.MARINE...Relatively tranquil marine conditions are expected to
persist today. Winds are generally out of the east northeast at up
to 15 knots with higher gusts. Seas are generally 4 feet or less
across the local waters. However, seas up to 5 feet are forecast for
the Mona Passage and the Offshore Caribbean waters today. Early next
week, winds will diminish. This will result in subsiding seas across
the waters, with heights of generally 3 feet or less, including
our offshore waters.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 79 90 78 / 10 20 20 20
STT 91 81 91 80 / 20 20 20 10

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
514 AM AST Mon Aug 30 2021

.SYNOPSIS...Saharan dust concentrations will gradually lessen over
the workweek. Light winds and slowly increasing moisture will
allow shower activity to increase from nearly none to scattered
showers during the passage of a tropical wave Friday through
Saturday. No other impacts from tropical systems are expected
during the next 7 days.

&&

.SHORT TERM...Today through Wednesday...

Saharan Air Layer (SAL) continues to be present across the area,
however dust concentrations will diminish gradually throughout
the week. This will continue to maintain mid-levels dry mostly
inhibiting shower activity through the short-term period. This is
shown by the TJSJ 30/00Z sounding with below normal precipitable
water and an inversion cap around 850 mb with drier air just above
it. This has caused generally fair weather conditions with little
to no rain throughout the night and is expected to persist
through the morning hours.

At upper levels, a TUTT centered north of eastern Cuba and western
Hispaniola will continue to move away from the area as a ridge moves
in and further increases stability aloft through Wednesday. At low
levels, the TUTT has induced a perturbation over the western
Atlantic, north of Puerto Rico. This perturbation along with a
surface high pressure across the eastern seaboard has shifted
winds to northeast with gradually decreasing speeds. For the next
couple of days, moisture content will be low and rather shallow
with patches of moisture filtering in from time to time. These
weather condtions are not generally conducive for shower and
thunderstorm development. However, the low steering winds in
combination with surface heating and local effects could be enough
to overcome the inversion cap and somewhat enhance shallow
convection and rainfall accumulations during afternoon hours. In
fact, several Hi-Res models are suggesting increased shower
activity along the interior and southern sections of Puerto Rico
each afternoon. If this forecast materializes, expect low to
moderate rainfall accumulations in localized places with this
activity. At other times, expect mainly fair weather conditions
with hazy skies under a light to gentle northeasterly wind flow.

.LONG TERM...Thursday through Monday...
The passage of a tropical cyclone well to our northeast as well as
a migrating 700 mb high pressure north of the area will cause the
fairly chaotic flow on Thursday to become more southeasterly as a
weak tropical wave approaches the area. The wave passage on
Friday will bring an increase in moisture and southeasterly flow
on Friday night and Saturday. This will represent some of the best
rain chances of the week. On Friday and Saturday mornings
precipitable water values will also peak at their highest values
for the next 7 days with 1.7 and 1.9 inches respectively.
Southeast flow, however, will also cause temperatures to warm--at
least on the north coast--driving highs back into the low 90s.
Temperatures relax almost immediately afterward with northeast
flow resuming. Models appear to be doing this in response to
another tropical cyclone that will move past the area on Monday
almost 1000 miles to the east northeast. Although confidence in
the strength and location of this cyclone is low, it is relatively
certain that no impacts from tropical systems will occur with in
the next 7-10 days, except for a mild increase in showers late
Friday and Saturday due to the aforementioned tropical wave.
Concentrations of Saharan dust will likely increase modestly next
weekend.

&&

.AVIATION...Mainly VFR conditions expected across all terminals. HZ
due to Saharan dust will continue, although diminishing in
concentration, and VISs should remain at P6SM. Winds will be from
the NE remaining near or below 10KT, with sea breeze variations.

&&

.MARINE...Winds and seas will continue to diminish in the near-
term. A northeasterly swell will cause seas to increase somewhat
next weekend but seas will remain below 5 feet for the next 8 days
or so. The risk of rip current will remain low through Thursday
night. Then a moderate risk of rip currents will spread around the
coasts of the local islands over the weekend.


&&

.Fire Weather...With northeasterly winds, downslope flow on the
southeastern coastal plains of Puerto Rico will cause highs to
push into the low to mid 90s with relative humidities expected to
fall into the 40s. With strong heating and a weakening cap induced
by the Saharan Air layer, some stronger winds from the northeast
could break through and cause conditions approaching, but not
reaching red flag warning levels. The National Weather Service in
San Juan Puerto Rico will continue to monitor these conditions and
issue any necessary statements should conditions deteriorate.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 78 90 78 / 20 20 20 20
STT 91 80 91 80 / 20 20 20 20

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
429 AM AST Thu Sep 2 2021

.SYNOPSIS...

A typical shower pattern will continue to affect the region, with
patches of moisture streaming over the islands today. Tomorrow into
Saturday, a tropical wave will bring more active weather conditions
to the area. Early next week, drier, more stable conditions are
expected; by midweek, however, active weather is likely to
continue, under the influence of Tropical Cyclone Larry, which
will pass by the region well to the northeast. This system is
expected to have no direct impacts to the area.

&&

.SHORT TERM...Today through Saturday...

A surface high pressure system located across the north central
Atlantic and an upper level high will remain as the dominant
features today, maintaining a generally moderate easterly wind flow
and somewhat unfavorable conditions with drier air aloft across the
region. Although this scenario may not necessarily support shower and
thunderstorm development, local and diurnal effects along with a
slight increase in the available moisture (increasing from 1.40
inches by 02/12Z to around 1.60 inches by 02/18Z) will support
convective development this afternoon. This activity will cluster
across the northwestern quadrant and interior of Puerto Rico,
where moderate to locally heavy rains could result in ponding of
water on roadways and poorly drained areas to localized urban and
small stream flooding. Isolated shower activity is possible
elsewhere, particularly downwind from El Yunque and the local
islands, between the late morning and early afternoon hours.
Daytime highs are forecast to peak into the upper 80s to lower
90s, with a slight threat for heat due to heat indices reaching
the lower to mid 100s across various sections of the local
islands.

A transition is anticipated by Friday into Saturday, when
the upper level high will be gradually replaced by a westward moving
Tropical Upper Tropospheric Trough (TUTT). The latter will hold over
the region through the rest of the short-term forecast period,
promoting favorable conditions aloft for shower and thunderstorm
development. At the surface, a tropical wave and associated wind
surge is forecast to reach the area on Saturday. Thus, expected
moderate to locally fresh winds as well as an increase in the
available moisture. In fact, model guidance suggest that
precipitable water values will peak around 2.10 inches by Friday
night, which is well above normal values for the season. This
scenario will support enhanced shower and thunderstorm activity
across the region, and the potential for urban and small stream
flooding. In addition, a slight threat for heat will continue
throught the rest of the forecast period.

.LONG TERM...Sunday through Thursday...

Much of the long-term period is dominated by the (indirect) impacts
of currently-Tropical-Storm Larry. The system is expected to pass by
the region well to the northeast during the first half of next week.

Patches of moisture will make their way into the region from time to
time during the first part of the week. Precipitable water values
are forecast to remain near normal during this time, or possibly
just below normal. Meanwhile, ridging aloft will limit instability;
on the whole, a typical shower pattern is expected, but activity is
likely to be somewhat inhibited.

Increasing moisture is expected on Wednesday, as moisture is dragged
up from the ITCZ towards Larry, which is likely to be to our
northeast/north-northeast at that point. This moisture will likely
be dragged across our region during the second half of the week.
Additionally, conditions aloft are anticipated to become less stable
during this time, under the influence of troughing associated with
the tropical cyclone. This will aid in the development of
convection, as well as in the generation of a deeper moist layer
over the area. As such, increased shower activity is expected. There
is higher than normal uncertainty in the forecast, however, for a
few reasons. One is the general dependency on the track and
intensity of Larry, as well as the conditions on the periphery and
with the tail of the system. Another is also tied to the effects
of Larry, less directly - winds over the region will be influenced
by this system, especially by the system`s interaction with the
surface high pressure that will be to the north of the islands. As
such, the area most likely to be affected by the increased
rainfall could change in future forecasts, and likely will change.

