Caribbean - Central America Weather

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
411 AM AST Tue May 4 2021

.SYNOPSIS...A mid to upper-level ridge will hold over the region
through Thursday, resulting in a relatively stable weather pattern
for Puerto Rico and the U.S. Virgin Islands. Conditions are
forecast to become unsettled late this week into early next week
as a polar trough moves near the area.

&&

.SHORT TERM...Today through Thursday...
High pressure over the central Atlantic at lower levels will
continue to generate moderate--locally fresh--trade winds through
the period that will carry shallow patches of moisture across the
area at fairly regular intervals for night and morning showers in
eastern Puerto Rico and around the U.S Virgin Islands and
afternoon showers in western Puerto Rico during each afternoon.
The atmosphere will be generally dry above 12 thousand feet today
limiting showers. A weak short wave trough at upper levels will
cause weakness in the mid level pressures and cooling of several
degrees C on Wednesday which will increase the chances of
thunderstorms over western Puerto Rico during the afternoon.
Little change at any level is foreseen at this time for
Thursday.

&&

.LONG TERM...Friday through Wednesday...
Current model guidance continues to depict an unsettled weather
pattern for most of the region on Monday through Tuesday, with
moderate to heavy rainfall and isolated thunderstorms. The deep
tropical moisture will yield urban and small stream flooding and
ponding of water on roadways for many areas. However, before the
deep tropical moisture reaches our area on late Sunday night
through Tuesday, a fairly active weather pattern is expected
Friday through Sunday, with showers across eastern and northern
Puerto Rico and the U.S. Virgin Islands during the morning hours,
then showers and possibly isolated thunderstorms across the
interior and western areas of Puerto Rico during the afternoon.
The GFS and ECMWF move a polar trough into the western Atlantic
waters on Friday, the polar trough will weaken the mid-level ridge
and cause the winds to become lighter and veer to the east to
southeast. Convection that develops during the afternoon will move
slowly due to the lighter wind flow across the region; at this
time deep convection is not expected due to the dry mid to upper-
levels of the atmosphere. The GFS cross-section shows the
relative humidities ranging from 15 to 30 percent from 500 to 250
mb. The dry layers should limit the vertical growth of showers and
limit the development of deep convection during the afternoons.

Monday and Tuesday, The GFS keeps the base of the polar trough to
the southwest of Hispaniola which will cause upper-level divergence
across Puerto Rico which leads to upward motions which will aid in
the vertical development of thunderstorm. The GFS and ECMWF has 500
mb temperatures cooling to minus 8 degrees Celsius on Monday and
Tuesday. An easterly perturbation moving across the southern
Caribbean waters coupled with deep tropical moisture and great
instability will create favorable conditions for the development
of deep convection across most of Puerto Rico and the U.S. Virgin
Islands. The GFS shows precipitable water reaching 2.2 inches,
which will rank near the 99th for the month of May. Therefore, we
expect urban and small stream flooding and ponding of water on
roadways during the beginning of the work-week.

On Wednesday, deep moisture and modest instability will remain
across the region and aid in the development of deep convection
during the afternoon over Puerto Rico and the U.S. Virgin Islands.
Urban and small stream flooding and ponding of water on roadways
is expected from showers and isolated thunderstorms.

&&

.AVIATION...VFR conds to prevail across the forecast area. Sct SHRA
across NE PR thru 04/15Z and dvlpg SHRA across wrn PR aft 04/16Z
will cause brief MVFR and mtn obscurations. Some SHRA will affect
TJMZ. Sfc winds less than 10 knots in land breezes bcmg aft 04/14Z
12-18 kt and higher gusts with sea breeze influences. Max winds NW
35-45 kt btwn FL360-480.

&&

.MARINE...A northerly swell will cause seas to increase to 6 feet
across the offshore Atlantic waters and promote a high risk of
rip currents for the northwestern to central beaches of Puerto
Rico. Seas up to 5 feet expected elsewhere and winds up to 20 kts
for some of the local waters.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 88 77 87 77 / 20 60 40 30
STT 88 75 88 76 / 30 40 30 20

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
422 AM AST Wed May 5 2021

.SYNOPSIS...A mid to upper-level ridge will continue to dominate
the local weather through Thursday. A surface high pressure north
of the area will continue to result in easterly trade wind flow,
which will drive patches of low-level moisture into the area. The
easterly trade winds and local and diurnal effects will induce
isolated to scattered showers. Conditions are forecast to become
unsettled this weekend and early next week as a polar trough moves
near the area.

&&

.SHORT TERM...Today through Friday...
High pressure over the central Atlantic at lower levels will
continue to generate moderate--locally fresh--trade winds through
the period that will carry shallow patches of moisture across the
area at fairly regular intervals for night and morning showers in
eastern Puerto Rico and around the U.S Virgin Islands and afternoon
showers in western Puerto Rico during each afternoon. The atmosphere
will be generally dry above 12 thousand feet limiting showers. A
weak short wave trough at upper levels was moving through earlier
this morning and will cause less effect than earlier believed. The
GFS still expects mid level cooling of several degrees C which
should still increase the chances of thunderstorms over western
Puerto Rico during the afternoon. Little change at any level is
foreseen at this time for Thursday and Friday, but stability will
increase and thunderstorm formation will become more difficult.

&&

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

The global models continue to adhere to an unsettled weather
pattern commencing late Sunday and persisting through the middle
of the work week. A polar trough, currently over the eastern U.S.
will push across the western Atlantic waters on Friday and slowly
deepen west of Puerto Rico through early next week. The polar
trough will weaken the upper-level ridge across the region and
cause the winds to become lighter and veer to the east to
southeast this weekend. The veering winds will slowly moisten the
atmospheric column during the weekend. The GFS has precipitable
water peaking to 2.0 inches late Sunday night and precipitable
water values are forecast to remain above climatology through the
middle of the work-week.

The polar trough will be the trigger that will promotes the
development of deep convection across Puerto Rico, U.S Virgin
Islands, and the local waters late this weekend through the
middle of next week. The GFS and ECMWF are in agreement showing
the polar trough southwest of Hispaniola. The position of the
trough will cause upper-level divergence across Puerto Rico and
the U.S. Virgin Islands, and trigger upward motion. This vertical
motion will cause showers to mature to thunderstorms during the
long-term period. The GFS and ECMWF are in agreement showing the
500 mb temperatures cooling to minus 7 to 8 degrees Celsius
starting on Sunday and continuing through Monday.

The strong instability across the region due to the position of the
polar trough and the robust moisture will yield moderate to heavy
rainfall and isolated thunderstorms for most of the region through
Monday evening. The moderate to heavy rainfall will cause Urban and
small stream flooding across Puerto Rico and the U.S. Virgin
Islands. Furthermore, winds will be relatively light across the
region; the light winds will cause convection to move slowly across
the local area which could further exacerbate the flooding for many
areas.

The GFS and ECMWF solutions begin to diverge late Monday through
Thursday. The GFS weakens the polar trough and lifts its energy
away from Puerto Rico. The ECMWF deepens the trough as several
short- waves deepens the through west of Puerto Rico.
Nevertheless, the weather pattern will remain unsettled with
showers with showers across eastern and northern Puerto Rico and
the U.S. Virgin Islands during the morning hours, then showers and
possibly isolated thunderstorms across the interior and western
areas of Puerto Rico during the afternoon.

&&

.AVIATION...Mainly VFR conditions will prevail across the forecast
area. Minor -SHRA to cont arnd the USVI and ern PR, then afternoon
SHRA dvlpmnt in the vcty of TJSJ, TJPS, TJBQ, TJMZ and the USVI
terminals through 05/22Z with areas of MVFR/IFR and mtn
obscurations. Isold TSRA psbl. Sfc winds 10-20 knots with gusts up
to 28 kt and sea breeze variations btwn 05/14-22Z, becoming light E
with land breezes after 06/02Z. Max winds NW 25-35 kt btwn
FL390-450.

&&

.MARINE...Small craft operators are urged to exercise caution
due to winds up to 20 kts causing choppy seas for some of the
local waters. Seas are up to 5 feet across the Atlantic and
Caribbean waters and local passages. There is a moderate risk of
rip currents for the local beaches of Puerto Rico and the U.S.
Virgin Islands.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 87 77 87 77 / 30 30 30 20
STT 88 76 88 75 / 30 20 20 20

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
447 AM AST Thu May 6 2021

.SYNOPSIS...Scattered showers in modest areas of moisture will
move through on moderate to locally fresh easterly trade winds
today through Saturday. Winds are expected to turn more
southeasterly on Friday and tropical moisture from the southeast
is still expected to move into the area on Sunday. A trough
reminiscent of a tropical wave will move through the U.S. Virgin
Islands and Puerto Rico Sunday afternoon through early Monday
morning and spread showers and thunderstorms across the area with
urban and small stream flooding likely in localized areas.
Improvement should begin on Monday, but moisture Tuesday through
Thursday of next week will remain more abundant than that of the
next few days.

&&

.SHORT TERM...Today through Saturday...
A mid to upper-level ridge will promote a fairly stable and dry
weather pattern through the short-term period due to an adequate
trade wind cap that is encompassing the region. An east to northeast
wind flow from the surface to 700 mb will drag shallow moisture
across Puerto Rico and the U.S. Virgin Islands periodically. The
shallow moisture and local and diurnal effects will induce showers
mainly over the western, northwestern, and interior areas of
Puerto Rico. A few streamers developing this afternoon near the
U.S. Virgin Islands and metro San Juan are possible.

Friday, the surface winds will become lighter and take an east to
southeast component. The east to southeast winds on Friday and
Saturday will cause afternoon temperatures to soar. Although,
moisture content will be limited; local and diurnal effects will
yield a few showers across Puerto Rico and the U.S. Virgin Islands
each day during the afternoon hours. At this time due to the warm
500 mb temperatures and limited moisture content, no significant
wetting rains are expected on Friday and Saturday.

