Area Forecast Discussion...CORRECTED
National Weather Service San Juan PR
544 AM AST Tue Oct 15 2019
.SYNOPSIS...
Similar conditions are expected for today as occurred yesterday.
Moisture from a tropical wave will combine with instability, local
effects, and diurnal heating to bring increased shower and
convective activity, especially to western and interior portions
of Puerto Rico during the afternoon. Potential impacts include a
risk of urban and small stream flooding, as well as rapid river
rises this afternoon. Somewhat drier conditions are anticipated
for tomorrow, but afternoon showers and isolated thunderstorms are
still likely. Another tropical wave is expected to provide us with
another increase in moisture for Friday.
&&
.SHORT TERM...Today through Thursday...
Light easterly winds and near normal low level moisture will
continue across the region today. The TUTT and weak upper low
northeast of the northern Leewards will remain in place, and is
still expected to become amplified and sink southwest across the
northern Leewards and northeast Caribbean by Wednesday while
strengthening. This will maintain a NW-N wind flow in the upper
levels today and Wednesday. Forecast soundings are suggesting that
the instability for today through Thursday will be similar to that
recently observed over the local area, which means that good
convection could occur over the next few days. An induced surface
trough and a tropical wave approaching the area will favor a fairly
unstable environment today. A gradually drying trend is expected to
start late on Wednesday as a drier air mass accompanying a saharan
air layer will move across the region.
The forecast for today is for mostly scattered showers with isolated
thunderstorms across the local waters during the rest of the
morning, some of those could affect the USVI and eastern PR.
Afternoon thunderstorm activity over land is expected across the
interior and western PR, but some showers and thunderstorms may be
observed near the San Juan metro area. The pattern today is not too
different to that observed yesterday, wit the exception that is is
expected to cover a larger area, so at least some urban and small
stream flooding as well as rapid river rises are expected this
afternoon.
On Wednesday, a drier airmass is forecast as an area of saharan dust
moved in behind the tropical wave. But the upper trough will still
be in place and the moisture will linger overnight from Tuesday into
early Wednesday. Therefore, early morning showers and thunderstorms
are expected across the local waters and portions of souther and
eastern PR as well as the USVI. Then in the afternoon, the local
effects will help in the development of showers and thunderstorms
across the interior and northern slopes of PR as the local winds will
be more southerly. Then for Thursday, the shower activity overnight
should be isolated to scattered as drier air prevails over the area,
then some afternoon convection is possible across central and
western PR, but the coverage and duration is expected to be more
limited compared to what we expect today.
.LONG TERM...Friday through Wednesday...
A tropical wave is expected to push into the forecast region late on
Thursday or early Friday, bringing increased moisture. This moisture
will aid in the development of showers and isolated thunderstorms
caused by local effects and diurnal heating, especially in western
and interior portions of Puerto Rico, as well as in the San Juan
metro area.
Potentially also aiding in the development of shower and
convective activity is a cutoff upper-level low. While there remains
some uncertainty with respect to the location of this feature, the
current most-likely scenario is that it will be to the south or
southwest of Puerto Rico moving to the southwest and then shifting
to a more westward or northwestward direction. With there still
being a lack of good agreement on this low from the guidance, the
uncertainty with respect to how much it will impact conditions
locally is high enough that a close eye will need to be kept on this
system and its trajectory between now and the end of the week. With
or without the assistance of this cutoff TUTT low, there remains the
potential for potentially hazardous conditions caused by locally
heavy rainfall, including the risk of urban and small stream
flooding.
The moisture will remain at relatively high levels through the
weekend, with another surge of moisture forecast to make its way
over the local islands on Sunday. Shower and isolated thunderstorms
are likely each afternoon, especially in, but not limited to western
and interior portions of Puerto Rico. Potential impacts include
locally heavy rain with the risk for urban and small stream flooding
as well as frequent lightning and locally gusty winds
A somewhat drier airmass is expected to make its way into the area
late Monday and Tuesday. Even with comparatively decreased moisture
available, local effects and daytime heating are still expected to
lead to the development of showers and isolated thunderstorms.
Wednesday into Thursday will see the arrival of another tropical
wave to the Caribbean waters. While it currently appears that the
bulk of the moisture will stay to the south and southeast of the
local islands by Thursday, it is likely that there will be at least
a slight increase in moisture over the area, which could aid in the
development of showers and isolated thunderstorms caused by local
effects and diurnal heating.
&&
.AVIATION...
SHRA activity expected through the forecast period across the local
flying area. This will cause at least VCSH across most of the local
terminals for the next 24 hours, with brief VCTS possible at the
USVI terminals. TSRA is expected over central and western PR today,
for that reason VCTS across all of the terminals in PR is likely
after 15/16Z, though TEMPO MVFR conds at TJMZ are possible. The
local winds will be easterly at 10kt or less, with sea breeze
variations.
&&
.MARINE...
The northwesterly swell will continue to diminish this morning.
Wave heights are expected to remain generally below five feet over
the next several days. There will remain a moderate risk of rip
currents on north-facing beaches in Puerto Rico, Culebra, and St.
Thomas today, subsiding a little tonight; there will be a low to
moderate risk tomorrow at north-facing beaches on mainland Puerto
Rico and Culebra.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 89 78 88 77 / 60 50 40 30
STT 88 77 87 77 / 50 50 20 10