&&

.AVIATION...
VFR conds will prevail at all TAF sites through 02/16Z. VCTS/SHRA
are forecast to develop across the interior and western areas of
PR by 02/16Z. This activity could cause brief MVFR conds. Sfc
winds will be light and variable to calm early in the morning
becoming mainly from the east around 10 knots with sea breeze
variations aft 02/14z.

&&

.MARINE...

Relatively tranquil marine conditions continue today. Seas of around
3 feet or less are anticipated across the local waters. Winds are
generally out of the east at up to 10 to 15 knots.

There is a moderate risk of rip currents for portions of the north
coast of Puerto Rico, from around the San Juan Metro through north
central Puerto Rico, as well as for Culebrita Beach in Culebra.

Building seas are expected by this weekend, as winds increase; early
next week, swell from Tropical Cyclone Larry is expected to make its
way into the local waters, and hazardous conditions currently appear
likely. Please monitor future forecasts for updates.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 78 90 78 / 20 20 20 60
STT 89 78 89 78 / 20 20 20 60

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
510 AM AST Fri Sep 3 2021

.SYNOPSIS...

A tropical wave is expected today; this, combined with an increase
in instability, is expected to cause more active weather
conditions, including scattered to numerous showers and isolated
thunderstorms, especially for northwestern and interior Puerto
Rico. Lingering moisture is expected tomorrow, with drier
conditions for the start of the week. Midweek next week brings
another influx of moisture, as moisture is pulled up from the ITCZ
towards Hurricane Larry, as it passes by well to the northeast of
the region.

Swell and hazardous marine conditions are expected for the early
part of the next workweek.

&&

.SHORT TERM...Today through Sunday...

A weak tropical wave will move across the eastern Caribbean and near
Puerto Rico and the US Virgin Islands. This wave will increase Total
Precipitable Water to around 2 inches, as well as instability.
GFS and ECMWF suggest unstable lapse rates from low to mid-levels,
and cooling 500 MB temperatures, which could promote favorable
conditions for the development of afternoon thunderstorms along
and north and west of the Cordillera Central in Puerto Rico. The
Galvez-Davison index also supports this scenario. Therefore, this
afternoon, there is a slight to moderate risk of observing
thunderstorms and minor, urban, and small stream flooding, mainly
across the interior, north, and west of Puerto Rico. Although the
best chance for those threats is over PR, thunderstorms, and
ponding of water in poorly-drained areas, remain possible across
portions of the US Virgin Islands. Though this scenario is likely
to result in afternoon showers, the urban and the coastal regions
will observe above-normal maximum temperatures and unpleasant
heat indices between 100 and 107 degrees Fahrenheit.

The tropical wave will quickly move away from the islands early
Saturday, replaced by a drier air mass with African Dust
Particulates. However, daily heating, local effects, sea breeze
variations, and the lingering tropical moisture will feed
additional convection Saturday afternoon, with the most intense
convection affecting the northwest quadrant of Puerto Rico. On
Sunday, the skies will remain hazy due to the Saharan Air Layer;
but, as winds shift from the northeast, afternoon convection may
affect the southwest quadrant of Puerto Rico and downwind from the
US Virgin Islands.

.LONG TERM...Monday through Friday...

Much of the long-term period is dominated by the (indirect) impacts
of Hurricane Larry. The system is expected to pass by the region
well to the northeast during the first half of next week.

Patches of moisture will make their way into the region from time to
time during the first part of the week. Precipitable water values
are forecast to remain near normal during this time, or possibly
just below normal. Meanwhile, ridging aloft will limit instability;
on the whole, a typical shower pattern is expected, but activity is
likely to be somewhat inhibited.

Increasing moisture is expected late Wednesday, as moisture is
dragged up from the ITCZ towards Larry, which is likely to be to our
northeast/north-northeast at that point. However, model guidance has
backed off on how much moisture makes it over the region, and slowed
down the arrival compared to yesterday`s model runs. Even so, some
moisture is likely during the second half of the week; uncertainty
is substantial with respect to how much and for when. In addition to
the moisture increase, conditions aloft are anticipated to become
less stable during this time, under the influence of troughing
associated with the tropical cyclone. This will aid in the
development of convection, as well as in the generation of a deeper
moist layer over the area. As such, increased shower activity is
expected. There is above-normal uncertainty with respect to the
forecast, as alluded to previously. This is largely due to the
general dependency of conditions here on the track and intensity of
Larry, as well as conditions from the periphery of the system, and
how much moisture the system is able to drag up with it. Confidence
with respect to the location of peak activity is also below normal,
due to the effects of Larry on the winds over the region, largely
associated with the system`s impacts on the location of the surface
ridge to the north. As such, the details with this forecast are
likely to change going forward.

Behind this increased moisture is a drier air mass with patchy
moisture. Currently this looks likely to affect the region Thursday
into Friday, with decreased shower activity.

On Friday, a tropical wave is forecast to bring moisture to the
region. Meanwhile, conditions aloft will be more conducive to
convective development. An increase in shower activity is expected,
lasting into the weekend.

&&

.AVIATION...

Through at least 03/16z, expect VFR conditions. VCSH at NCM/KPK
throughout the day and will later move near USVI/E-PR terminals by
03/15z. SHRA/TSRA will then develop across the interior, north, and
west of the Cordillera, affecting JSJ/JBQ and the VCTY of JPS. The
most intense activity will develop between 03/17-22z. Calm to light
and variable winds will continue thru 03/13z, increasing between 10
and 18 knots with higher gusts and sea breeze variations afterward.

&&

.MARINE...

Relatively tranquil marine conditions persist over the local waters
today, with seas of 4 feet or less. Winds remain out of the east-
southeast at up to around 15 knots. Increasing winds are anticipated
for this weekend, associated with a tightening pressure gradient
induced by the surface high to the north of the area. Additionally,
swell associated with Hurricane Larry, which is forecast to pass by
the region well to the northeast, is expected to push into the local
waters on Monday. Hazardous marine conditions are expected for the
start of the next workweek.

There is a moderate risk of rip currents for most beaches of
northern Puerto Rico, northern and eastern Culebra, and eastern St.
Croix today.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 80 91 80 / 50 50 40 30
STT 89 81 89 80 / 50 30 20 30

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
523 AM AST Sat Sep 4 2021

.SYNOPSIS...A Saharan Air Layer is expected to continue through
the weekend. Seasonable weather conditions are expected through
early next week, with diurnal shower activity developing over the
western sections of PR each afternoon. Hazardous marine and
coastal conditions are expected through much of next week, due to
swells generated by Hurricane Larry.


&&

.SHORT TERM...Today through Monday...

Moisture is decreasing across the region. Precipitable water imagery
from GOES-16 shows that the bulk of the moisture from yesterday`s
tropical wave is now west of the local islands, with a drier air
mass making its way into the region in its place. Precipitable
water values of around 1.5 inches are anticipated, which is well
below seasonal normals. Occasional patches of slightly increased
moisture are likely from time to time, with which precipitable
water values may approach normal levels. At the low- and lower-
mid-levels, the ridge reasserts itself through the day today, and
model guidance suggests the development of a relatively strong cap
below 850 hPa; this appears reasonable, considering the Saharan
Air Layer, with a modest amount of dust, that is expected. On the
whole, convection will be inhibited, and is likely to be generally
limited to northwestern and interior Puerto Rico.

