Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 270901

Area Forecast Discussion
National Weather Service San Juan PR
501 AM AST Fri Apr 27 2018

.SYNOPSIS...Strong surface high pressure across the central
Atlantic will prevail for the next several days, causing easterly
winds over the local area. Upper ridge will also prevail over the
local area for the next several days while a mid level ridge is
also expected, which will keep a relatively stable atmosphere for
the next few days. Drier air is moving in and the available
moisture is expected to be below normal for the next several days
as well. Locally induced showers are expected, isolated
thunderstorms will not be ruled out.


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

Although rain activity was almost undetectable overland, cloudy
skies were detected across the islands overnight. The Doppler
radar detected a cluster of showers across the Atlantic water and
the Mona Passage. Satellite imagery detected a patch of clouds
embedded in the trade winds. This patch is expected to reach the
islands through the morning and could produce passing showers at
times. In general, fair weather conditions are expected to prevail
across most of the region, but the combination of the available
moisture, with daytime heating and local effects will result in
showers across the northwest quadrant of Puerto Rico.

A ridge is forecast to build and persist aloft through the upcoming
week. Under this ridge pattern a trade wind cap will limit most of
the activity to just shallow convection. Model guidance suggested a
dry air mass filtering from the east through the weekend. As these
ingredients mix together a fair weather pattern can be expected,
with little or no shower activity during the overnight and morning
hours and shallow convection confined across the western sections of
Puerto Rico during the afternoon hours. In addition, under a
southeast wind flow maximum temperatures could reach upper 80s or
even low 90s along the north coast of Puerto Rico and the U.S.
Virgin Islands.

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

Middle and upper level ridge is forecast to prevail over the local
area through midweek, keeping a relatively stable atmosphere, but
the long range model guidance suggests an upper low will approach
the local area, causing instability. Available moisture will also
remain below normal through Tuesday but then increase to normal
levels on Wednesday and Thursday, then increasing even more late
next week. Under the forecast weather pattern, the expected
weather is for the early part of the upcoming workweek will have
limited shower activity except for the western sections of PR in
the afternoon due to locally induced showers. Wednesday looks like
it could have more rainfall coverage as moisture increases,
Thursday and Friday looks even wetter with more thunderstorm
activity across the local area and then Saturday looks like a
rather rainy day across the entire forecast area, due to deep
moisture moving in and an upper low positioned just right to give
us persistent showers and thunderstorms. However, confidence in
the forecast past Wednesday is low due to the recent model trends
of being too wet late in the forecast period.


.AVIATION...VFR conditions will prevail across all TAF sites through
27/17Z. A few passing -SHRA could affect the VCTY of TJSJ/TIST
through the morning hrs. Increasing cloudiness with SHRA are fcst
across TJMZ and TJBQ from 27/17-28/00z. As a result, brief periods
of MVFR conditions could be expected. Low level winds will be mainly
east southeast at 10 to 15 kts with higher gusts.


.MARINE...Choppy seas up to 6 feet are expected today across the
local waters and winds up to 20 knots. Small craft should exercise
caution. Seas will be at 6 feet or less for the next several days.


SJU  88  75  89  75 /  20  40  20  40
STT  86  76  86  75 /   0  40  20  30




LONG TERM....JA is the U.S. government's official web portal to all federal, state and local government web resources and services.