.LONG TERM...Sunday through Wednesday...
An approaching cold front over the Atlantic will pull air out of
the southeast. The GFS still shows a band of moisture reminiscent
of a tropical wave moving up with it and entering the forecast
area on Sunday. This band is fairly quick moving and will pass
through Puerto Rico before dawn on Monday. Although the airmass
then dries out by Tuesday, precipitable water values will not
return to current levels before the end of the period. 500 mb
temperatures plunge to minus 8 degrees overnight on Sunday and are
still as low as minus 7 degrees on Monday, so there should be
enough instability to generate at least isolated thunderstorm
Sunday night through Monday evening, especially since mid-levels,
once they moisten on Sunday, will not dry out again until the end
of next week. Upper levels are generally suppressing convective
activity and will do so until a trough deepens over Cuba on
Sunday. The trough is blocked by upper level high pressure to our
southeast and is not able to progress into the area. Also the
ridge from the high to the southeast will force anti-cyclonic flow
over the area and push much of the jet coming out of the trough
to our north. This will also help to limit showers and
thunderstorms Monday afternoon or overnight into Tuesday. In fact,
these unfavorable conditions may actually commence on Monday so a
close watch for improvement then will need to be made with
successive model runs.

&&

.AVIATION...Mainly VFR conditions will prevail through 06/17Z, then
SHRA/VCSH are forecast develop near terminal sites TJBQ, TJMZ, and
portions of nwrn PR. The activity this afternoon could cause
brief MVFR conditions near the above TAF sites diminishing by
07/01Z. Sfc winds from the east at 10-20 knots with higher gusts
and sea breeze variations btwn 05/14-22Z. Max winds WNW-NW 35-50
knots btwn FL 410-500.

&&

.MARINE...Seas remain 3 to 5 feet over most open waters through
the next 5 to 7 days. A swell from the northeast will enter the
local Atlantic waters and come through the Anegada Passage on
Wednesday, but even then small craft advisories are not expected.
Thunderstorms are expected over the local waters Sunday through
Monday and may present the greatest threat to mariners of the
next 7 to 10 days.

&&

.FIRE WEATHER...Winds and low fuel moisture levels will keep the
threat of dangerous fire behavior at elevated levels on the south
coast of Puerto Rico. Most other areas have had some wetting rains
to dampen fuels.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 87 77 89 76 / 30 30 20 20
STT 88 75 87 76 / 20 20 20 20

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
525 AM AST Fri May 7 2021

.SYNOPSIS...Isolated showers in the night and morning periods for
the waters around the U.S. Virgin Islands and eastern Puerto Rico,
followed by scattered to numerous showers in the west and interior
portions of Puerto Rico each afternoon will characterize the
weather today and Saturday. Moisture will increase considerably
on Sunday and continue to be generous Tuesday and Wednesday,
following an easterly wave passage on Sunday. Showers and
thunderstorms will diminish through the rest of next week.

&&

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

Isolated to scattered showers are expected across eastern Puerto
Rico and the U.S. Virgin Islands during the morning, then showers
will develop across the northwestern and interior areas of Puerto
Rico during the afternoon. Afternoon, convection could cause
urban and small stream flooding especially in western Puerto which
has received one to three inches of rain over the last several
days. Late tonight into Sunday the surface winds will turn to the
east to southeast. The east to southeast winds will cause daytime
high temperatures to soar with temperatures raging from the upper
80s to lower 90s near lower elevations and the upper 70s to lower
80s in higher elevations.

On Sunday, a mid to upper-level ridge will hold over the region
through Saturday morning then weaken due to a deepening polar
trough across the western Atlantic waters. The base of the polar
trough is forecast to be southwest of Hispaniola by Sunday
afternoon. The position of the trough will cause upper-level
divergence across Puerto Rico and the U.S. Virgin Islands and
trigger upward motion. The vertical motion will cause showers to
mature to thunderstorms on Sunday across most of Puerto Rico and
the local islands on Sunday. The GFS and ECMWF are in agreement
showing the 500 mb temperatures cooling to minus 7 to 8 degrees
Celsius Sunday. Therefore, we expect moderate to heavy rainfall
and isolated thunderstorms for most of the area on Sunday. Winds
will be relatively light across the region and the lighter winds
will cause convection to move slowly and lead to urban and small
stream flooding and ponding of water on roadways for most of the
local islands.


.LONG TERM...Monday through Thursday...
The long wave polar trough that dug into the Windward Passage will
pull out and north of the forecast area Monday through Wednesday.
Winds aloft will become lighter and migrating surface highs will
produce a gradient sufficient to maintain moderate to locally
fresh easterly trade winds through the period. Moisture will
remain relatively high Monday through Tuesday night, but will
begin to decline significantly Wednesday through Thursday and
beyond. The passage of the easterly wave on Sunday will mark the
peak moisture at lower levels Sunday and Sunday night, and
widespread showers with isolated thunderstorms will linger into
Monday. Shower and thunderstorm activity will decline Tuesday and
Wednesday as temperatures stop warming in the lower levels and
the 500 mb temperatures climb causing an increase in overall
stability. Then decreasing moisture will hasten the trend toward
diminished shower activity later in the week as temperatures
return to a warming trend. Although showers and thunderstorms
will diminish, some could still be persistent enough to cause
urban and small stream flooding in localized areas most days.

&&

.AVIATION...VFR conditions will persist through 07/16Z. Then SHRA
are forecast to develop across terminal sites TJBQ and TJMZ. The
afternoon convection could cause brief MVFR conditions across
terminal sites TJBQ and TJMZ through 07/23Z. Convection could impact
TJSJ/TIST/ISX thru 07/23z. Winds will continue from the east at 10
to 15 knots with local sea breeze variations and higher gusts. Winds
will become east to southeast by 07/15Z across the region.

&&

.MARINE...Seas will increase somewhat with the passage of an
easterly wave Sunday and Monday, but conditions are expected to
remain below small craft advisory criteria through the next 7
days. Mariners will see an increase in thunderstorm activity
Sunday and Monday. The threat of rip currents will be low to
moderate through mid week next week.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 76 89 76 / 20 40 50 40
STT 87 76 86 77 / 20 40 30 30

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
425 AM AST Sat May 8 2021

.SYNOPSIS...A diurnal weather pattern will continue to prevail
today through Sunday afternoon, with passing showers affecting
portions of the eastern Puerto Rico and the USVI during the
overnight and morning hours followed by afternoon convection
across northwestern Puerto Rico. By Sunday night into Monday, a
tropical wave is forecast to move across the area, with more
widespread shower and isolated thunderstorm activity expected.

&&

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

A broad surface high pressure across the central Atlantic will
promote light to moderate east to southeast trades through Sunday.
Winds will increase briefly and turn more from the east between late
Sunday night and early Monday, as another surface high over the
western Atlantic merges with the high over the central Atlantic and
a tropical wave moves across the region. Meanwhile, a polar
trough will move over Hispaniola today and remain west and over
portions of the local area through early Monday.

For today, the combination of the available low-level moisture with
day time heating and the sea breeze convergence will result in
afternoon showers over west/northwest PR and from streamers
developing off the USVI and eastern mountains of PR. Maximum
temperatures are expected to range from the mid 80s to low 90s
across the lower elevations and urban areas of the islands.
Increasing moisture content and unstable conditions aloft will
promote better chances for showers and isolated thunderstorm
development across portions of the islands and regional waters from
late Sunday into Monday morning. Urban and small stream flooding can
be expected with the heaviest showers.

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

Weather conditions at this time are expected to improve by Tuesday
as the tropical wave moves away from the area and the moisture
content diminishes with low to mid-levels of the atmosphere drying
out. This is in response to a mid-level ridge expected to build
in from the western tropical Atlantic into the area. Therefore,
shower activity is expected to be limited across most of the local
area. The mid-level ridge is expected to hold through the end of
the long term period, limiting the potential for organized and
widespread convective activity from materializing. Meanwhile, at
lower-levels, a series of surface high pressures will move across
the Atlantic basin north of the area. This will result in a
moderate easterly wind flow, which will push patches of low-level
moisture into the area from time to time. Therefore, the typical
diurnal weather pattern is expected with some passing showers
during the overnight and morning hours across the USVI and eastern
Puerto Rico, followed by the development of afternoon convection
across the western half of Puerto Rico. The afternoon convection
across western Puerto Rico may be heavy enough to result in some
urban and small stream flooding.

&&

.AVIATION...Mainly VFR conditions expected to prevail across all
terminals. However, SHRA is expected to develop in and around
TJSJ/TJBQ btw 16z-22z, this could cause MVFR conds. VCSH mainly
across the USVI terminals. Low-level winds ESE at 10-15 kts with sea
breeze variations aft 14z.

&&

.MARINE...Mostly tranquil seas of between 2 and 5 feet are
expected to prevail across the regional waters today and Sunday.
More choppier seas are possible by early next week as winds
increase due to passage of a tropical wave, however, no small
craft advisory conditions are expected at this time. With the
passage of the tropical wave, shower and isolated thunderstorm
activity is expected to increase across portions of the local
waters. For beachgoers, there is a moderate risk of rip currents
for some of the northern beaches of Puerto Rico as well as the
eastern tip of Culebra, Vieques and Saint Croix.

&&

.FIRE WEATHER...Latest surface observations continue to indicate
very dry soils across the southern coastal plains of Puerto Rico
with KBDI values over 700 in Guanica and Camp Santiago. Very
similar weather conditions are expected today compared to previous
days, with little to no rainfall activity across the southern
coastal plains. Relative humidity values are forecast to drop into
the mid 40s to low 50s across most areas with winds expected to
peak at around 15 mph with higher gusts. Therefore, the
combination of these factors will once again result in an elevated
fire danger threat across the southern coastal plains and a Fire
Danger Statement was issued accordingly.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 74 87 75 / 30 20 60 70
STT 86 78 86 78 / 30 20 50 50

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
524 PM AST Sun May 9 2021

.SYNOPSIS...

An easterly wave with an axis now crossing the Lesser Antilles
will approach the region overnight through Monday while
interacting with an upper trough west of the region. The influx of
tropical moisture accompanying the disturbance, along with the
positioning of the upper trough will favor a diffluent pattern
aloft along with good low level moisture convergence and
instability to favor increasing cloudiness and convection across
the forecast area overnight through Monday. A return to a more
seasonal and diurnal weather pattern is then expected by midweek,
when a mixture of sunshine and clouds will prevail each day.

&&

.SHORT TERM...Rest of today through Tuesday...

The aforementioned easterly wave and associated moisture field,
now over the Lesser Antilles, will enter the eastern Caribbean
and cross the local area overnight through Monday. In the upper
levels, an amplified trough with base now extending southwards
between Cuba and Hispaniola, will promote unstable conditions
aloft through at least Monday and possible into early Tuesday. The
total precipitable water (PWAT) content is still forecast to
gradually increase to above 2.00 inches through Monday, with winds
generally from the east to southeast. In addition, the 500 mb
temperatures are forecast to range between minus 7 to minus 8
degrees C, also favoring instability aloft. Consequently,
increasing tropical moisture and enhanced shower and isolated
thunderstorm activity is forecast to unfold across the region,
beginning later this evening and continuing through Monday. Under
this moist and unstable environment, urban and small stream
flooding is, so far, expected to be the main threat across all
the islands. However, some localized flash flooding and quick
river rises will be possible where thunderstorms develop over
mainland PR, especially on Monday.