The drier air mass will persist over the region into early next week,
with increasing patches of moisture. Shower activity will remain
limited, though thunderstorms cannot be ruled out. Winds shift
tomorrow, becoming more east-southeasterly by the late afternoon
or evening. There remains some uncertainty on the magnitude of
moisture during this time, as well, owing to the indirect impacts
of Hurricane Larry.


.LONG TERM...Tuesday through Saturday...

The main threat during the long term period will be the swells
generated by Hurricane Larry, which are expected to bring life-
threatening rip currents and high surf conditions along the north
and eastern beaches of the islands.

On Tuesday, a low to mid-level ridge north of the area is
expected to move further west, and instability should increase
somewhat over the region due to a lingering TUTT-low over the
ridge. This will favor mainly scattered shower with possible
isolated thunderstorms over the west/southwestern interior
sections of PR, due to northeast steering winds and diurnal
effects.

During the middle part of the week, the forecast will be driven by
track of Hurricane Larry, which currently is forecast to pass well
north of the region between Wednesday and Thursday. Latest
guidance suggest a weak trough pattern at the surface, increasing
PWAT content and turning winds from the southeast. Based on this,
the best chance for showers and thunderstorms will be over the
west/northwest quadrant of PR during the afternoons, with a
slight increase in shower activity across the USVI and
east/southeast sections of PR during the overnight hours.

A weak SAL event could reach the area between late Thursday night
into Friday, limiting briefly shower activity. However, a tropical
wave is forecast to stream across the region, with the bulk of
moisture passing south of the area.


&&

.AVIATION...Generally VFR conditions are expected for the next 24
hours across all terminals. Winds increasing this morning around
04/14Z, out of the east-southeast, up to 10 to 15 knots, with sea
breeze variations, then diminishing after sunset, about 04/23Z.
VCTS conditions possible at TJBQ due to afternoon convection.


&&

.MARINE...Relative tranquil seas and winds are expected through
the rest of the weekend, with seas at 5 feet or less and winds
generally around 15 kts. This will change quickly during the
workweek, as swells generated by Hurricane Larry are forecast to
move across the regional waters through much of the week. Seas
could build to around 10 feet across the Atlantic waters on
Tuesday and Wednesday, and up to 8 feet across the open waters and
passages. Small Craft Advisory conditions are expected with this
event starting as early on Monday and lasting through Thursday.

These swells will also generate large breaking waves along some
of the north and eastern exposed beaches of the islands, creating
life-threatening rip currents and possible minor beach erosion.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 91 80 89 79 / 20 20 20 40
STT 90 79 90 79 / 20 20 30 40

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
420 AM AST Sun Sep 5 2021

.SYNOPSIS...Hazy skies will continue today across the islands due
to a lingering Saharan Air Layer. Afternoon showers with possible
isolated thunderstorms cannot be ruled out over western PR today.
Diurnally induced afternoon showers with isolated thunderstorms
expected through much of the workweek, mainly over the interior
and western PR. Higher moisture content and more unstable
conditions are expected later in the weekend.


&&

.SHORT TERM...Today through Tuesday...

Precipitable water values over the region are expected to remain
generally below normal today, around 1.3 to 1.5 inches. What
moisture makes its way over the region will be patchy and generally
constrained to the lower levels. The 0Z upper air sounding shows a
relatively strong cap near 900 hPa, above which there is mostly dry
air. There is sufficient moisture to sustain some shower activity,
though it will be inhibited. Isolated thunderstorms do remain
possible, with a typical amount of instability; 500 hPa temperatures
will be between -6C and -7C, which is within seasonal normals.
However, thunderstorms are likely to be limited, should they occur,
to interior Puerto Rico and western into southwestern Puerto Rico.
Winds will shift, becoming more east-northeasterly, under the
influence of Hurricane Larry, passing by the region well to the
northeast, and as the surface high to the north moves westward.

Modest increases in moisture are expected for the start of the
workweek. The mid-levels are also expected to become somewhat less
dry, as the influence of the Saharan Air Layer weakens. With a
seasonally typical amount of instability, and moisture near the low
end of normal, an increase in shower activity is likely.

LONG TERM...Wednesday through Sunday...

Winds are forecast to shift from the east to southeast during the
second part of the workweek, as Hurricane Larry tracks well north
of the region. A col region is expected to set up just northeast
of the Leeward Islands on Wednesday and Thursday, and mostly light
steering winds will allow for afternoon showers to last longer
than usual, increasing the flood threat. However, moisture content
is currently forecast to remain below normal values (1.50-1.75
inches), with models showing a peak of precipitable water (PWAT)
favoring the diurnal cycle. Warmer than normal temperatures are
expected with the light southerly winds.

On Friday, a Saharan Air Layer will promote hazy skies and drier
air intrusion, with PWAT dropping to around 1.40 inches. This
drying trend is expected to be short lived, as a weak tropical
wave propagates across the Caribbean Sea by Friday night into
Saturday, and an upper level low builds just southeast of the
local area. This will bring a small surge in moisture content to
favor diurnal shower and thunderstorm development over portions of
western PR, and in trade wind shower activity during the
overnight/early morning hours. Furthermore, another tropical wave
with higher PWAT content (2.00-2.20 inches) is forecast to reach
the area on Sunday.


&&

.AVIATION...Generally VFR conditions are expected across all TAF
sites next 24 hours. Winds picking up generally out of the east
after 05/13Z to 10 to 15 knots, with sea breeze variations and
locally stronger gusts. Diminishing winds expected after sunset,
becoming generally light overnight. VCSH possible for
TJSJ/TIST/TISX after 05/23Z, with minimal operational impacts
expected at the terminals.


&&

.MARINE...Relative tranquil seas and light to moderate winds are
expected to prevail across the regional waters today. Seas will
gradually build during the first part of the workweek as long
period swells generated by Hurricane Larry move across the
Atlantic waters and local passages. Hazardous seas around 8 feet
are expected mainly across the outer waters. The long period
swells between 12-14 seconds will cause dangerous rip currents
and possibly high surf conditions across the northern and eastern
beaches of the islands, especially on Wednesday when the swell
has a more northeasterly direction.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 91 81 90 80 / 20 50 50 40
STT 90 79 91 79 / 10 50 40 40

&&

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
500 AM AST Mon Sep 6 2021

.SYNOPSIS...

A seasonal weather pattern will prevail early this week, with the
advection of patches of moisture from time to time and afternoon
convection enhanced by sea breeze variations. Warmer than normal
temperatures will persist across Puerto Rico and the U.S. Virgin
Islands. Hurricane Larry, located way off to the northeast over
the Atlantic Ocean, will produce a swell deteriorating marine and
coastal conditions tonight into mid-week.

&&

.SHORT TERM...Monday through Wednesday...

Mostly clear skies were seen across the local islands overnight.
There were only a few showers over the local waters, with no
measurable rain over land. Winds were generally light, at around 5
mph out of the northeast. Minimum temperatures were in the mid 70s
to near 80 in coastal areas and in the upper 60s to low 70s at
higher elevations.

A weakening TUTT low will continue to meander north of the local
islands through Wednesday. At lower levels, as Hurricane Larry
continues to lift towards the Central Atlantic, the steering winds
will continue to prevail from the east northeast. The northeasterly
wind flow will continue to drive the Saharan air layer away from the
islands while simultaneously it is to enhance low level moisture
convergence Tuesday and Wednesday. Nevertheless, precipitable water
values over the region are expected to remain below normal today, at
around 1.3 to 1.5 inches. Under this evolving pattern, continue to
expect locally induced afternoon showers and thunderstorms across
west areas of Puerto Rico each day as well as a higher incidence of
early morning showers to affect the USVI and Atlantic coastal areas
of Puerto Rico on Tuesday. Heat indices will again push into the
100s with temperatures in many areas ranging from the upper 80s to
lower 90s.

