Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 210859

Area Forecast Discussion
National Weather Service San Juan PR
459 AM AST Wed Mar 21 2018

.SYNOPSIS...Strong surface high pressure ridge across the north central
Atlantic will maintain a light to moderate east to southeast
windflow across the region through Thursday. Strong mid to upper
level ridge will also hold across the northeastern Caribbean
through Thursday.The ridge aloft will erode by Friday and into the
weekend, as a deepening polar trough will enter and move across
the western Atlantic. Surface winds are also forecast to become
more northeasterly by Friday through the weekend, as a cold front
and associated frontal boundary is expected to move across the
region, as a surface high pressure ridge builds across the
southwestern Atlantic.


.SHORT TERM...Today through Friday...
As expected, mostly clear to partly cloudy skies prevailed across
the islands overnight. A few passing showers were detected across
the local waters with just a few of them moving inland over the
eastern sections of Puerto Rico and U.S. Virgin Islands.

Satellite imagery depicted an area of clouds and showers approaching
from the east. Mimic and GOES-16 TPW products indicated values
between 1.3 and 1.5 inches, which are around the normal values of
March Climatology. As this air mass approaches, passing clouds with
isolated showers are possible across the U.S. Virgin Islands and
eastern Puerto Rico during the morning hours at times. However,
rainfall amounts are expected to be less than one-tenth of an inch.
Then showers are expected to develop over the western interior and
western sections of Puerto Rico, as well as downwind from El Yunque
into the San Juan Metro area late this morning into the afternoon
hours. Elsewhere, mostly sunny to partly cloudy skies with little or
no shower activity.

A mid to upper level ridge with near normal PW values will continue
to prevail across the forecast area through at least Thursday
morning. As a result, a seasonal weather pattern will prevail
through this period.

Moisture is expected to pool over the islands by late Thursday
afternoon into the weekend as a frontal boundary (which is
associated to a polar trough) and associated surface trough move
closer to the region. The chance for shower activity and
thunderstorm development will increase after Thursday afternoon as
instability aloft, surface convergence and moisture increase across
the islands.

.LONG TERM...Sat thru Thu...Surface winds will become more east to
southeast as the cold front and associated frontal boundary is still
forecast to move across the area through the weekend. By Sunday, winds
are to become more east northeast then predominantly northeasterly
Monday through Thursday as a surface high pressure ridge will build
and spread across the western Atlantic. The remnants of the aforementioned
cold front/frontal boundary and associated moisture field is expected
to sink farther southwards. This in turn will bring a significant increase
in cloudiness and shower activity as well as better chance for thunderstorm
activity across the region.

The mid to upper level high pressure ridge is forecast to erode by over
the weekend as a polar trough deepens and moves across the west and
central Atlantic. This pattern will increase instability aloft
and provide good ventilation, while eroding the trade wind cap
inversion over the upcoming weekend. The combination of the
prevailing northeast winds, upper level instability and good low
level moisture convergence will support an unsettled weather
pattern with increasing shower activity and chance for isolated
thunderstorms across the coastal waters and local islands
especially during the early morning and afternoon hours.
Conditions are to then gradually improve by late Monday through
Thursday as a deep layered High pressure ridge builds once again
across the southwest Atlantic.

Near the end of the period expect a gradual tightening of the local
pressure gradient and thus and increase in the northeast trade winds.
As a result expect occasional patches of shallow low level moisture
advection across the coastal waters and parts of the islands. By
then the associated passing shower activity should be mainly during
the early morning and late afternoon hours with mostly fair weather
skies to prevail once again during the daytime as the building ridge
aloft will suppress convective development.


.AVIATION...VFR conds are expected to prevail through the morning
hours. Passing showers and cloud are expected to affect E-PR/USVI
btwn 21/12z-18z, which could bring VCSH at TJSJ/TIST/TISX. Then
afternoon SHRA/-SHRA will develop across the interior and the W-PR
btwn 21/17-22z resulting in brief MVFR conds at TJBQ and possibly at
TJMZ. Sfc winds will be calm to light and vrb becoming from the E at
10-15 kt with sea breeze variations aft 21/14Z. Gusty winds btwn 20-
27 kts are possible.


.MARINE...Overall tranquil marine conditions will prevail across
the local waters at least through Thursday with seas of 3 to 5
feet and east to southeast winds between 10 to 18 knots. Seas will
increase by the upcoming weekend, as a high pressure ridge builds
across the southwest Atlantic and a northerly swell reaches the
regional waters and some of the local passages.


SJU  88  75  87  74 /  20  50  50  40
STT  86  76  84  75 /  30  50  50  40




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