Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 250859

Area Forecast Discussion
National Weather Service San Juan PR
459 AM AST Mon Oct 25 2021

.SYNOPSIS...A ridge aloft will persist into Tuesday, resulting in
stable conditions aloft and thus limiting the potential for deep,
organized convective activity. However, patches of low-level
moisture riding the trade wind flow will be sufficient to generate
isolated to scattered shower activity from time to time during
the next couple of days. The ridge aloft is expected to weaken by
the middle to latter half of the week as a shortwave trough moves
across and depending on the amount of moisture, it may allow for
better convective activity.



A trade wind perturbation will continue to result in passing showers
across the islands today. While a ridge aloft will limit the
vertical development of these showers. However, residents in the
Virgin Islands and the east and north portions of Puerto Rico can
expect showery weather this morning. These showers may produce
ponding of water on roadways and in low-lying areas. Later in the
afternoon, local effects, diurnal heating, and sea breeze variations
will give way to convection across the western sections of PR,
downwind from the El Yunque and the Virgin Islands.

A drier air mass with Suspended Particles from the Africans` Deserts
will limit moisture and produce hazy skies through at least mid-
week. Nevertheless, the trade winds will bring patches of moisture
at times; therefore, pesky showers cannot be ruled out through the
forecast period.

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

Model guidance continues to be consistent on a marginally unstable
weather pattern for most of the long term period. Thursday starts
with an upper-level short wave trough crossing the area and low-
level winds shifting to southeast through Friday. Discrepancies
still exist regarding the amount of moisture convergence through the
end of the workweek as mentioned in previous discussions. GFS
continues to have better moisture advection due to a southerly flow
than ECMWF. However, winds are still expected to weaken as pressure
gradient decreases over the area. Therefore, uncertainty still
exists on how much moisture will converge over the forecast area.

Recent GFS model runs are consistent that the bulk of the
moisture plume will stay south of the region but close enough for
above normal moisture content to reach the area. If this plays
out and with enough instability in the atmosphere, widespread
convective development would be likely with the potential for
isolated to scattered thunderstorms across the area. The bulk of
the activity could be observed by Thursday night with showers and
thunderstorms developing over the Caribbean waters and being
advected towards the southern and southeastern coastal
municipalities of Puerto Rico and St. Croix. During the day,
surface heating and sea breeze convergence could produce showers
and scattered thunderstorms over interior and northern half of
Puerto Rico and occasionally affecting the local islands. Light
steering winds will increase the potential of flooding threat
especially in urbanized regions such as the greater Bayamon and
San Juan Metropolitan area. If the ECMWF run plays out instead,
then a more less widespread and more localized activity could be
observed as moisture intrusion over the forecast area will be
greatly reduced.

The weekend also looks marginally unstable with the northeast
Caribbean under the boundary of an upper-level building ridge and
trough. By Sunday through the end of the forecast period, both
global models are consistent in the development of an upper cut-off
low lingering over Puerto Rico and U.S. Virgin Islands but strength
and positioning still not completely agreeing within each model
solution. Moisture still does not look impressive, at times both
models showing below normal values. Hence, continue to expect a more
seasonal weather pattern for the weekend through Tuesday with deep
convection developing during each afternoon depending on the
available moisture and the evolution of unstable weather conditions
aloft. Although models are consistent on the progress of weather
features for this week and early next week, location differences of
these features continue to bring forecast uncertainty. Therefore,
confidence continues to be low to moderate.


.AVIATION...Prevailing VFR conds expected through the forecast
period. However, VCSH will continue to arrive across the local
terminals, and there is a possibility of brief SHRA at times. SHRA
may form later in the afternoon across the interior and W-PR. A
Saharan Air Layer will result in hazy conditions late this afternoon
through the rest of the forecast period. Wind will remain from the
east at around 10 to 15 knots with sea breeze variations after


.MARINE...The combination of a long dominant period northerly
swell and moderate easterly winds will result in seas between 3
to 6 feet through at least this afternoon, with the highest seas
expected across the offshore Atlantic waters. Seas are expected to
subside tonight through Tuesday as the wind and swell decrease. A
high risk of rip currents will continue across the northwestern
beaches of Puerto Rico and Culebra through this afternoon.


SJU  88  76  88  76 /  50  30  30  30
STT  88  77  87  78 /  50  40  30  30


PR...High Rip Current Risk until 6 AM AST early this morning for
     North Central-Northeast-San Juan and Vicinity-Southeast.

     High Rip Current Risk through this afternoon for Culebra-

VI...High Rip Current Risk until 6 AM AST early this morning for St



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