.LONG TERM...Thursday through Monday

Local weather will be tight at some degree to the final track of
Hurricane Larry. As Larry moves across the Atlantic Ocean (in a
safe position for PR/USVI), winds will be predominantly from the
east to southeast through at least Saturday, resulting in a weak
steering wind flow on Thursday. This weak steering wind flow will
be allowing afternoon showers to last longer than usual,
increasing the threat of flooding. However, the limiting factors
seem to be below- normal available moisture. In addition, a
Saharan Air Layer is forecast to arrive Thursday into Friday,
promoting hazy skies.


A retrograding TUTT-Low moving over the Northeast Caribbean will
induce surface perturbation on Saturday, increasing instability
and convective activity Saturday and Sunday. Atmospheric
conditions will remain favorable for the development of rain
showers and thunderstorms with the arrival of a tropical wave
Sunday afternoon, with its moisture lingering through Monday.

&&

.AVIATION...

VFR conditions are expected at all TAF sites during the
forecast cycle. Northeasterly winds at 10 to 15 knots with higher
gusts and sea breeze variations aft 06/13z.

&&

.MARINE...

Hurricane Larry, which is moving across the Atlantic Ocean far to
the northeast, will deteriorate the Atlantic Waters marine
conditions through mid-week. Larry`s swell will increase seas
between 6 and 8 feet across the Offshore Atlantic waters and the
Anegada Passage throughout the day. Small Craft Advisories are in
effect for these waters. Then, as additional swell energy arrives
Tuesday and Wednesday, hazardous seas will spread into the coastal
Atlantic waters of PR and the U.S. Virgin Islands, causing
dangerous coastal conditions. Marine conditions are forecast to
improve around Thursday night or early Friday.

Beachgoers, Cramer Park has a high risk of rip currents.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 80 90 80 / 20 50 50 30
STT 91 79 89 79 / 20 50 50 30

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
437 AM AST Tue Sep 7 2021

.SYNOPSIS...

Once again, the typical weather pattern is expected, with the
advection of patches of moisture at times and afternoon convection
enhanced by sea breeze variations. Warm to hot temperatures will
persist across Puerto Rico and the U.S. Virgin Islands. A swell
produced by Hurricane Larry will result in a high risk of rip
currents for the north and some of the east-facing beaches.

&&

.SHORT TERM...Tuesday through Thursday...

Short term forecast is still on track...A weakening TUTT low will
continue to meander north of the islands through early Wednesday. At
lower levels, as Hurricane Larry continues to lift towards the
Central Atlantic, the steering winds will continue to prevail from
the east northeast. However, on Wednesday, as Larry moves to a
position well north-northeast of the area, the low-level flow will
become east-southeast. The aforementioned wind pattern will enhance
somewhat low level moisture convergence across the local islands.
Nevertheless, precipitable water values over the region are expected
to remain between 1.6 and 1.8 inches which is near normal. As a
result, continue to expect a seasonable weather pattern with locally
induced afternoon showers and thunderstorms across west areas of
Puerto Rico each day. A higher incidence of early morning showers
affecting the USVI and Atlantic coastal areas of Puerto Rico is
possible under the east northeast wind flow throughout the day
today. Some haziness due to Saharan dust particles is possible on
Thursday. Heat indices will again push into the 100s with
temperatures in many areas ranging from the upper 80s to lower 90s.

.LONG TERM...Friday through Tuesday

The arrival of a Saharan Air Layer combined with an east to
southeast wind flow will promote warm to hot temperatures across
the islands, with maximum temperatures in the 90s and heat indices
in the 100s Friday and even Saturday. The SAL will remain over
the region through early Saturday, resulting in hazy skies. As the
dry air exit the area, a retrograding TUTT-Low moving across the
Northeast Caribbean will induce surface perturbation through the
weekend, increasing instability and convective activity.
Atmospheric conditions will remain favorable for the development
of rain showers and thunderstorms with the arrival of a negative
tilted tropical wave late Sunday afternoon, with its moisture
lingering through Monday. GFS suggested Total Precipitable Water
ranging between 1.90 and 2.30 inches through Tuesday, but the best
moisture pooling over the islands on Monday.

&&

.AVIATION...

VFR conditions are expected at all TAF sites during the forecast
cycle. Northeasterly winds at 10 to 15 knots with higher gusts and
sea breeze variations aft 07/13z.

&&

.MARINE...

A swell generated by Hurricane Larry is moving across the local
Atlantic Waters and Anegada Passage. Seas will range between 6 and
10 feet across the Atlantic Offshore Waters and between 3 and 7
across the coastal waters. However, swell periods between 12 and
15 seconds will create breaking waves between 6 and 9 feet across
the north-facing beaches from the northwest coast to San Juan and
vicinity area and the north and eastern shores in St Croix.
Breaking waves will peak by Wednesday when swells between 5 and 7
feet (or even higher) hit the Atlantic shoreline of Puerto Rico
and the northern and eastern shoreline of St Croix.

Small Craft Advisories are in effect for the Atlantic Waters and
local passage through at least mid-week. Then, marine conditions
are forecast to improve Thursday into the weekend.

Beachgoers, Larry`s swell is producing dangerous breaking waves,
resulting in a high risk of rip current across the northern
shoreline of Puerto Rico and the north and eastern coast in St
Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 80 93 79 / 20 40 40 20
STT 89 79 90 80 / 20 40 20 20

[cycloneye]

Area Forecast Discussion...CORRECTED
National Weather Service San Juan PR
501 AM AST Wed Sep 8 2021

.SYNOPSIS...

Another day with above-normal maximum temperatures and heat
indices in the 100s will prevail across Puerto Rico and the U.S.
Virgin Islands. Located well to the north across the Atlantic
Ocean, Hurricane Larry will promote a light east-southeast wind
flow. Under this wind flow and enough moisture, expect afternoon
showers and thunderstorms to develop over and north of the
Cordillera Central. In contrast, the U.S. Virgin Islands can
expect passing showers at times. A swell produced by Hurricane
Larry will result in life-threatening rip currents and dangerous
surf, mainly across the north and east-facing beaches.

&&

.SHORT TERM...Wednesday through Friday...

As a TUTT low north of the area continues to fill, a weak mid level
ridge will extend from the east across the northeast Caribbean. At
lower levels, Hurricane Larry will continue to move northwest and
away from the area, resulting in light east-southeast winds across
the local islands. Latest guidance suggested precipitable water
values remaining between 1.5 to 1.7 inches through the forecast
cycle. However, by the end of the cycle, a deeper TUTT low
retrogresses across the tropical Atlantic with the closed upper low
moving north of the forecast area. This feature is then expected to
bring better moisture advection with precipitable water values near
2.0 inches.

As a result, under enough moisture and light east-southeast winds,
expect showers and thunderstorms developing over and north of the
Cordillera Central each afternoon. Heavy rains could lead to urban
and small stream flooding across the interior and the northern
slopes of the island including portions of the San Juan Metro Area.
As far as the US Virgin Islands, mainly fair weather conditions will
continue to prevail. The chance for shower activity is expected to
increase Friday night as the aforementioned deeper TUTT enhances
early morning convection.

With east-southeast winds, heat indices will again push into the
100s with temperatures in many areas ranging from the upper 80s to
lower 90s today. In addition, Saharan dust particles continue this
way, and some haziness is expected late Thursday and Friday.

.LONG TERM...Saturday through Wednesday...

Hazy skies will remain through early Saturday as the SAL exit the
area. A retrograding TUTT-Low will swing across the Northeast
Caribbean, inducing surface perturbations through the weekend,
increasing instability and convective activity. Model guidance
persists in suggest favorable atmospheric conditions for the
development of rain showers and thunderstorms with the arrival of
a negative tilted tropical wave late Sunday afternoon, with its
moisture lingering through Tuesday. GFS indicated, once again,
above-normal Total Precipitable Water through mid-week. Also, the
GFS Galvez-Davison suggests favorable conditions for the formation
of thunderstorms across the islands through that period.