For the rest of the evening and overnight into early Monday,
cloudiness, showers and thunderstorm activity will diminish over
the west and northwest, and the focus of shower development will
shift to in and around across the USVI and eastern sections of PR.

By Tuesday afternoon, a drier and more stable airmass is expected,
as the upper trough weakens and the easterly disturbance and
associated moisture passes. A mid-level ridge is forecast to then
gradually build across the northeastern Caribbean, as well as
weak ridging in the upper levels. This will promote decreasing
PWAT values, which are forecast to to drop to near 1.60 inches,
along with warmer and more seasonal 500 mb temps. This expected
pattern should then favor increasing easterly trades and therefore
limit widespread shower development and rainfall accumulations,
as well as lessen the potential for afternoon convection across
the islands, except for some locally and diurnally induced shower
activity mainly over the west interior sections of Puerto Rico
and downwind of the islands.

.LONG TERM...(From previous discussion/issued 402 AM AST Sun May
8 2021/...Wednesday through Sunday...

A mid-level ridge is expected to prevail through most of the long-
term period. This will result in mostly dry and stable conditions
aloft, limiting the potential for any widespread and organized
convective activity from materializing across the local area.
Meanwhile, at lower-levels, a broad surface high pressure will be
situated across the central Atlantic. This will drive moderate
east to east-southeast winds across the region on Wednesday and
Thursday. Then, on Friday and next weekend, winds are expected to
become more southeast as the surface high pressure moves farther
east into the Atlantic and a surface low pressure and an
associated frontal boundary, which will be located well north of
the area, develops over the western Atlantic. Patches of low-level
moisture riding the trade wind flow will be moving across the
area throughout the period. This will result in a typical diurnal
weather pattern with some passing showers across eastern and
southeastern Puerto Rico and portions of the USVI during the
overnight and morning hours followed by the development of
afternoon convection across the northwestern quadrant of Puerto
Rico. Additional afternoon showers in the form of streamers may
also develop downwind of the USVI and also downwind of El Yunque,
affecting portions of the San Juan metro area. The afternoon
activity across the northwestern quadrant of Puerto Rico may be
heavy enough to result in areas of urban and small stream
flooding. As winds veer from the southeast on Friday and next
weekend, afternoon high temperatures across northern Puerto Rico
may reach above-normal levels, in the lower 90s.

&&

.AVIATION...A tropical wave will bring SHRA/TSRA across the region,
and some of them will move over the terminals. MVFR condtions are
likely over JBQ, with VCSH/VCTS elsewhere. JBQ could expect brief
MVFR conds due to SHRA/TSRA thru 09/23z. SHRA/TSRA will spread
across most of the local flying area overnight, aft 09/23z. Surface
winds are expected from the ESE at 10-20 kts with sea breeze variations
and higher gusts, around 10 kt overnight, and increasing once again
after 10/14z.

&&

.MARINE...An approaching tropical wave will increase winds and
create choppy seas across the local waters. Expect seas up to 5
feet and winds between 15 and 20 knots. Therefore, mariners should
exercise caution across most of the regional waters and passages
overnight.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 76 83 74 88 / 60 70 50 40
STT 77 84 77 87 / 70 70 60 40

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
326 AM AST Mon May 10 2021

.SYNOPSIS...A tropical wave moving across the Caribbean Sea and an
upper-level trough west of the region will continue to promote
cloudy skies and scattered shower activity through at least today.
A mid-level ridge moving from the east will promote drier air
intrusion and more stable conditions across the islands on Tue-Wed.
A weak upper level trough will move briefly by midweek, while a
ridge builds from the western Caribbean into the region and holds
over the area through the weekend. Afternoon showers are expected
each day mainly over portions of western PR, due to local effects.


&&

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

Significant adjustments were made to the forecast for today. An
easterly wave will be moving across the region. This wave is bring
with it deep tropical moisture with precipitable water values
hovering at around 2.00 inches through early this afternoon.
However, despite the deep moisture currently present, latest global
and high-res model guidance are now much less bullish on rainfall
activity across the local area. The most logical explanation to this
shift to much less rainfall activity has to do that the upper-level
trough that was going to interact with the wave is located too far
west that all the best dynamics are now expected to be located west
of the area. As a result, conditions do not appear to be as
favorable for organized convective activity to materialize and
affect the area. Therefore, the forecasted rainfall probabilities as
well the accumulations have been adjusted downward. Nevertheless,
given the deep moisture still present, scattered shower activity is
still expected to affect portions of eastern Puerto Rico and as well
as in and around the USVI during the morning hours. Depending on how
much heating there is through the morning hours, afternoon shower
activity along with isolated thunderstorms may develop across
northwestern Puerto Rico. Given that the rainfall activity will not
be that impactful, the flooding potential will be limited today with
ponding of water on roadways and in low-lying areas expected to be
the main hazard.

By later this afternoon into tonight, dry air intrusion at around
700 mb is expected as a mid-level ridge begins to build overhead.
This ridge will then prevail through the rest of the short-term
period, resulting in a drier and more stable weather pattern across
the local area. Therefore, shower activity will be limited on
Tuesday and Wednesday. Nevertheless, there may still be sufficient
moisture in the low-levels to combine with local and diurnal effects
to result in some afternoon convection across western Puerto Rico.
Some passing showers cannot be ruled out at times across portions of
eastern Puerto Rico and the USVI during the overnight and early
morning hours.

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

An upper-level trough is forecast to move from the northwest
quickly on Thursday, as a mid-to-upper level ridge builds over the
western Caribbean and across the eastern Caribbean through the
end of the long term period. On Thursday, 500 mb temperatures are
forecast to remain around -8 degrees celsius due to the proximity
of the upper level trough. Meanwhile, at lower-levels moisture
content is expected to gradually decrease as the trade wind cap
increases in response to a ridge across the central Atlantic.
Having said that, left isolated thunderstorms in the forecast for
Thursday afternoon over western PR in diurnal activity.

For the rest of the long term period, drier air intrusion and
mid-level ridge will dominate the local weather conditions. Mostly
fair weather conditions should prevail across all the islands,
with the exception of afternoon showers developing over portions
of the western interior each day. Long term model guidance suggest
that a Saharan Air Layer may reach the local area by early next
week. An overall drying trend and above normal temperatures are
expected during the weekend, as precipitable water content drops
to around 1.30-1.50 inches and the ridge over the central Atlantic
moves further east and in response winds turn more southeasterly.


&&

.AVIATION...Mainly VFR conditions expected. Sct SHRA will be moving
across the area from time to time throughout the period resulting
VCSH across all of the local terminals. VCTS are possible across
TJBQ between 10/16z and 10/22z. Brief MVFR conditions are possible
if the SHRA activity moves over the terminals. BKN layer expected
mostly between FL050-FL080. Winds will continue from the E-ESE below
10 knots through 10/12z, increasing to around 15-18 kts with some
sea breeze variations after 10/14z.


&&

.MARINE...A tropical wave will continue to move mainly south of
the area and exit the local area later today. Easterly winds near
20 kts will cause choppy seas up to 6 feet across most of the
local waters due to the wave passage. Therefore, small crafts
should continue to exercise caution. There is a moderate risk of
rip currents for most of the beaches of Puerto Rico and the U.S.
Virgin Islands. A small northerly swell is expected to reach the
local Atlantic waters by midweek, this swell could cause a high
risk of rip currents along the northern beaches of Puerto Rico.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 86 76 88 78 / 50 30 30 40
STT 86 76 86 77 / 40 30 30 40

&&

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
302 AM AST Tue May 11 2021

.SYNOPSIS...A mid-level ridge is expected to continue over the
region during the next few days. Mid-to-upper level cloudiness
associated to a trough west of the region will continue today into
Wednesday. Afternoon shower development over western Puerto Rico
is expected through the long term period due to the combination of
the available low-level moisture and local effects. Passing
showers will continue to move at times across the U.S. Virgin
Islands and eastern PR.


&&

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

Latest GOES-16 infrared satelite imagery shows an abundance of mid
to upper-level cloudiness prevailing over area in response to an
easterly wave located west of the area and a weakening upper-level
trough located near the windward passage. Latest model guidance
suggests that the mid to upper-level cloudiness will prevail today
through at least Wednesday afternoon maintaining variably to
mostly cloudy skies. Meanwhile, a dry layer is noted between the
850-500 mb layer in response to a mid-level ridge building in.
This will basically limit the intensity as well as areal coverage
of shower activity across the local area through the short-term
period. Nevertheless, some patchy low-level moisture riding the
easterly winds will aid in sustaining some passing showers across
portions of eastern Puerto Rico and in and around the USVI during
the overnight and early morning hours. Then, depending on the
amount of daytime heating that occurs, some locally induced
showers and possibly a few thunderstorms may develop over western
Puerto Rico during the afternoon hours. As the aformentioned
upper- level trough continues to weaken and lift away late
Wednesday into Thursday, drying is expected in the mid to upper-
levels. Therefore, on Thursday, more significant heating is
expected and thus afternoon convection across western Puerto Rico
will have a better chance of developing. However, limited low-
level moisture will limit the longevity on any convection that
develops. Overall, given the limited moisture and unfavorable
conditions aloft, widespread as well as significant rainfall
activity is not expected through the period.

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

A mid-to-upper level ridge is forecast to hold over the region
through the long term period. A polar trough and associated
surface front are expected to remain north of area, while a
surface high pressure over the central Atlantic moves further east
late in the period. This will cause winds to turn more from the
southeast during the weekend. Warmer than normal temperatures are
expected with the southeasterly winds. Hazy skies due to Saharan
dust are possible early next the week, as a Saharan Air Layer is
forecast to enter the southeastern Caribbean. The upper level
ridge will promote drier air aloft and more stable conditions in
general.

At lower levels, moisture is expected to be capped below 700 mb,
and this shallow layer of moisture will bring occasional passing
showers across the USVI and eastern sections of PR during the
overnight/early morning hours. During the afternoon hours, the
combination of the available low-level moisture, daytime heating
and the sea breeze convergence will cause showers to develop
mainly over the west/northwest sections of PR, and from streamers
downwind of the USVI.


