Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 170849

Area Forecast Discussion
National Weather Service San Juan PR
449 AM AST Sat Mar 17 2018

.SYNOPSIS...Drier weather will prevail over the next several days
with a possibility for an isolated shower across northwest Puerto
Rico today and Sunday. The next weather maker will be a cold
front that will move off the east coast of the United states on
Wednesday with the front nearing the islands next Saturday.


SHORT TERM...Today through Monday...
Weak high pressure at the surface rests just north of the Bahamas
and will drift to north of the local area at 25 north. Clouds from a
shearline drape across Hispaniola and stretch northeast into the
cold front about 375 miles north of San Juan. This cloudy area
should remain just northwest of the forecast area through 18/03Z and
then blow through to the Caribbean waters on northwest flow at 700
mb. by Sunday night. Yesterday no stations reported rainfall except
Maricao with 0.03 inches. Our forecast area remains under this same
relatively dry air mass--due to the proximity of the front.  Also
the 17/00z relative humidity was over forecasted over the area by
the GFS. Therefore, based on model performance yesterday which
had areas of very high POPs during the afternoon over northwest
Puerto Rico, (Hi-RES models) and a general area of higher POPs
(GFS) over central Puerto Rico, and model output today, which
continues to develop very high POPs over the western coast (HI-
RES) and nearly 100 percent POPs just east of the center of Puerto
Rico (GFS) am greatly limiting expectations to around 30 percent.
(Tool rounding may show a different field than forecast.) Do
consider that weak moisture increases could lead to more shower
development today than yesterday, but it should be limited due to
dry mid layers.

This consideration will play into the forecast for Sunday also,
except that the model is bringing the clouds associated with the
shearline through and moisture is forecast to be higher. Also the
presence of an upper level jet entrance just north of the area
should enhance shower activity over the area.  Current indications
suggest that the front may not break through the area as modeled so
have scaled back the precipitation forecast somewhat while allowing
the area affected to expand. As the surface flow turns more easterly
from Sunday to Monday we will see a more typical distribution of
precipitation should moisture move through as expected. Hence on
Sunday night and Monday morning scattered showers are expected on
the eastern slopes of Puerto Rico with isolated showers in the U.S.
Virgin Islands, followed by some convection over the inland portion
of western Puerto Rico in the afternoon with little shower activity
in the U.S. Virgin Islands.

.LONG TERM...Tuesday through Sunday...A surface and mid level
ridge of high pressure will create quiet conditions during the
beginning of the long term period.

Wednesday, a cold front will move off the east coast of the
United States and will continue to move south into the Atlantic
waters. Ahead of the front east southeast winds will pull moist
air over Puerto Rico and the U.S. Virgin Islands with scattered
showers across northwest, central and southwestern portion of
Puerto Rico through Friday.

Saturday, the front will be to the northwest of Puerto but with
southerly moist flow combining with orographic lift will create
showers across the offshore waters.

Sunday, a diffuse front moves over Puerto Rico bringing moisture
across portions of northwest and northern Puerto Rico during the
afternoon this moisture will continue to spread east across
northeast Puerto Rico and the U.S. Virgin Islands.


.AVIATION...VFR conds are expected to prevail through 18/12Z. Some
SHRA are psbl in NW PR with mvfr conds and mtn obscurations from
17/17Z thru 17/22Z, but TAF sites may only be brushed by this if
affected. Sfc winds will be southeast 6-12 kt, sea breezes will
dominate aft 17/14Z. Winds alf W 60-80 kt btwn FL310-510 winds incrg
thru 18/12Z. Maximum winds FL410.


.MARINE...Seas will range from 4 to 6 feet across the Atlantic waters and Mona
passage, and between 3 and 5 feet elsewhere. A high risk rip
currents risk is in effect for the northern beaches of Puerto RIco
until Sunday.


SJU  89  74  87  74 /  10  10  20  20
STT  85  74  85  73 /  10  10  10  20


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




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