Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 170836

Area Forecast Discussion
National Weather Service San Juan PR
436 AM AST Mon May 17 2021

Hazy skies are expected again today due to Saharan dust. A drier
air mass is expected for today, but moisture should increase by
midweek due to an approaching cold front, resulting in an
increase in shower activity. This feature is expected to stall
north of the area through the weekend.


.SHORT TERM...Today Through Wednesday...

GOES Total Precipitable Water imagery indicated a drier air mass
over the Eastern Caribbean and extending east northeast into the
western tropical Atlantic. The southeast trade wind flow is expected
to continue over the area. A low pressure at the surface of 1006 mb
about 1400 miles northeast of Puerto Plata in the Dominican Republic
will cause a trailing cold front to approach the local waters during
the period and the possibility of weak low just north of Puerto Rico
on Wednesday. This will be accompanied by a sharp rise in
precipitable water over the area and an increase in shower activity.
Because of the southeasterly flow most of the activity will be north
of the Cordillera Central and concentrated in northwest Puerto Rico
during the afternoons, but some shower activity will also begin to
be seen in the overnight and early morning hours of eastern Puerto
Rico Tuesday. At mid levels, weak  troughing continues just west of
Puerto Rico, and at upper levels some cyclonic curvature in the
mainly west northwest flow may also be seen in the same general
area. Although this will not help much with rainfall today, it will
favor the slight increase in shower activity on Tuesday and a
better chance of showers on Wednesday.

Aerosol models suggest that light concentrations of Saharan Dust
will spread over the region today resulting in hazy skies.

.LONG TERM...Thursday through Monday...

At the mid and upper level, a polar trough will continue to take
control of the weather over the western and central Atlantic,
while a surface to mid level ridge will hold over the western
Atlantic, making its way into the eastern Caribbean. The frontal
boundary that extend from the trough should stall north of the
islands for much of the long-term forecast period. This feature
should provide moisture to increase shower activity around the
region as well, but there is some uncertainty on how much (and
when) this system will affect the area. On Thursday, the latest
run of the GFS seems more aggressive into increasing shower
activity over northwestern Puerto Rico, while the ECMWF and the
NBM had a more typical, and perhaps more plausible, weather
pattern with showers affecting the eastern portion of the forecast
area through the day, and an increase in shower in the afternoon
over northwestern Puerto Rico. Although not explicit in the
forecast for now, isolated thunderstorms may develop over
northwestern Puerto Rico in the afternoon. Then, by late Thursday
into Friday, a drier air mass is expected to filter into the mid-
levels (850 to 500 mb) from the south. This should limit somewhat
the shower south of the region, but some activity should still
occur over the Atlantic waters due to influence of the frontal

For the weekend, the pattern is still unclear, with the GFS
bringing the front over the islands, but the ECMWF still having
the system to the north. If the GFS is right, then shower activity
should increase over the area, especially on Saturday. Even with
the drier solution that the ECMWF model is showing, at least
afternoon activity due to local effects should develop on these
days. By the end of the period the NASA GEOS dust model shows more
Saharan dust advancing over the eastern Caribbean, while both
models also show drier air coming from the east. This should
result in hazy skies, as well as a reduction in shower activity
across the islands.


.AVIATION...VFR conditions are expected to prevail across all
terminals thru the forecast period. Brief SHRA are possible in and
around TJBQ/TJMZ btwn 17/17z-22z. HZ due to the presence of Saharan
dust will cont today with vsbys btwn 9-15 SM. SE winds 10-15 kt
continues below FL100. Max winds WNW 35-45 kts btwn FL415-510.


Tranquil marine conditions will continue today with seas up to 4
feet and winds up to 15 knots. A northerly swell is expected for
mid-week, increasing seas up to 6 feet. For the beaches, there is
a moderate rip current risk for the northern coast of Puerto Rico,
Culebra and eastern St. Croix.


SJU  90  77  90  76 /  20  20  20  20
STT  86  77  87  77 /  20  20  30  30




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