Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 202101

Area Forecast Discussion
National Weather Service San Juan PR
501 PM AST Sat Apr 20 2019

High pressure at the surface will maintain moderate easterly trade
winds across the forecast area through early Monday. The dominant
high pressure ridge will maintain mostly fair weather skies during
the rest of the weekend. A surface low and associated trough will
however develop northeast of the area late Monday through mid-
week and will induce light to moderate northeasterly winds. This
in turn will bring low level moisture remnants from a cold front
across the region resulting in an increase in low level moisture,
as well as instability aloft due to an approaching upper level


.SHORT TERM...Rest of today through Monday...

Broad surface high pressure spread across the west and central
Atlantic will continue to promote a moderate easterly winds across
the region through Sunday. Under this flow, occasional patches of
low- level moisture will move across the area, briefly enhancing
cloud cover and shower activity. Overnight and early morning showers
may stream across the waters from time to time brushing mainly the
windward areas of some of the islands. Meanwhile the locally and
diurnally induced afternoon convection will be limited and
focused mainly over parts of the interior and western sectors of
Puerto Rico. Streamer- like afternoon showers will remain possible
around the San Juan metro area, eastern Puerto Rico downstream
from El Yunque. A few showers will be possible mainly downstream
of the remainder of the surrounding islands including the USVI
where mostly sunny skies should prevail. Significant rainfall
accumulations are not expected due to the overall dry and stable
conditions expected for the rest of the weekend.

Overnight through early Sunday, a mid-to upper level short-wave
trough will continue to weaken and lift farther into the northeast
Caribbean as a weak upper level ridge establishes across the forecast
area. This will be short lived as another area of low pressure
and associated amplifying trough will sweep eastward across the
west and central Atlantic late Sunday through Monday. This will be
supported by an induced low level trough which is forecast to
develop and linger across the region through the early part of the
upcoming week. This combination will result in the weakening of
the trade wind inversion and enhance mid- level moisture
convergence while supporting deeper convective instability late
Sunday and Monday. Favorable conditions aloft will support
enhanced shower coverage and intensity with moderate to locally
heavy rainfall possible in isolated areas at least through Monday.
The forecast still calls for the most intense activity on Monday
with an estimated precipitable water increasing to around 1.50
inches. Most of this convection should however favor eastern
Puerto Rico and the U.S. Virgin Islands during the morning hours
and portions of the interior and southwestern Puerto Rico during
the afternoon.

.LONG TERM...Tuesday through Saturday (from previous Discussion)

The upper level low and associated trough is forecast to be closest
to the region by Tuesday. Meanwhile an upper level jet will round
the base of the upper trough and pass just north of Puerto Rico
and the U.S. Virgin Islands placing the region on the divergent
side of the upper trough. The proximity of the upper level trough
and the approach of the mid level trough Tuesday and Wednesday
will bring 500 mb temperatures close to minus 11 degrees C late
Tuesday night and around minus 9 degrees on Tuesday. Therefore
the possibility remains for isolated thunderstorms in southwest
Puerto Rico along and around the Cordillera Central on Tuesday
afternoon, since sufficient moisture should still be present to
generate convection that could break through 20 to 25 kft. On
Wednesday 500 mb temperatures warm slightly and moisture is weaker.
Although the forecast does not have afternoon thunderstorms at this
time, will check runs 24 hours hence to see if conditions persist.
The upper level trough will be past by Thursday and total precipitable
water will reach a minimum on Thursday morning. Moisture begins to
rise again Thursday through Friday, as a patch of low-level moisture
moves west through the area, but dry mid layers and an atmosphere
that is just barely unstable will counteract most convection.
Nevertheless,showers will increase then.

A tropical wave will move through the tropical Atlantic mostly
below 10 north Tuesday and Wednesday, sending a plume of moisture
northeast behind it. It will enter the Caribbean well south of
the area, but that plume will move across Puerto Rico and the U.S.
Virgin Islands Saturday and Sunday and reinforce some of the low
level moisture that arrived the days before. Thus the period will
end with some of the best moisture of the entire week, but still
not completely up to what is usually seen by the end of April.


.AVIATION...VFR conditions will prevailed across the local flying
area. SHRA are expected in the VCTY of Mayaguez til 20/23z, and
possibly SHRA at TJMZ/TJBQ. Brief passing SHRA cannot be ruled
out VCTY E-PR and around the USVI and the Leeward Islands durg
prd. Winds will be fm the E at 15 to 20 knots with higher gusts
and sea breeze variations, bcmg 10 knots or less aft 20/23z, then
increasing 15-20 kt aft 21/14z.


.MARINE...Moderate to locally fresh easterly winds will create choppy seas
across the regional waters through the rest of the weekend. Mariners
are urged to exercise caution across the local waters and passages due
to winds up to 20 knots and seas up to 6 feet. Increasing trade winds
will create choppy and hazardous seas up to 7 feet on Sunday for the
outer Atlantic Waters through at least early Monday morning. Therefore,
a Small Craft Advisory will be in effect across these waters.


SJU  75  85  75  85 /  40  40  70  70
STT  75  86  73  86 /  20  20  40  40


AM...Small Craft Advisory from 8 AM Sunday to 8 AM AST Monday for
     Atlantic Waters of Puerto Rico and USVI from 10 NM to 19.5N.



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