&&

.AVIATION...Mainly VFR conditions expected. VCSH are expected at
times across most area terminals, however, no significant impacts to
operations are expected. SCT-BKN layer between FL050-080 will
continue through the period. Winds will continue from the E at less
than 10 kts through 11/12z, increasing to 15-18kts with some sea
breeze variations after 11/14z.


&&

.MARINE...Choppy wind driven seas up to 6 feet will continue today
across the offshore waters and local passages. East winds between
15 to 20 knots are expected in general, decreasing 10 to 15 kt
during the second part of the week. A 4-5 feet, long period
northerly swell is forecast to move briefly across the Atlantic
waters from Wed night through Thursday. A moderate rip current
risk will continue across the north, east, and western beaches of
Puerto Rico, and across all the beaches of the U.S. Virgin
Islands, Vieques and Culebra.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 88 78 87 77 / 30 40 30 30
STT 87 76 87 77 / 30 30 20 20

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
454 AM AST Thu May 13 2021

.SYNOPSIS...

Relatively calm, stable conditions are expected through early next
week. Though stability aloft will inhibit significant, organized
convective development, patches of low-level moisture embedded in
the trade winds will help to sustain shower activity in a typical
pattern. The best chance for rain over the next week and half is
late next week, with more favorable conditions aloft expected.

&&

.SHORT TERM...Today Through Saturday...

Upper level trough just west of the region extending southwards
across Hispanola will continue to weaken through the period. Mid
level ridge will hold in place while building across the area
through at least Saturday. This mid to upper level pattern will
aid in limiting convective development during the short term
period. A broad surface ridge extending across the region from the
north central Atlantic, will maintain light to moderate easterly
winds today but prevailing winds are expected to become more
southeasterly by Friday into Saturday. During this time, an
induced easterly perturbation is forecast to cross the region and
bring a slight increase in trade wind moisture. This will result
in better potential for shower development with periods of
moderate to locally heavy rains and consequently increased chance
for minor ponding of water on roadways and poor drainage areas can
be expected.

Based on most recent model guidance, moisture transport is forecast
to be minimal, except for the aforementioned easterly perturbation.
However, occasional patches of shallow moisture in the prevailing
easterlies will bring occasional showers to the local waters and
parts of the coastal municipalities during the late evening and
early morning hours. Each afternoon, locally and diurnally induced
isolated to scattered showers can be expected mainly over the
central interior and western Puerto Rico. Elsewhere, mostly sunny
skies and fair weather conditions will prevail with no significant
operational or weather impacts forecast at this time. A few
afternoon showers may develop in the form of streamers mainly on the
west end and just downwind of the islands. Minimal rainfall
accumulations are so far expected.

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

Relatively stable conditions will dominate across the region for the
first part of the week next week. A mid- to upper-level ridge is
expected to remain in place over the area into Tuesday. However,
some signs of weakness in this ridge will start to appear Monday
around 500 hPa. The ridge will erode, and increasing moisture and
instability is expected for the upper mid-levels.

Then, Wednesday into Thursday, a mid- to upper-level trough will
approach from the west, associated with a polar trough moving off of
the coast of North America. At lower mid-levels, however, model
guidance suggests that the ridge will be slower to yield to the
trough, which could inhibit shower activity. On the whole, however,
more unstable, wetter air will be in place over the region for the
second half of the week. Precipitable water values are forecast to
remain within normals, however, and the lingering ridge around 850
to 700 hPa are likely to result in conditions remaining typical for
mid-May.

&&

.AVIATION...

VFR conds durg prd. SCT ocnl BKN lyrs nr FL025...FL050.
Mstly Isold SHRA ovr regional waters and en route btw islands. Brief
Mtn Top obscr ovr E interior sections of PR due to low clds and
-SHRA. Sfc winds FM E less than 10 kts bcmg 10-20 kt with ocnly hir
gusts and seas breeze variations. L/Lvl wnds fm E 15-20 kts BLO 700
millibars then backing and incr w/ht ABV. No sig operational wx
impacts attm.

&&

.MARINE...

Choppy seas up to 6 feet remain possible this morning, mostly in the
offshore Atlantic waters and Anegada Passage. There is a lingering
weak northerly swell that will continue to gradually subside
today, as well. Winds generally out of the east up to 15 to 20
knots.

There is a high risk of rip currents for beaches of northwestern and
north central Puerto Rico. The rest of the north coast of Puerto
Rico, as well as the beaches of Vieques, Culebra, the USVI, and
southeastern Puerto Rico have a moderate risk of rip currents.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 88 77 88 77 / 20 20 20 20
STT 87 78 86 76 / 20 30 40 40

&&

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
451 AM AST Fri May 14 2021

.SYNOPSIS...

An increase in shower activity, in a typical pattern, is expected
for today compared to yesterday, with increasing moisture over
the region. Drying is expected for the early part of the next
workweek, with hazy conditions presently expected for Monday, as a
Saharan Air Layer makes its way across the local islands. Moisture
and instability are forecast to increase around midweek next week,
and another increase in shower activity is anticipated.

&&

.SHORT TERM...Today Through Sunday...

Upper level trough axis now extends across the northern Leeward
islands, with a mid to upper level ridge forecast to hold in place
across the forecast area at least through Saturday. A short wave
trough is however expected to cross the west and southwest Atlantic
over the next few days and pass just north of the region by Sunday
while eroding the upper level ridge. This will result in marginally
unstable condition aloft on Sunday. In the low levels, the
combination of a broad surface high pressure ridge extending
southwest across the region from the northeast Atlantic, and a
surface trough across the eastern Caribbean, will promote an east
southeast wind flow across the region through the period. This will
favor fairly warm daytime temperatures especially along the north
coastal areas each day. In addition, occasional patches of trade
wind moisture enhanced by the lingering upper trough just east of
the region will bring periods of passing low level clouds and
showers to the local waters and parts of the islands especially
during the overnight and early morning hours.

For today through Saturday, recent model guidance and precipitable
water values all suggest that there will be sufficient moisture
transport availability each day to combine good daytime heating and
local effects to support afternoon shower development mainly over
parts of the central interior and northwest sections of Puerto Rico,
as well as in and around parts of the San Juan metro. Some of the
afternoon showers will produce locally heavy rains and could lead
to minor localized urban and small stream flooding and ponding of
water on roadways and in poor drainage areas. Plenty of sunshine
is expect for the U.S. Virgin Islands but brief periods of early
morning passing showers and isolated afternoon showers can be
expected each day, mainly on the west-end and just downwind of the
islands carried by the southeasterly tradewinds.

On Sunday more of the same pattern is expected, however with the
approach of the short wave trough and the weakening of the upper
level ridge, there is a better chance for enhanced afternoon showers
with an isolated thunderstorm or two possible mainly over parts of
the northwest and northern half of Puerto Rico, as the low level
steering winds are forecast to continue from the southeast. The
chance for urban and small stream flooding as well as minor ponding
of water on roadways in isolated areas will therefore continue for

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

On Monday morning, the dominant feature over the region remains the
mid- to upper-level ridge, providing stability. Furthermore, a
Saharan Air Layer is forecast to make its way over the region during
the day on Monday; haze is expected with the dust associated with
the SAL, and will likely linger into Tuesday morning.

Monday into Tuesday, shortwave troughs aloft are forecast to cross
the region, mostly affecting the upper mid-levels, around 500 hPa
and above. The ridge aloft will be gradually eroded due to a polar
trough. This will allow for increasing instability, and the vertical
extent of the moisture will increase. The lower mid-levels are where
the hold of the ridging is the strongest, however, which may cause a
small layer of drier air between around 850 and 700 hPa. There is
good agreement from the model guidance on this general trend, though
there is disagreement on timing and the magnitude of the impacts.
For example, the GFS is moister, with less of a dry layer, with the
impacts being late Wednesday into Thursday, while the ECMWF is about
half a day ahead, and retains a decent cap in the moisture at the
lower mid-levels. With recent model runs of the GFS, however, the
trend has been towards less moisture, and the 850-700 hPa dry layer
has a bit more staying (and returning) power.

Meanwhile, at the surface, increasing moisture is expected for
Wednesday. This is partially due to increased moisture as the winds
shift to be more southeasterly, dragging additional moisture over
the area, from an area of enhanced moisture associated with an
easterly perturbation that is forecast to pass by largely to the
south of the area. Precipitable water values are forecast to
increase, but remain within climatological normals. The end result
is that an increase in shower activity is likely for Wednesday.
Confidence is medium-high for the general scenario; for the timing
and exact effects on the conditions for the region, confidence is
low-medium.

The ridging aloft reasserts itself in the lower mid-levels on
Thursday. As such, there will be a moisture cap at around 850 hPa,
though there will remain some good moisture beyond that layer. This
will likely lead to a decrease in shower activity into the end of
the week. That being said, model guidance has trended wetter with
the 0Z runs - this is true for the GFS, ECMWF, and CMC/GDPS - for
late in the week, compared to previous runs. Considering the lead
time and the fact that this is a pretty good change compared to
previous solutions, confidence in the forecast rapidly decreases
beyond midweek.

&&

.AVIATION...

VFR durg prd. Wdly SCT SHRA ovr regional waters and en route btw
PR and the Northern Leeward islands. VCSH at mainly TJSJ/TJNR/TIST/TISX
til 14/14z. Brief MVFR and Mtn Top Obscr due to -SHRA/Low clds
ovr the east interior sections of PR til 14/12Z. SFC wnds calm to
lgt/vrb bcmg fm east at around 15 knots with occasional gusts and
sea breeze variations aft 14/14Z. Fm 14/17Z- 14/22Z SHRA ovr W
interior of PR with VCSH AT TJSJ/TJBQ/TJMZ.

&&

.MARINE...

Seas continue to subside, though there remains seas of up to 6 feet
for the offshore Atlantic waters during the day today. For the rest
of the local waters, seas of 5 feet or less are forecast for the
next several days. Easterly winds of up to 15 knots are expected to
continue today, as well.

For the beachgoers, there remains a moderate risk of rip currents
for most beaches of the USVI, Culebra, and Vieques, as well as the
north coast and some southeastern beaches of Puerto Rico.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 88 77 89 76 / 40 20 30 20
STT 86 76 86 77 / 50 50 50 30

&&

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
417 AM AST Sat May 15 2021

.SYNOPSIS...