Although the GFS and ECMWF models forecast a moist environment in
the long range (Monday-Wednesday), the NASA Dust Extinction
Aerosol guidance suggests an extensive area with African Dust
Particulate across the Atlantic Basin. If that drier airmass
materializes, it could negatively impact the solution of these
guidances. Therefore, we incline our forecast to a more
climatological pattern, consisting of passing showers overnight
and early in the morning across the windward sections, followed by
afternoon convection enhanced by daily heating and sea breeze
effects across the interior leeward portions of the islands.

&&

.AVIATION...
Mainly VFR conditions expected. SHRA/TSRA activity expected
across interior and western PR between 08/16z and 08/22z,
resulting in VCSH across TJBQ and TJSJ. Winds increasing up to 15
kts from the E-ESE with seas breeze variations after 08/14z.

&&

.MARINE...

A swell generated by Hurricane Larry is moving across the local
Atlantic Waters and Mona Passage. Seas will range between 6 and
10 feet across the Atlantic Waters and Mona Passage through
tomorrow. However, swell periods between 12 and 15 seconds will
create hazardous breaking waves between 8 and 12 feet across the
Atlantic shoreline of Puerto Rico. The north and east-facing
beaches in St Croix will also observe life-threatening rip
currents. Small Craft Advisories are in effect for the Atlantic
Waters and local passage through at least Thursday. Then, marine
conditions are forecast to improve Friday into the weekend.

Beachgoers, Larry`s swell will produce life-threatening breaking
waves, resulting in a dangerous rip current across the northern
shoreline of Puerto Rico and the north and eastern coast in St
Croix later today.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 93 79 94 79 / 50 20 40 20
STT 90 80 91 80 / 20 20 20 20

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
453 AM AST Thu Sep 9 2021

.SYNOPSIS...

Heat indices will again push into the 100s, with temperatures in
many areas (especially across urban and coastal locations) ranging
from the upper 80s to lower and mid-90s. African dust particulate
will arrive later today, creating hazy skies. Diurnal heating and
sea breeze variations will result in the formation of showers and
thunderstorms during the afternoon. Urban and small-stream
flooding will be possible.

&&

.SHORT TERM...Thursday through Saturday...

A weak mid level ridge will prevail across the local islands through
at least Friday. This feature will erode as a retrogressing TUTT
reaches the northeast Caribbean late Friday into Saturday with the
closed upper low north of the area by Saturday afternoon. At lower
levels, east southeast winds will prevail through Friday, becoming
more easterly on Saturday. The precipitable water values over the
region will continue to range between 1.5 to 1.7 inches. However, by
the end of the cycle, the aformentioned retrogressing TUTT will
bring better moisture advection.

As a result, under moisture near normal values and southeasterly
winds, expect sea breeze convergence and diurnal convection
resulting in afternoon showers and thunderstorms over and north of
the Cordillera Central Thursday and Friday. Heavy rains could lead
once again to urban and small stream flooding across the interior
and the northern slopes of the island including portions of the San
Juan Metro Area. On Saturday as the wind flow shifts to the east,
the diurnal convection is then expected over the interior and west
portions of Puerto Rico. As far as the US Virgin Islands, mainly
fair weather conditions will continue to prevail. The chance for
shower activity is expected to increase Friday night into Saturday
morning as the TUTT enhances early morning passing showers.

With east-southeast winds, heat indices will again push into the
100s with temperatures in many areas ranging from the upper 80s to
lower and mid 90s. In addition, Saharan dust particles continue this
way, and some haziness is expected late today into Friday.

.LONG TERM...Sunday through Thursday...

A negative tilted wave will bring an increase in showers and
thunderstorms from Sunday to Monday. Another pulse with Saharan
dust particulate will move across the region by Monday, suggested
by the NASA Dust Extinction Aerosol guidance. However, the global
models continue to foresee a wet and unstable pattern early next
week with the arrival of a couple of tropical waves and favorable
dynamics aloft through at least mid-week. Once again, the GDI
forecast is indicating an increase in convective instability
across the Northeast Caribbean.

Although ECMWF and GFS have consistently suggested the region
moving into an unstable weather pattern, the possibility to
observe a drier air mass with African Dust Particulate could limit
the development of showers and thunderstorms if we receive an
extensive SAL event. NASA Extinction Aerosol guidance consistently
indicates a vast concentrations of this air mass in the Atlantic
Basin.

&&

.AVIATION...

Mainly VFR conditions are expected through the forecast cycle.
SHRA/TSRA activity expected across interior and western PR between
09/16z and 09/22z, resulting in VCSH/VCTS across TJBQ and TJSJ.
Winds increasing up to 15 kts from the ESE with seas breeze
variations after 09/14z.

&&

.MARINE...

A swell generated by Hurricane Larry is moving across the local
Atlantic Waters and Passages. It will spread and fade across the
waters throughout the day. Therefore, Small Craft Advisories
remain in effect through this afternoon due to seas up to 7 feet.
Later this afternoon, seas will diminish at 6 feet or less, and
mariners should exercise caution. Marine conditions will slowly
improve later tonight into the weekend. The Caribbean coastal
waters are observing calm marine conditions, and it will prevail
over the rest of the forecast period. However, local effects could
increase winds between 15 and 20 knots from time to time,
especially during the afternoons.

Beachgoers, Larry`s swell will continue to produce life-
threatening rip currents across the islands due to dangerous
breaking waves across the exposed north-facing beaches of Puerto
Rico and the north and east beaches of St Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 93 79 89 79 / 50 30 20 30
STT 90 80 89 79 / 20 30 20 40

&&

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
440 AM AST Sat Sep 11 2021

.SYNOPSIS...A seasonable weather pattern will prevail across the
local area for today with afternoon convection across the interior
and southwestern quadrant of Puerto Rico. Conditions will turn
more unsettled for Sunday as a tropical wave in combination with
a Tropical Upper Tropospheric Trough (TUTT) will result in more
widespread shower and thunderstorm activity. Unsettled weather
conditions look to continue into the middle of next week as the
TUTT and additional deep tropical moisture will enhance shower and
thunderstorm activity.

&&

.SHORT TERM...Today through Monday...

An upper trough and an increase in moisture due to an approaching
tropical wave, means a transition to a wetter pattern through the
short term period, though mainly starting late tonight through
Monday. For today, we have an upper trough that is moving west to
the north of the local islands, which is also reflected in the mid
levels with a weak trough. This is causing widely scattered showers
and a few thunderstorms across the local waters early this morning,
and is expected to provide support for thunderstorm development this
afternoon across the interior into the southwestern quadrant of
Puerto Rico. Not only the mid and upper level troughs may play a
role in the afternoon convection over Puerto Rico, the sea breeze
convergence is the main reason why the most significant showers are
expected to be in the west to southwest sections of Puerto Rico,
with a lower chance of rain across the coastal areas of PR and
across the USVI.

That said, a tropical wave could start causing an increase in
moisture tonight, and much higher than normal moisture could be over
the local areas by late Sunday into Monday. The precipitable water
is forecast to be near 1.8 to 1.9 inches this evening, decreasing in
the late evening, but then increasing again by Sunday morning to
around 2 inches, persisting through Sunday, but increasing to about
2.25 inches by Monday afternoon. This significant increase in
moisture, combined with the upper trough, which is expected to be
just to the west northwest of the local islands, could cause a
significant increase in instability and shower activity across the
local area. The local guidance suggests that eastern PR and in
particular the northern USVI could see a decent increase in shower
activity on Sunday, while Monday favors central and western PR.