A typical shower pattern is expected for today into tomorrow.
Afternoon convection is expected in northwestern Puerto Rico and
in streamers affecting eastern Puerto Rico, including the metro
area. Drying is expected for Monday, with the arrival of a Saharan
Air Layer and some dust. Hazy skies are anticipated Monday into
Tuesday. Increasing moisture, and therefore shower activity, is
expected for midweek onward.

&&

.SHORT TERM...Today Through Monday...

IR satellite imagery as well as satellite derived moisture analysis
indicated a surface trough moving across the northeast Caribbean
this morning. Moisture convergence with the aforementioned trough
will favor the development of scattered to numerous showers across
the windward side of the islands the rest of the morning. Then,
afternoon showers will form downwind of the small islands and El
Yunque during the afternoon hours. This will likely result in a
streamers affecting eastern PR, portions of the San Juan Metro Area
and northwest PR. Lightning activity appears to be limited today as
mid to upper level ridge holds over the forecast area keeping the
mid level temperatures fairly warm. Although moisture is relatively
high due to the passage of the surface trough, conditions aloft will
not favor widespread convection.

On Sunday, the mid-upper level ridge erodes as a short-wave trough
swing across the northeast Caribbean. This trough aloft will provide
additional instability across Puerto Rico and the U.S. Virgin
Islands. Moisture level remains near normal, enough to favor another
round of convection over the northern half of Puerto Rico during the
afternoon hours. Northwestern and western interior of Puerto Rico
will again be the favored locations for the heaviest rain.

Then, a Saharan Air Layer will reach the local islands on Monday
morning. Increasing concentrations of dust are possible late in the
afternoon. This dry and dusty air mass will likely produce hazy
skies, reduce visibilities and limited shower activity early next
week.

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

The mid-level ridge persists on Tuesday, especially in the lower mid-
levels. Dry air aloft is also expected, with the Saharan Air Layer
lingering. The ridge will continue to be eroded as a mid- to upper-
level trough approaches from the west. Even so, it is forecast to
persist in the lower mid-levels; a relatively dry layer is forecast
to persist through most of the period at around 850 to 700 hPa.

A disturbance is expected to pass by the region, generally to the
south, around midweek. It is forecast to then shift northwestward,
tilting. The shift of winds to be more southeasterly will increase
moisture advection in the region. Meanwhile, an old frontal boundary
stalled north of the region will provide additional support for
moisture convergence. The front will linger, as the disturbance, and
its moisture, merges into it. Model guidance continues its trend
from yesterday of keeping the moisture into the end of the workweek,
and possibly into the weekend. As such, favorable conditions for
showers will likely persist beyond midweek. Wednesday and Thursday
appear to be the best days for shower activity. The GFS remains as
the "wettest" of the model guidance, but it does continue its
gradual trend towards a drier solution, especially in the "dry"
layer in the lower mid-levels. There is still good agreement on the
general scenario, and now continues, for the most part, into the end
of the workweek. Confidence is medium-high.

A drying trend appears likely for Friday night into Saturday. There
is disagreement as to how much drying, especially aloft, though
timing looks to be in decent agreement. Forecast confidence
decreases for Friday into the weekend.

&&

.AVIATION...

SCT SHRA moving in route to USVI and PR may produce brief MVFR
conditions at TIST, TISX and TJSJ thru 15/14Z. Then, afternoon
SHRA will form over northwest PR and VCSH may affect TJSJ between
15/17-21Z. But overall VFR conds will prevail most of the time
across the terminals. ESE winds at 15 to 20 knots with some sea
breeze variations are forecast across most of the TAF sites.

&&

.MARINE...

Seas will be relatively tranquil for the next several days, up to
around 5 feet. Winds continue out of the east to east-southeast, at
up to 15 knots, across the local waters.

There is a moderate risk of rip currents for the north coast of
Puerto Rico, beaches of Culebra, as well as some beaches of St.
Thomas and St. Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 77 90 78 / 40 30 20 20
STT 86 78 87 77 / 40 30 30 30

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
404 AM AST Sun May 16 2021

.SYNOPSIS...

A drying trend is expected for the next couple days, with a SAL
moving over the area; decreased shower activity is expected into
midweek. Hazy skies are also anticipated. Later this week, the
combination of more favorable conditions aloft, an easterly
disturbance, and a stalled frontal boundary will likely lead to an
increase in shower activity. The best days for rain this week are
likely Friday into Saturday.

&&

.SHORT TERM...Today Through Tuesday...

GOES Total Precipitable Water imagery indicated a drier air mass over
the Eastern Caribbean and the Lesser Antilles. The southeast trade wind
flow will slowly bring that dry and stable air mass in over the local
islands as the day progresses. At mid to upper levels, the ridge will
continue to weaken as a long wave trough moves across the western
Atlantic. Although mid level dynamics are slightly better than
yesterday, moisture will be somewhat limited. Therefore, the afternoon
convection will be focused over northwest Puerto Rico, with little
shower activity expected elsewhere. Aerosol models suggest that light
concentrations of Saharan Dust will spread over the region today
resulting in hazy skies at times.

On Monday, the atmosphere dries out a bit more and dust concentrations
will increase slightly with a second pulse of Saharan Dust expected
later in the day. Convection, if any, will develop again the northwest
sections of Puerto Rico in the afternoon. High temperatures will be
above 90 degrees in coastal sections and urban areas of northern Puerto
Rico.

A jet segment will pass just north of the region on Tuesday increasing
the cloudiness across the local islands. These mid-upper level clouds
will reduce the solar radiation delaying the afternoon convection over
northwest Puerto Rico on Tuesday. Also the cloudiness will likely
maintain the maximum temperatures below 90 degrees in most locations.
Dust concentrations are forecast to decrease late Tuesday.

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

The general scenario that has been forecast remains the most likely,
with dust and stability aloft giving way to moisture and increasing
instability as a deep-layer trough swings from out of the northwest
to approach and then traverse the region. Still expecting a drier,
more stable layer around 850 to 700 hPa, as well. An old frontal
boundary will approach the region before stalling out to the north.
Meanwhile, a disturbance will pass by the region to the south
then shift northwestward, approaching the frontal boundary.
Moisture convergence is expected over the region. With the
addition of low- level moisture and instability, increasing shower
activity is likely.

That being said... Details and timing look less certain than they
did yesterday. Model guidance generally agrees with pushing this
shift to increasing moisture to later in the week than previously
anticipated. This is reflected in the GFS, Euro, and CMC;
additionally, the Goddard Earth Observing System (GEOS) model
suggests that there will still be some dust lingering over the
region around midweek, lending additional support to a delay in
the increased moisture. This change has been reflected in the
forecast; both Wednesday and Thursday look like they will be drier
than was previously expected, with Thursday being the transition
day. The trough aloft is forecast to basically stall out in the
vicinity of the CWA, mostly remaining west of the area, with
occasional shortwave troughs traversing the area. With the upper-
level trough expected to camp out over and north of Hispaniola,
divergence aloft is anticipated, aiding in developing instability
over the region. The weaker troughing that quickly yields to a
ridge in the lower mid- levels, however, will help to maintain a
cap/dry (drier) layer, which may act to inhibit convective
development somewhat.

The questions surrounding timing are a significant source of
uncertainty with the forecast. Model guidance has progressively
shifted the timing of a potential peak in showers later into the
week over the last several runs. With the features aloft forecast to
stick around for a couple days (or more), there is a better chance
for favorable dynamics and moisture to coincide over the region.
But, shortwave development/splitting off of the main trough and the
slow movement of the SAL/dust could complicate things.

Overall, relatively dry conditions are expected for Wednesday into
Thursday, with improving potential for rain starting later in the
day on Thursday. Enhanced activity, in a typical pattern, is then
expected for Friday into Saturday. Drying is expected for late
Sunday/early Monday. Confidence is low to medium.

&&

.AVIATION...

Mainly VFR conditions are expected to prevail across all terminals
thru the forecast period. Brief SHRA possible in and around TJBQ
btw 16/17z-22z. HZ due to the presence of Saharan dust is possible
on Sunday, but vsby should remain P6SM. SE winds 10-15 kt
continues below FL100.

&&

.MARINE...

East to east-southeast winds up to 15 knots continue across the
local waters, and seas will remain relatively tranquil for the next
few days, around 5 feet or less.

There is a moderate risk of rip currents for the north coast of
Puerto Rico and Culebra, and beaches of eastern St. Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 77 90 77 / 20 10 20 20
STT 86 78 86 77 / 30 10 20 20

&&

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
436 AM AST Mon May 17 2021

.SYNOPSIS...
Hazy skies are expected again today due to Saharan dust. A drier
air mass is expected for today, but moisture should increase by
midweek due to an approaching cold front, resulting in an
increase in shower activity. This feature is expected to stall
north of the area through the weekend.

&&

.SHORT TERM...Today Through Wednesday...

GOES Total Precipitable Water imagery indicated a drier air mass
over the Eastern Caribbean and extending east northeast into the
western tropical Atlantic. The southeast trade wind flow is expected
to continue over the area. A low pressure at the surface of 1006 mb
about 1400 miles northeast of Puerto Plata in the Dominican Republic
will cause a trailing cold front to approach the local waters during
the period and the possibility of weak low just north of Puerto Rico
on Wednesday. This will be accompanied by a sharp rise in
precipitable water over the area and an increase in shower activity.
Because of the southeasterly flow most of the activity will be north
of the Cordillera Central and concentrated in northwest Puerto Rico
during the afternoons, but some shower activity will also begin to
be seen in the overnight and early morning hours of eastern Puerto
Rico Tuesday. At mid levels, weak troughing continues just west of
Puerto Rico, and at upper levels some cyclonic curvature in the
mainly west northwest flow may also be seen in the same general
area. Although this will not help much with rainfall today, it will
favor the slight increase in shower activity on Tuesday and a
better chance of showers on Wednesday.

Aerosol models suggest that light concentrations of Saharan Dust
will spread over the region today resulting in hazy skies.