The daytime high temperatures are expected to be near 90 degrees
once again today across the lower elevations of PR and the USVI,
with heat index values in the low 100s. However, given the amount of
moisture and possible cloudiness expected on Sunday and Monday, it
looks like the daytime high temps could peak in the upper 80s,
especially for the San Juan and St Thomas airports, but St Croix
could still get to 90 degrees this weekend.

.LONG TERM...Tuesday through Saturday...

Model guidance continues to suggest unsettled conditions will
prevail Tuesday and Wednesday. The aformentioned TUTT will be
nearly stationary near the Dominican Republic, with the area on
the favorable side for divergence aloft. This in combination with
deep tropical moisture with precipitable water values expected to
range between 2.0-2.2 inches will favor rounds of enhanced shower
and thunderstorm activity, with the potential for urban and small
stream flooding to materialize in areas that see the heaviest and
most persistent rainfall activity.

By Thursday and continuing through the end of the long term
period, as the TUTT weakens and a mid and upper-level ridge builds
overhead, a drier air mass will be moving in aloft. In addition, a
Saharan Air Layer will also be moving in promoting somewhat drier
air at low-levels as well. Therefore, a decrease in the areal
coverage of shower activity seems likely. Nevertheless, strong
daytime heating and local effects in combination with sufficient
low-level moisture will be sufficient to generate afternoon
convection, mainly across the western half of Puerto Rico.

&&

.AVIATION...Brief SHRA could cause VCSH across the local terminals in the
morning. Winds will be light and variable through 11/13Z, but will
be from the E-NE thereafter at around 15KT with occasional gusts.
Sea breeze variations in wind direction expected at TJPS after
11/13Z. VCTS possible at TJPS after 11/17Z. Afternoon cloud
development will cause mountain obscurations across the higher
elevations. Winds will decrease after 11/23Z.

&&

.MARINE...Mainly tranquil seas of up to 5 feet will prevail during
the next several days along with generally moderate easterly winds.
A tropical wave will move across the area tonight into Sunday,
increasing shower and thunderstorm activity across the regional
waters. For beachgoers, there is a moderate risk of rip currents
for most of the north and east-facing beaches of the local
islands for today.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 91 80 90 78 / 40 30 50 40
STT 89 79 88 78 / 40 50 50 50

[cycloneye]

AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
436 AM AST Mon Sep 13 2021

.SYNOPSIS...
Unstable conditions aloft will enhanced once again the afternoon
shower activity over the local area. As a result, urban and small
stream flooding is expected with the heaviest activity, mainly
over Puerto Rico. For Tuesday into Wednesday, the unsettled
conditions continue as above normal moisture continues over the
region and combine with the upper level trough.

&&

.SHORT TERM...Today through Wednesday...

Essentially an unsettled and rainy pattern is expected in the
short term. An upper trough is still causing instability across
the local area, as it remains nearly stationary to the north of
Hispaniola. An increase in moisture will occur gradually, and the
shower and thunderstorm activity will gradually increase in the
early morning hours. Therefore, today`s forecast has showers and
thunderstorms in the morning for eastern PR as well as areas near
and around the USVI. Also, the mid levels temperatures are
slightly colder than normal, and are expected to get about a
degree colder by this evening, from -7 to -8, which would
contribute to enhanced thunderstorm potential this afternoon and
into Tuesday. For this reason, the forecast for today has showers
with isolated to scattered thunderstorms across several sectors of
PR, with a risk of urban and small stream flooding. Eastern PR is
expected to observe the most rainfall starting around the
mid-morning hours into the early afternoon, while central and
western PR would have the peak of the rainfall in the afternoon
hours. The USVI could observe moderate showers and nearby
thunderstorms as well, but the rainfall is expected to cause
mainly ponding of water on roadways and poor drainage areas. Most
of the activity near the USVI is expected to be in the morning
hours, with a brief break in the early afternoon, then passing
showers and isolated thunderstorms again in the late afternoon
hours.

The upper trough will remain nearly stationary today into Tuesday,
but it is forecast to be over Cuba by Wednesday, which it could
still cause instability over the local area, but perhaps less of
an impact than what is expected today and Tuesday. The available
moisture will pool over the local area, which will cause
above normal moisture through Wednesday. The model guidance is
still suggesting the precipitable water values to remain near 2
inches. This pattern is expected to continue causing rainy
conditions with thunderstorms as the instability of the upper
trough will combine with the deep moisture. Any diurnal heating
and sea breeze convergence over PR would cause areas of enhanced
thunderstorm potential. Some areas across southern PR have only a
slight chance of showers over the next few days. The USVI will
have mainly passing shower activity, but some of the showers and
thunderstorms could be heavy for brief periods.

The daytime high temperatures are forecast to be near 90 degrees
once again today across the lower elevations of PR and the USVI,
with heat index values in the low 100s. However, given the amount of
moisture and possible cloudiness expected today, it would not be
surprising for the daytime high temps to peak in the upper 80s.

.LONG TERM...Thursday through Monday...

An upper level high pressure will be to the north of the local
area starting in the long term period. In addition, at the mid
levels, a high pressure will be present, resulting in relatively
stable conditions in the mid and upper levels. Meanwhile, at the
surface, a weak trough located well to northwest will maintain
plenty of tropical moisture with precipitable water values above
climatological normals. As a result, for Thursday, expect
afternoon convection with isolated thunderstorms mainly over
western and northwestern sections of Puerto Rico. For late
Wednesday into Friday, according to NASA Dust Extinction Aerosol
Model, low to moderate concentrations of Saharan dust will
move through the local islands. As a result, hazy skies are
possibles over the forecast area.

For the upcoming weekend, a drier pattern is expected, due to a
drier airmass that filters into the region. Therefore, this
airmass, in combination with stable conditions aloft, will
result in mainly fair weather. However, patches of low level
moisture, the local effects, and diurnal heating can cause
isolated to scattered showers across western Puerto Rico. Monday
is expected to be a transition day, as the model guidance
suggests that the available moisture will increase, due to an
approaching tropical wave, currently being monitored by the
National Hurricane Center.

&&

.AVIATION...SHRA is gradually increasing, and will cause at least
VCSH or VCTS across the USVI terminals early this morning, while
VCTS is forecast at TJSJ after 13/14Z. Winds will be light and
VRB through 13/13Z, but gusty near the SHRA/TSRA, otherwise winds
will increase to around 10-15KT from the east. Sea breeze
variations in wind direction expected at TJPS after 13/13Z. VCTS
possible at TJPS and TJBQ after 13/15Z. Winds will decrease after
13/23Z. Overall, passing SHRA/TSRA are expected across the local
flying area today and tonight.

&&

.MARINE...

Across the regional waters, tranquil marine conditions will
prevail for the next several days. Seas of up to 5 feet are
expected. Winds are expected to be from the east, up to 15 knots.
For today and tomorrow, isolated showers and thunderstorms will
persist over the local waters. There is a moderate risk of rip
currents for the northern beaches of Puerto Rico, as well as the
beaches located over the eastern tip of Culebra, Vieques, and St.
Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 78 86 77 / 60 40 50 50
STT 88 79 89 79 / 60 50 60 60

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
424 AM AST Tue Sep 14 2021

.SYNOPSIS...
Another wet pattern is expected today with afternoon thunderstorm
activity over the forecast area. As a result, a moderate threat
of flooding is expected today. By Thursday, conditions will
improve as less moisture and some particles of Saharan dust
filters into the region limiting the shower activity. For the long
term period, expect shower and thunderstorm activity due to an
upper level trough moving over the area. There is some
uncertainty for the latter part of the long term period. Tranquil
marine conditions will prevail with seas up to 5 feet or less
across the local waters.


&&

.SHORT TERM... Today through Thursday...

Added uncertainty to the short term forecast, as the model guidance
started to divert a little from each other. The higher resolution
models are a bit more bullish in the rainfall coverage and amounts
than the global models. Moreover, the rainfall that has occurred
over the past few days has caused some areas to have saturated
soils, which will cause quicker runoff with significant rainfall
over the next few days. That said, the forecast itself has not
changed much for today, we still expect an unsettled and rainy
pattern, as a tropical wave moves in today into early Wednesday, but
perhaps with less rainfall and cloud cover on Thursday. The upper
trough that has cause thunderstorms over the past few days will
continue to cause an enhanced thunderstorm potential today, but will
have less of an impact on Wednesday and Thursday as the trough
continues moving west and an upper ridge starts to move in. In terms
of moisture, the above normal precipitable water will continue
through the short term period, with values ranging from around 1.8
to 2.15 inches, probably peaking late tonight into early Wednesday
with the trailing edge of the tropical wave. Also, the mid levels
temperatures are slightly colder than normal, and are expected to
range between -7 and -8 today and Wednesday, which would contribute
to enhanced thunderstorm potential. So, overall we still have the
ingredients for showers and thunderstorms over the area, but the
upper level support might be better today than on Wednesday and
Thursday if the models are correct. Therefore, the forecast for
today has showers with isolated to scattered thunderstorms across
several sectors of PR, with a risk of urban and small stream
flooding. The most significant showers and thunderstorms over PR are
expected to be across the northwestern quadrant of the island in the
afternoon hours. The USVI could observe moderate showers and nearby
thunderstorms as well, but the rainfall is expected to cause mainly
ponding of water on roadways and poor drainage areas. Most of the
activity near the USVI is expected to be in the morning hours, with
a brief break in the early afternoon, then passing showers and
isolated thunderstorms again in the late afternoon and in the
evening hours.

With the trailing edge of the tropical wave, the shower and
thunderstorm activity on Wednesday morning is expected to be mainly
over the local waters, southern and eastern PR and around the USVI.
Becoming less rainy over PR though lingering showers over the USVI as
Wednesday progresses. However, that`s according to the global
models, while the higher res models still favor plenty of activity
over land areas in PR into the afternoon hours. because of this
uncertainty, we split the difference in the models, putting showers
and thunderstorms in the forecast but with modest PoP to account for
uncertainty. Thursday looks slightly drier, with Saharan dust moving
in, but possible significant rainfall over localized areas, while
brief scattered showers are forecast over the local waters and
around the USVI.

The daytime high temperatures are forecast to be near 90 degrees
once again today across the lower elevations of PR and the USVI,
with heat index values in the low 100s. However, given the amount of
moisture and possible cloudiness expected today, it would not be
surprising for the daytime high temps to peak in the upper 80s at
the San Juan and Saint Thomas airport, similar to what was
observed yesterday.

.LONG TERM...Friday through Tuesday...

On Friday, according to both models (GFS and ECMWF) a high
pressure reflected in all the levels will dominate the weather
pattern. Meanwhile, at the surface, several patches of low-level
moisture with precipitable water values above climatological
normals will move into the area. As a result, afternoon convection
over the interior and western sections of Puerto Rico is
expected. Late Saturday into Sunday, a more east-northeasterly
wind flow will dominate the area and will push some fragments of
low level moisture, with precipitable water values from 1.6 to
1.9 inches over the area. Shower activity is expected over the
local area, as the available moisture combines with a Tropical
Upper Tropospheric Trough (TUTT) moving southwest. Additionally,
mid levels temperatures are expected to range from -6 to -7
Celsius, which is slightly below normal, and could contribute to
enhanced thunderstorms potential. So far, if the model guidance
were to verify, the best time for widespread activity is Sunday
and Monday.

For Tuesday, there is still uncertainty with this forecast, as
the different model guidance is not in agreement with the
position of the upper level trough. Nevertheless, an increase in
tropical moisture is expected with a wind surge from an
approaching tropical wave, currently being monitored by the
National Hurricane Center with a medium chance of formation (50%)
in the next 48 hours and a high chance (80%) of formation in the
next 5 days.

&&

.AVIATION...SHRA over the local area is causing VCSH or VCTS across
the USVI terminals early this morning, while VCTS is forecast at
TJBQ after 14/16Z. Winds will be light and VRB through 14/13Z, but
gusty near the SHRA/TSRA, otherwise winds will increase to around 10-
15KT from the east. Sea breeze variations in wind direction expected
at TJPS after 14/13Z. Overall, passing SHRA/TSRA are expected across
the local flying area today and tonight.

&&

.MARINE...Relatively tranquil marine conditions will prevail for
the next several days across all of the local waters and passages
with seas up to 5 feet. Wind flow will be mostly from east up to
15 knots. For today, isolated showers and thunderstorms are
forecast over the local waters. There is a moderate risk of rip
currents for the north exposed beaches of Puerto Rico, as well as
the beaches located over the eastern tip of Culebra, Vieques, and
St. Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 78 88 78 / 50 60 50 40
STT 89 78 87 79 / 40 50 60 40

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
354 AM AST Wed Sep 15 2021

.SYNOPSIS...Surface high pressure across the eastern Atlantic,
and a surface trough just north of the Bahamas, will cause SE
winds today, becoming ESE on Thursday. Upper level low over
Hispaniola will continue to slowly move west, keeping instability
over the local area through today, which will combine with the
above normal moisture, to cause showers and thunderstorms over the
local area. For the latter part of the workweek, Saharan dust is
expected to start moving in on Thursday, persisting through
Friday, while an upper level high pressure moves over the local
area, causing an increase in stability and decreasing the
thunderstorm coverage. Another upper level low will start moving
in late in the weekend into early next week.


&&

.SHORT TERM...Today through Friday...

Another unsettled day is forecast across the local area. A tropical
wave streaming westward across the area along with a surface trough
east of the Bahamas is resulting in an east-southeast to southeast
wind flow as well as pulling in deep tropical moisture with
precipitable water ranging between 2.0-2.2 inches, which is slightly
above normal for this time of the year. A Tropical Upper
Tropospheric Trough (TUTT) located over the island of Hispanola will
continue to provide divergence aloft as well as enhanced upper-level
instability with 500 mb temperatures expected to range between -7
and -8 degrees Celsius. The combination of these features will
continue to result in enhanced shower and thunderstorm activity
across portions of the local area for today. Thunderstorm activity
has been active during the overnight hours across the Caribbean
waters and the waters between eastern Puerto Rico and the USVI. Some
of this activity is expected to stream into portions of eastern
Puerto Rico and the USVI during the early through mid-morning hours.
If enough diurnal heating materializes, which is uncertain at this
time due to mid and high level clouds currently over the area, then
strong shower and thunderstorm activity is likely to develop across
the interior and northwestern quadrant of Puerto Rico during the
afternoon hours with heavy rainfall, frequent lightning, and gusty
winds being the main impacts. Additional activity is also possible
across the eastern half of Puerto Rico and the U.S. Virgin Islands.
Urban and small stream flooding as well as mudslides in areas of
steep terrain will be possible with the heaviest and most persistent
rainfall activity.

Thursday will be a transition day to a less active weather pattern.
The TUTT will be moving further westward, with a mid to upper-level
high pressure building in. This will result in a drier and more
stable air mass aloft. At the same time, a weak Saharan Air Layer
will be moving in providing some drier air at low-levels.
Nevertheless, moisture content will continue to remain at near
normal levels with precipitable water values expected to range
between 1.8-1.9 inches. This will be more than sufficient to combine
with daytime heating and local effects to result in another round of
afternoon convection across the interior and northwestern sections
of Puerto Rico as well as across portions of the San Juan metro
area. Across the USVI, just a few showers are expected.