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

At the mid and upper level, a polar trough will continue to take
control of the weather over the western and central Atlantic,
while a surface to mid level ridge will hold over the western
Atlantic, making its way into the eastern Caribbean. The frontal
boundary that extend from the trough should stall north of the
islands for much of the long-term forecast period. This feature
should provide moisture to increase shower activity around the
region as well, but there is some uncertainty on how much (and
when) this system will affect the area. On Thursday, the latest
run of the GFS seems more aggressive into increasing shower
activity over northwestern Puerto Rico, while the ECMWF and the
NBM had a more typical, and perhaps more plausible, weather
pattern with showers affecting the eastern portion of the forecast
area through the day, and an increase in shower in the afternoon
over northwestern Puerto Rico. Although not explicit in the
forecast for now, isolated thunderstorms may develop over
northwestern Puerto Rico in the afternoon. Then, by late Thursday
into Friday, a drier air mass is expected to filter into the mid-
levels (850 to 500 mb) from the south. This should limit somewhat
the shower south of the region, but some activity should still
occur over the Atlantic waters due to influence of the frontal
boundary.

For the weekend, the pattern is still unclear, with the GFS
bringing the front over the islands, but the ECMWF still having
the system to the north. If the GFS is right, then shower activity
should increase over the area, especially on Saturday. Even with
the drier solution that the ECMWF model is showing, at least
afternoon activity due to local effects should develop on these
days. By the end of the period the NASA GEOS dust model shows more
Saharan dust advancing over the eastern Caribbean, while both
models also show drier air coming from the east. This should
result in hazy skies, as well as a reduction in shower activity
across the islands.

&&

.AVIATION...VFR conditions are expected to prevail across all
terminals thru the forecast period. Brief SHRA are possible in and
around TJBQ/TJMZ btwn 17/17z-22z. HZ due to the presence of Saharan
dust will cont today with vsbys btwn 9-15 SM. SE winds 10-15 kt
continues below FL100. Max winds WNW 35-45 kts btwn FL415-510.


&&

.MARINE...
Tranquil marine conditions will continue today with seas up to 4
feet and winds up to 15 knots. A northerly swell is expected for
mid-week, increasing seas up to 6 feet. For the beaches, there is
a moderate rip current risk for the northern coast of Puerto Rico,
Culebra and eastern St. Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 77 90 76 / 20 20 20 20
STT 86 77 87 77 / 20 20 30 30

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
401 AM AST Wed May 19 2021

.SYNOPSIS...
Hazy skies due to Saharan dust and warm temperatures under a
southeasterly wind flow are expected to continue for the next
several days. A frontal system stalling north of the islands may
increase the potential for showers and thunderstorms by the end of
the week. A small northerly swell will increase the risk of rip
currents to high for today across the northern coast of Puerto
Rico.

&&

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

Fair weather conditions prevailed overnight and early this morning
with a few sprinkles observed across the Atlantic waters. Overnight
temperatures were mostly in the 70s in the lower elevations and in
the mid and upper 60s in the higher elevations. Winds were light and
variable.

Ridge aloft will continue to weaken as a mid to upper level trough
amplifies across the western Atlantic into the Greater Antilles. At
lower levels, a meandering surface front north of the area will
approach the local islands Thu-Fri as the aforementioned trough
continues to dig southward. The low level winds will remain east
southeast through the forecast cycle.

This east southeast wind flow will advect some low level moisture
with precipitable water values reaching 1.75 inches later today. As
a result, available moisture and diurnal heating will promote shower
and isolated thunderstorm development this afternoon across the
northern slopes of Puerto Rico. Elsewhere, mainly fair weather is
expected. Additional moisture advection is possible Thu-Fri as
the front moves closer to the forecast area. Nevertheless, aerosol
models suggest African dust particles reaching the area by the
end of the workweek. Therfore, there is low confidence in the
intensity and areal coverage of shower and thunderstorm activity.
At this time, best PoPs were included across western areas of
Puerto Rico each afternoon.

Warm temperatures will be expected once again today particularly
across the norther slopes of Puerto Rico.

.LONG TERM...Saturday through Wednesday...
The mid to upper level trough will remain in place across the
western and central Atlantic Ocean during the weekend, with a
surface low pressure well north of the islands. A frontal system
will continue to stall north of the area, still providing some
moisture across the region. However, a drier air mass at the mid
levels (from 850 to 600 mb) is also expected to filter in on
Saturday. This could work to hamper the intensity of the rain
activity across the area. Nevertheless, due to moisture
convergence, there is a potential for isolated thunderstorm
activity, especially north of the area. In the afternoon, adding
the local effects and diurnal heating to the mix, additional
showers and thunderstorms are expected to develop over the
islands, especially over the interior and northwestern Puerto
Rico.

For Sunday, the forecast is still a bit tricky. On one hand, the
GFS still forecast a drying trend, with limited shower activity,
but on the other hand, the ECMWF is showing a more bullish
solution, although less than on previous runs. Additionally, the
aerosols models are forecasting the presence of some Saharan dust
as well. For these reasons, the forecast reflects the bulk of the
activity occurring over the Atlantic waters and some areas of
eastern PR/USVI during the morning, with another round of
afternoon showers and thunderstorms for the interior and
northwestern Puerto Rico.

Moving into the first half of the next workweek, the guidance are
more in tune, with drier conditions expected in general, and
still hazy skies due to Saharan dust. Some showers will still be
possible, but rainfall accumulation are expected to be limited
during this period. And finally, by Wednesday, another mid to
upper level trough is expected to develop over the western
Atlantic, which should help to increase instability aloft and the
potential for shower activity.

&&

.AVIATION...VFR conditions will continue to prevail at all TAF sites
with SHRA/TSRA possible in and around JSJ/JBQ after 19/17z. ESE
winds at 10 to 15 knots with some sea breeze variation.


&&

.MARINE...
A small northerly swell will continue to invade the local waters
today through Friday, but seas should remain around 5 feet. Winds
will continue from the southeast at 5 to 15 knots. There is a
high rip current risk for the northern coast of Puerto Rico.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 92 76 89 76 / 30 40 50 50
STT 87 76 86 75 / 30 40 50 50

[cycloneye]

Area Forecast Discussion...CORRECTED
National Weather Service San Juan PR
503 AM AST Fri May 21 2021

.SYNOPSIS...Moisture associated with a surface boundary and
instability provided by the proximity of an upper level trough
should aid in the development of showers and thunderstorms, mainly
across central and west Puerto Rico each afternoon today and into
the weekend. Hazy skies due to African dust will continue. Warm
temperatures will be expected once again today particularly across
the northern slopes of Puerto Rico.

&&

.SHORT TERM...Friday through Sunday...

A broad mid to upper level trough continues over the central and
western Atlantic with an associated surface low pressure well north
of the local area and a stalling frontal boundary just north of the
islands. In the meantime, a high pressure over the eastern Atlantic
will continue to generate a southeasterly wind flow for the rest of
the short-term forecast period. Moisture associated to the front and
instability aloft provided by the trough should aid in the
development of showers and thunderstorms, mainly over the Atlantic
waters, and in the afternoon, over the interior and northwestern
municipalities of Puerto Rico. The aerosol models, such as NASA GEOS
Dust, still show suspended Saharan dust over the region, but should
decrease a little bit by the afternoon and evening hours, before
another denser pocket of dust moves by late Saturday into Sunday
morning.

Throughout the weekend, instability should remain high, with 500 mb
temperatures around -8 to -9 degree Celsius, and divergence aloft
due to the presence of a jet. On Saturday, although the Saharan dust
may work to limit the coverage of the rain activity, still some
showers and thunderstorms are expected to develop during the
afternoon over the interior and western Puerto Rico. Urban and small
stream flooding is possible in areas that receives the heaviest
rainfall amounts. On Sunday, aside from the moisture associated with
the frontal system, additional moisture is expected to be advected
from the southeast. As a result, the forecast soundings are showing
precipitable water content increasing to around 2.0 inches. Hence,
shower and thunderstorm activity may be more widespread, also
developing over portions of eastern Puerto Rico and the U.S. Virgin
Islands, with heavy rain possible at times.

Under the influence of this southerly wind flow, expect temperatures
to remain on the warm side, with highs climbing to the low and mid-
90s at lower elevations, and heat indices around the 100 degree mark.

.LONG TERM...Monday through Saturday...

Overall, the long term forecast is still on track. Latest guidance
continues to suggest trofiness aloft much of the next week with
best upper level dynamics likely to be observed on Wednesday. At
lower levels, moderate easterly winds will continue to prevail
during the forecast cycle. Regardless the trofiness aloft,
moisture across the area is expected to remain near or below the
normal range, except on Monday and Wednesday with precipitable
water values exceeding 1.75 inches. Although there is a caveat,
aerosol models continue to suggest another pulse of African dust
particles reaching the area early the next workweek.

Under this evolving pattern, the most likely outcome is to
observe the highest chance for showers and thunderstorms Monday
and Wednesday mainly across central and west Puerto Rico. For
Tuesday, Thursday and into the end of the upcoming week, mainly
fair weather conditions with locally induced afternoon showers
and isolated thunderstorms focused across west Puerto Rico.
Seasonable temperatures are expected to prevail across the
forecast area. Periods of haziness to continue.

&&

.AVIATION...Mainly VFR conditions expected across all terminals.
VCTS are expected for TJBQ after 21/17Z, which could result in
MVFR to IFR conditions due to low ceilings and reduced VIS.
Saharan dust remains across the region, but VIS should stay P6SM.
An ESE wind flow will prevail at 10 to 15 knots with stronger
gusts and sea breeze variations.

&&

.MARINE...A small northerly swell will continue to affect the
local Atlantic waters throughout the day today with a second
pulse reaching the local waters and passages on Sunday.
Nevertheless, seas will remain below Small Craft Advisory
criteria. There is a high risk of rip currents for northern
beaches of Puerto Rico with a low to moderate risk of rip
currents for the rest of the local beaches.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 90 77 90 76 / 40 30 40 40
STT 87 79 87 79 / 30 30 30 50

&&

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
509 AM AST Sat May 22 2021

.SYNOPSIS...

The broad mid- to upper-level trough will continue to promote unstable
weather conditions trough the weekend. A combination of clouds and
haze will be observed across the forecast area due the arrival of
suspended African dust particles on Sunday. A more typical
weather pattern will be possible by the end of the week due to a
high pressure system building near the area. Seas will also
increase by Sunday due to the arrival of a second pulse of the
northerly swell.