Even drier conditions are expected for Friday, with the
aformentioned mid to upper-level high pressure dominating aloft,
creating stable conditions. This will result mainly fair weather
conditions with limited shower activity across most of eastern
Puerto Rico and the USVI, however, afternoon convection due to local
and diurnal effects and near normal moisture content can still be
expected across the southwestern quadrant of Puerto Rico as the low-
level flow switches from the east-northeast.

.LONG TERM...Saturday through Thursday...

Higher than normal uncertainty in the long term period, as the local
guidance is trying to solve a few low pressure systems that could
become tropical cyclones during this time period. Depending on the
development and eventual trajectory of these systems, the local
forecast will have to change accordingly. At this time, the global
and long range models are suggesting that a surface high pressure
will be over the NE Atlantic, while a SFC low, probably a tropical
cyclone develops across the western Atlantic, and another tropical
cyclone develops across the tropical Atlantic. At this time, the
long range models are not suggesting a direct impact to the local
islands from a tropical cyclone. However, the National Hurricane
Center is closely monitoring these systems and they will provide the
official forecasts on these systems.

Having said that, the best possible forecast at this time is that
essentially every night and early morning, we can expect isolated to
scattered showers, then locally induced showers and thunderstorms
each afternoon over areas of sea breeze convergence in Puerto Rico,
while isolated to scattered brief showers are forecast elsewhere. An
upper low is expected to develop to the NE of the local islands by
Saturday, then move west and meander to the north of Hispaniola
until Thursday. Near to slightly below normal moisture is forecast
this weekend, and the overall moisture thereafter will start to
depend on the development and trajectory of the tropical wave that
is currently a few hundred miles to the south of the Cabo Verde
Islands. We decided to go with the GFS model forecast, having a
tropical cyclone several hundred miles to our east and northeast
early next workweek, causing mainly E-NE winds on Monday into
Tuesday, then more northerly for Wednesday and Thursday. The shower
activity with that scenario will be mainly of scattered showers over
the waters, USVI, and across northern PR.

As far as temperatures, we can expect near normal highs and lows in
the long term period. Highs in the upper 80s to near 90 across the
lower elevations, with lows in the mid 70s to near 80. The higher
elevations could observe highs and lows about 5 to 10 degrees cooler
than the lower elevations, even slightly cooler in a few isolated
areas.

Once again, this forecast has higher than normal uncertainty, so
lower than normal confidence for the long term period. Please
monitor the Tropical Weather Outlook and other products from the
National Hurricane Center regarding the tropical waves int he
Atlantic.


&&

.AVIATION...SHRA and TSRA could move into the USVI and eastern PR
through at least 15/14z, resulting in VCTS across TJSJ/TISX/TIST.
Between 15/16z and 15/22z, SHRA and TSRA activity across interior
and western Puerto Rico resulting in VCTS across the western PR
terminals. MVFR to possibly IFR conditions will be possible with
any activity moving over the terminals. Winds will generally be
light and variable through 15/12z, except near TSRA, increasing to
10-15 kts from the ESE with sea breeze variations after 15/14z.


&&

.MARINE...Seas of 4 feet or less and winds up to 15 knots are
expected today and through the weekend. There is a low risk of rip
currents today across all of the local beaches.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 78 90 78 / 50 40 50 30
STT 88 79 90 78 / 50 30 30 30

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
436 AM AST Thu Sep 16 2021

.SYNOPSIS...

Drier and stable conditions aloft are expected during the short
term period. However, a gradual erosion of low level moisture
across the forecast area will promote shower and thunderstorm
development over the interior and west to northwest sections of
Puerto Rico, and also from El Yunque toward the San Juan metro
area today. Long term period looks very uncertain as the local
guidance continue to try solving a few low pressure systems that
could become tropical cyclones.

&&

.SHORT TERM...Today through Saturday...

Tutt low now well west of the region , as a mid to upper level ridge
will gradually build across the region today through Saturday. This
will result in drier and stable conditions aloft during the period.
The broad surface low over the western Atlantic will continue to
lift farther northward while a surface high pressure ridge will
build north and east of the region through Saturday. This will
result in a gradual erosion of low level moisture across the
forecast area. The prevailing southeasterly wind flow will become
more easterly and increase Friday into Saturday as the high
pressure ridge builds across the Northeastern Caribbean. A low to
moderate concentration of Saharan dust particulates will spread
across the region during the next few days. This will also aid in
drying out out the low levels.

However, for the rest of the morning hours, the environmental
conditions will remain favorable enough to generate early showers
and isolated thunderstorms over the coastal waters and parts of the
north and east to southeast coastal sections of the islands. During
the afternoon hours, the available moisture along with daytime
heating and local effects will favor shower and thunderstorm
development over the interior and west to northwest sections of
Puerto Rico, and also from El Yunque toward the San Juan metro area.
Isolated afternoon shower development will also remain possible in
and around the USVI, but the activity should be of short duration.

Urban and small stream flooding will be possible during the
afternoon over Puerto Rico, with ponding of water on roadways and
in poor drainage areas likely with the moderate to locally heavy
rains during the morning hours. As mentioned , Saharan dust will
filter in from the east, resulting in hazy skies.

By Friday, mid-level ridge will strengthen across the region,while
the surface high pressure will generate an easterly wind flow while
slightly increasing. Somewhat drier conditions are expected for
Friday, with a more seasonal weather pattern expected. Passing
trade wind showers can therefore be expected during the morning
hours followed by hazy conditions during the day along with locally
and diurnally induced afternoon shower development. A similar
weather pattern is so far expected for Saturday, however the Saharan
air layer should diminish across the region by then.

.LONG TERM...Sunday through Friday...

The National Hurricane Center, which provides the official
forecast on tropical cyclone activity, continues to monitor
various areas of interest across the Atlantic basin with low to
high formation chance during the next 5 days. The western-most
feature, or Invest 95L, located more than 500 miles south-
southwest of the Cabo Verde Islands may pose a threat to the NE
Caribbean. Since global models continue to disagree on the
evolution of this system, the uncertainty for the long term period
remains higher than normal. Please monitor the Tropical Weather
Outlook and other products from the National Hurricane Center
regarding the tropical waves int he Atlantic. However, current
model guidance indicates that the best possible scenario for the
long term period will be controlled by the typical seasonal
weather pattern. Expect overnight and early morning isolated
showers over parts of eastern portions of Puerto Rico and the U.S.
Virgin Islands. Then, local effects, diurnal heating, ample low
level moisture and favorable dynamics aloft will result in showers
and thunderstorms across the areas of sea breeze convergence in
Puerto Rico. The upper low is expected to move west and meander to
the north of Hispaniola until Thursday. The overall moisture
thereafter will start to depend on the development and trajectory
of the tropical wave that is currently a few hundred miles to the
south of the Cabo Verde Islands.

&&

.AVIATION...

Brief periods of MVFR conds in SHRA/Isold TSRA will
continue across the regional waters and around PR and USVI at least
til 16/14Z. SCT-BKN lyrs nr FL025...FL040...FL100. Prevailing VFR
conds expected at all terminals with mainly VCSH psbl through the
morning hours. Mtn TOP obscr psbl with passing SHRA ovr eastern PR
due to low clds and SHRA. Sfc wnds light and variable to calm...
bcmg fm east to southeast 10-15 kts aft 16/14Z with sea breeze
variations. Aftn convection mainly ovr interior and W PR fm 16/17Z-
16/23Z with VCTS psbl at TJBQ/TJMZ and VCSH psbl at most of
remaining terminals.


&&

.MARINE...

Moderate trade winds are expected to continue for the next several
days. Generally tranquil marine conditions expected across the local
waters due to seas of 4 feet or less with winds between 5 and 15
knots. These conditions are expected to prevail through the
weekend. For the beaches, the rip current risk is low.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 78 88 78 / 40 40 40 40
STT 91 76 91 76 / 50 50 50 30