&&

.SHORT TERM...Today thorough Monday...
The base of the deepening mid to upper level trough continued to dig
southwards across Cuba and Hispaniola thus maintaining the region on
the unstable and divergent side of trough. In addition a subtropical
jet maxima will continue to round the base of the trough and pull up
across the region today into Sunday to maintain the region in the
right rear quadrant favoring instability aloft. In the low levels,
the proximity of a frontal boundary frontal just north of the region
and an induced low level trough will support good low level moisture
convergence and consequently promote an a very warm and unstable
environment through the weekend. This will therefore favor good
potential for enhanced late morning and afternoon shower and
thunderstorm development in and around portions of the islands.
Additionally, the 500 hPa temperatures will continue to range
between -7 to -9 degrees across the region with a continued dry
layer and still some suspended dust particulates between 700-500
millibars. With all the aforementioned ingredients in place, as well
as the intense daytime heating and light to moderate east southeast
wind flow, expect enhanced afternoon convection once again today and
also for Sunday.

Less explosive afternoon convection and a more seasonal pattern is
so far forecast for Monday as the current model guidance suggest
that the upper trough is to gradually lift slightly northward and
weaken. Also, with the present low to mid level flow, expect
afternoon convection to be focused along the northern half of Puerto
of Rico, especially over the central and northwest quadrant of the
islands, as well as in some areas of the San Juan metro. As the
result the potential for afternoon thunderstorms with heavy rains is
high and therefore there will be good potential for urban and small
stream flooding today and on Sunday. Elsewhere including in and
around the U. S. Virgin Islands, afternoon shower activity may be
locally heavy at times in isolated areas with a slight chance for an
afternoon thunderstorm but mainly on the west-end or just downwind
of the islands.

Forecast high temperatures will be in the low to mid 90s along the
coastal areas especially along the north coastal areas of Puerto
Rico, with heat indices to reach the high 90s to near 100 degrees in
some areas.


.LONG TERM...Tuesday through Sunday...

Model guidance suggest unstable weather conditions trough the end
of the long term forecast period. Dynamics at upper levels are
favorable for enhancement of showers and thunderstorm activity. At
lower levels, moderate easterly winds will continue to prevail
during this time. Also, precipitable water values are forecast to
exceed normal values for this time of the year. Aerosols models
continue to suggest a second impulse of African dust particles
reaching the area early the next workweek. However, the
interaction of these particles with the moist environment is not
expected to suppress convection significantly. Under this
evolving pattern, the most likely outcome is to observe diurnally
induced afternoon convection and isolated thunderstorm activity
over central and west Puerto Rico each day. A more typical weather
pattern is expected to return by the end of the week.

Satellite imagery indicates a weak tropical wave moving across the
tropical Atlantic near 47W south of 12N. This wave is expected to
stay well south of our forecast area on Sunday and thus, not
causing any direct impacts to PR/USVI. Seasonable temperatures
are expected to prevail across the forecast area.

&&

.AVIATION...
Prevailing VFR cond will prevail at all terminals durg the prd.
General wnd flow BLO FL150 will veer fm the east to southeast 10-15
kts then bcm fm SW-W and incr with height ABV. Fm 22/17Z-22/23Z
expect SHRA/Isold TSRA to affect the N central, and W-NW PR. This
will cause brief MVFR conds in and around TJBQ/TJMZ/TJSJ. Sfc winds
will be fm E-SE at 10 to 20 kts with higher gusts and sea breeze
variations. Afternoon convection may produce brief wind gust in
excess of 30 knots with strong TSRA.

&&

.MARINE...

A light to moderate east to southeast wind flow will continue to
across the local waters through early next week. Hazardous marine
conditions are expected late tonight into Sunday for the offshore
Atlantic waters with seas up to 7 feet due to the invading
northerly swell. Therefore, small craft advisories will go in
effect at least until Monday morning for the aforementioned marine
zone. We are closely monitoring this swell as model guidance
continues to suggest periods of around 12 to 14 seconds, which
could trigger breaking waves between 10 to 12 feet. There is still
a high risk of rip currents the along the northwestern to
northeastern beaches of Puerto Rico, including Culebra at least
until Monday afternoon.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 75 86 77 / 50 50 40 30
STT 88 77 86 77 / 30 50 50 40

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
510 AM AST Mon May 24 2021

.SYNOPSIS...
Low level moisture continued to erode along with the frontal boundary
that lingered across the region over the weekend. The mid-upper level
trough also continued to weaken and lift farther northeastward, now
placing the region on the subsident side of the upper trough. The
trough is forecast to be reinforced by yet another trough Tuesday
and Wednesday, therefore bringing a return of unstable conditions
aloft once again. In the meantime, sufficient moisture available
across the islands to favor afternoon convection driven by plenty
of sunshine, daytime heating, sea breeze convergence and local
effects. Urban and small stream flooding will therefore remain a
concern in some areas. A northeasterly swell will continue to
fade, but will maintain hazardous marine conditions over the local
Atlantic waters, and Mona Passage at least during eh morning
hours. There is a high risk of rip currents for northern beaches
of Puerto Rico, as well as beaches of Culebra, St. Thomas, and St.
Croix.

&&

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

As the frontal boundary continues to weaken and move away from
the forecast area, moisture will gradually diminish across the
local region trough the short term forecast period. Also, the mid-
to upper- level trough is eroding which will slightly decrease
instability aloft. However, precipitable waters values will
continue to remain near normal values. Thus, mostly fair weather
conditions will persist trough much of the morning hours. Then,
afternoon convection due to moisture confined between 1000 to 700
MB, local, and diurnal effects will produce scattered to numerous
showers over interior and western Puerto Rico. Due to somewhat
unstable upper level conditions, isolated thunderstorms cannot be
ruled out each day. The best chance for more widespread showers
and thunderstorms is on Wednesday due to an increase in moisture
at 700-500mb and instability aloft. Winds are turning now more to
the east-northeast and expected to continue like this trough the
rest of the short term cycle. Seasonal daytime temperatures can
also be expected.

.LONG TERM...Thursday through Tuesday...

Overall, model guidance continued to suggest a slight improvement
in weather conditions from Thursday and into the weekend. Light
concentrations of suspended Saharan dust particulates will linger
across the region accompanied by a drier mid- level airmass which
will continue to move in across the region. However, due to the
proximity of another upper level trough Thursday through Friday
and available trade wind moisture, shower activity and isolated
thunderstorm development will remain possible each afternoon. The
shower and isolated thunderstorm activity should be focused mainly
over parts of interior and western Puerto Rico but should be mainly
due to diurnally induced afternoon convection and local effects.

By the weekend, a easterly wave is forecast to enter the eastern
Caribbean and continue westward. So far all models suggest this wave
will move well south of the area with no direct impacts to the area
except for some peripheral tropical moisture which may be brought
across the region by the prevailing easterlies. Thereafter more
seasonal weather conditions are expected with periods of passing
morning showers affecting the east coastal sections followed by
afternoon convection focused mainly over the central and west
sections of Puerto Rico, and on the west-end or just downwind of
the U.S. Virgin Islands.

&&

.AVIATION...Mainly VFR conds are expected for all terminals for next
24 hours. However, afternoon convection will promote TEMP MVFR
conds due to - SHRA and possible VCTS btwn 24/14-18Z across TJBQ/TJSJ.
Winds will stay generally from the east btwn 10-15kts with possible
higher gusts and sea breeze variations.

&&

.MARINE...Small Craft Advisories remain in effect for zones 710,
712, and 741. Small craft operators should exercise caution
elsewhere due to choppy seas. Hazardous conditions will continue
the rest of the early morning hours for the Atlantic waters and
into the Mona Passage, associated with a fading northeasterly
swell. Gradual improvement is expected thereafter although choppy
conditions will continue. Moderate to locally fresh east to southeast
wind flow will continue across the local waters over the next several
days. Please refer to the latest Coastal waters Forecast (CWFSJU)
and Marine Weather Message (MWWSJU) issued by WFO San Juan PR for
the latest info on marine conditions.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 89 77 87 77 / 30 20 40 40
STT 87 77 88 77 / 30 40 40 40

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
503 AM AST Tue May 25 2021

.SYNOPSIS...
A short wave trough will move across the southwest Atlantic waters
and reinforce the mid to upper level trough which in turn will sag
southwards across the region once again by early Wednesday through
Thursday bringing unstable upper level conditions to the region once
again. Moderate east to northeast trade winds will prevail today as
an induced low level trough will approach the region, with winds
forecast to become more east to southeast on Wednesday and Thursday
as the aformentioned trough crosses the region, bringing an increase
in low level moisture. Overall fair weather but hazy skies will be
observed during the morning and early afternoon hours as some suspended
Saharan Dust particulates will persist over the local area. Locally
and diurnally induced afternoon convection can be expected over
portions of the island.

&&

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

Latest model guidance now suggest a gradual increase in moisture at
surface and instability aloft through the short term period. PW values
are forecast to be near normal -1.5 to 1.8 inches- for what is expected
during this time of the year. Ridging near our forecast area will
continue to promote moderate to fresh easterly wind flow. At the
mid- to upper- levels, 500 mb temperatures will be favorable for some
isolated thunderstorm activity each afternoon and troughiness near
the area will aid in the development of showers.

For today, hazy skies are expected due to the continuing incursion
of African Dust particles over the area. Moisture confined at the
low levels will be carried in by the trade winds to promote passing
showers early in the morning for parts of northeastern PR and the
U.S. Virgin Islands. Afternoon convection will lead to showers and
possible isolated thunderstorms over parts on interior and western
Puerto Rico. Elsewhere, isolated to scattered showers are expected.

Wednesday and Thursday are forecast to be the most active days in
terms of weather due to a weak easterly perturbation and another
upper level trough approaching the area. Model runs continue keeping
the easterly perturbation south of the islands, but associated moisture
in combination with the trough will enhance showers over the local area.
Afternoon convection is expected across interior and western parts
of Puerto Rico. The main threat for these areas will be least ponding
of water and minor flooding with the heaviest and most persistent
rains.

.LONG TERM...Friday through Wednesday...

Overall, model guidance continued to suggest an overall improvement
in weather conditions from Friday and into the weekend. Only light
concentrations of suspended Saharan dust particulates will linger
on Friday into the weekend, accompanied by a drier mid to upper level
airmass which will continue to move in across the region. However,
with the moderate easterly trades , expect periods of passing showers
to affect the windward side of the islands each day followed by locally
and diurnally induced afternoon convection mainly over parts of the
central and west sections of Puerto rico.

By the latter part of the weekend and into early next week, an easterly
wave is forecast to enter the eastern Caribbean and continue westward.
So far all models suggest this wave will move mainly south of the area
with only some peripheral tropical moisture expected to be transported
across the local waters and portions of the islands from time to time.
All in all for the remainder of the period a more seasonal weather
pattern is expected with periods of passing morning showers followed
by locally and diurnally induced afternoon convection mainly over the
central and west sections of the Puerto Rico during the afternoon hours.
For the U.S. Virgin islands, mostly fair weather skies should prevail
during the period expected for the passing early morning showers in
some areas followed by brief afternoon showers limited to the
west-end and just downwind of the islands.

&&

.AVIATION...Mainly VFR conds are expected during the 24 hr period
across all terminals. Sfc winds will remain light a generally
from the ENE til 24/10Z winds should pick up by the afternoon btwn
15-20 kts with possible higher gusts. Afternoon convection will
lead to possible VCSH/VCTS aft 25/17Z at TJBQ.

&&

.MARINE...Choppy seas of up to 6 feet and winds up to 20 knots are
expected over portions of the local waters and passages today. There
is a moderate risk of rip currents for most beaches today except for
some of the protected beaches in the south and west coast of Puerto
Rico. For additional info, please refer to the latest Coastal
Waters Forecast (CWFSJU) and the Surf Zone Forecast (SRFSJU)
issued by WFO San Juan PR.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 87 77 87 78 / 40 40 40 40
STT 88 77 87 76 / 40 40 20 30

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
509 AM AST Thu May 27 2021

.SYNOPSIS...A seasonal weather pattern, dominated by overnight and
early morning passing showers followed by locally and diurnally
induced shower and possible isolated thunderstorm activity, will
prevail during the next few days. Variations in the intensity and
spread can be expected, with the bulk of the activity favoring
the interior and western sections of Puerto Rico each afternoon.
As a result, ponding of waters to localized urban and small
stream flooding can be expected.


&&

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

Showers have formed over the Atlantic waters north of Puerto Rico
and also in the Mona Channel overnight. Almost no showers were seen
over land. In this the models have been greatly exaggerating the
chances of rain over eastern Puerto Rico.

The cutoff low at 250 mb will begin to pull away today and winds
aloft will begin to diminish. Nevertheless, current GFS shows the
trough is expected to pass today and very good divergence aloft and
local heating and convergence should be able to gain some traction
in the moderate moisture and modestly unstable atmosphere over the
area to provide areas of heavy showers and isolated thunderstorms.
On Friday a weakening ridge aloft will move over the local area but,
after an overnight minimum, moisture will begin rebounding to
current levels on Friday. By Saturday another trough will begin to
approach the local area to allow local effects to produce another
round of showers and thunderstorms during the afternoon.

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

Model guidance suggests that a slightly wetter seasonal weather
pattern, dominated by periods of overnight and morning passing
showers followed by locally and diurnally induced shower
development, will prevail through the long term forecast period.
Slight variations in the intensity and spread of the activity can
be expected due to fluctuations in moisture content and the
evolution of mid to upper level conditions, as well as the timing
of key driving features streaming across the region.

So far, a surface high pressure over the north central Atlantic
will maintain a moderate easterly flow through the period. Under
this flow, the passage of various weak easterly waves will
maintain generally normal precipitable water values due to
increasing tropical moisture as high as 1.90 inches. Despite mid
to upper level ridging expected to hold over the region through
at least Tuesday, these waves will enhance the frequency of
showers moving from the waters into eastern Puerto Rico and the US
Virgin Islands, as well as a better chance for afternoon shower
and isolated thunderstorm development along the interior and
western sections of Puerto Rico and portions of the San Juan
metropolitan area. By Wednesday and continuing through the end of
the workweek, an amplifying trough to the west will push the ridge
eastward and away from the local area; thus, expect a weakening
of the trade wind inversion and normal to above-normal moisture
content across the local area, with model-estimate precipitable
water peaking around 2.10 inches by Friday night. As a result,
ponding of water to localized urban and small stream flooding can
be expected, especially with the strongest activity each
afternoon. Since wetting rains are not expected to affect fire-
prone areas across southern Puerto Rico, further drying of soils
and fuels with a chance for elevated fire danger conditions will
continue.


&&

.AVIATION...Mainly VFR conditions are expected to prevail across all
terminals through the forecast period. However, SHRA/TSRA over
western PR and its higher terrain may cause mtn obscurations and
MVFR conds at TJMZ/TJBQ from 27/18Z till sunset. Low level winds
will continue from the east up to 15 kts, with gusts to 25 kts in
sea breezes less than 8 knots aft 28/01Z. Maximum winds WSW 50-75
knots btwn FL350-490 strongest at FL385. Winds diminishing during
the day to less than 60 knots.


&&

.MARINE...Tranquil marine conditions with seas up to 5 feet and
easterly winds up to 15 knots are expected to prevail during the
next few days. Locally higher winds and waves can be expected
across coastal waters of northwestern Puerto Rico and near any
shower and thunderstorm activity. A moderate risk of rip currents
continues for beaches along the north coast of Puerto Rico, as
well as a few beaches of southeastern Puerto Rico, Culebra and
Saint Croix.


&&

.WILDFIRES...Despite wetting rains observed across various
portions of the island, further drying of soils and fuels have
continued across the southern coastal plains of Puerto Rico, where
rainfall accumulations have not been reported in recent days. As
a result, recent KBDI and 10-hour fuel moisture values continue to
meet fire danger thresholds, with highest impact across the Camp
Santiago and Guanica area. Since the forecast calls for relative
humidities to fall into the mid 40s and sea breeze variations
will cause winds to peak above 15 mph with no chance for wetting
rains across these areas, a Fire Danger Statement (SJURFD) has
been issued for elevated fire danger conditions expected across
the southern coastal plains of Puerto Rico today.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 88 77 88 77 / 30 20 30 30
STT 86 76 88 77 / 30 20 30 30

[cycloneye]

Area Forecast Discussion
National Weather Service San Juan PR
430 AM AST Fri May 28 2021

.SYNOPSIS...A seasonal weather pattern, dominated by overnight
and early morning showers moving over windward areas from the
waters followed by enhanced afternoon showers and isolated
thunderstorms clustering along the interior, will prevail during
the next few days. Since soils are saturated, this could result
in urban and small stream flooding and possible mudslides near
areas of steep terrain. A slightly wetter pattern is expected for
the next work week following a similar shower pattern. Tranquil
marine conditions will prevail through early next week.


&&

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

Showers continued over the nearshore Atlantic waters around
central Puerto Rico and also around Saint Croix in the Caribbean.
A few of these showers were momentarily heavy, but were brief.
Fredericksted reported only traces.

Today, upper levels will see the approach of a weakening ridge
between several cut-off lows. The first low was the one that passed
on Thursday, the next is over the Bahama Islands. That low is
weakening and is expected to pass well north of the area on Sunday
morning. At mid levels, high pressure will come to dominate the west
central Atlantic over the next several days. At the surface the
pressure pattern changes very little with high pressure maintaining
a presence just west of the Azores. This high will drift further
west by the end of the weekend. Low level flow will alternate
between east and east southeast to maintain normal to slightly above
normal temperatures area wide, and slightly warmer on Puerto Rico`s
northern coast. Although precipitable water values will remain
fairly steady, patches of mostly low level moisture will cause
fluctuations between 1.4 and 1.75 inches. This in turn will cause
showers move onshore each day after midnight on the east coast, and
showers and thunderstorms in the west and central portions of Puerto
Rico. Passing showers will occur in the U.S. Virgin Islands but
accumulations will be light and generally brief. Heavy amounts will
occur over Puerto Rico during the afternoon periods but amounts will
be light on the south coast and near the coast on the northern
coasts. 500 mb temperatures will cool about a degree and a half
today through Sunday which could lead to somewhat more vigorous
thunderstorms, but convective activity will still have moisture as
the limiting factor. Nevertheless, a weaker wind flow will allow
heavy rains to linger longer in some areas that would produce urban
and small stream flooding.

A slight improvement in visibility values is expected now through
Tuesday as Atlantic air begins to replace Saharan Air over the local
area and the amount of dust in the air decreases.

.LONG TERM...Monday through Saturday...

Model guidance continues to suggest a near to above normal
moisture supporting a generally seasonal shower pattern. That is,
periods of overnight and morning showers moving from the waters
over eastern Puerto Rico and the US Virgin Islands, followed by
locally and diurnally induced shower development across the
interior and western sections of Puerto Rico and portions of the
San Juan metropolitan area. Nevertheless, slight variations in the
intensity, spread, and frequency of the activity can be expected
due to fluctuations in moisture content and the evolution of mid
to upper-level conditions, as well as the timing of key driving
features streaming across the region.

At the surface, high pressure over the north-central Atlantic will
maintain a generally moderate easterly flow through the period.
Under this flow, the passage of various weak easterly waves along
with a gradual increase in the available moisture is expected. For
Monday, mid to upper-level ridging will maintain a trade wind
inversion over the region. This may be strong enough to inhibit
organized deep convection, but not strong enough to suppress
shallow development. Even so, intense diurnal heating may aid in
the development of a few isolated thunderstorms in the afternoon.
On Tuesday and continuing through the rest of the workweek into
the weekend, an amplifying trough to the west will push the ridge
eastward and away from the local area. As a result, expect the
erosion of the trade wind inversion along with a significant
increase in tropical moisture as high as 2.10 inches by Friday
afternoon into Saturday. This scenario will support enhanced
shower and isolated thunderstorm development, resulting in the
potential for urban and small stream flooding and possible
mudslides near areas of steep terrain, especially with the
strongest activity each afternoon. Although global models have
favored a wetter pattern during the last few days and the forecast
follows this trend, there is still low to medium confidence on
whether it will fully materialize to the extent suggested by the
models.


&&

.AVIATION...Mainly VFR conditions are expected to prevail across all
terminals in the USVI but only until 28/16Z in PR. Aft 28/16Z
SHRA/TSRA will devlp ovr wrn and interior PR and downstream from El
Yunque causing areas of mtn obscurations and periods of MVFR conds.
This will most affect TJMZ/TJBQ with VCSH/VCTS at TJSJ/TJPS. Low
level winds will continue from the east up to 15 kts, with gusts to
25 kts in sea breezes, then less than 8 knots aft 29/01Z. Maximum
winds W 50-65 knots btwn FL380-480 strongest at FL450.


&&

.MARINE...Tranquil marine conditions with seas at 5 feet or below
and east winds up to 15 knots will prevail during the next few
days. There is a moderate risk of rip currents for north-facing
beaches of Puerto Rico and Culebra. Elsewhere, a low risk will
continue.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU 88 78 90 77 / 40 40 30 30
STT 88 76 88 77 / 30 30 30 40