Area Forecast Discussion
Issued by NWS San Juan, PR

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083
FXCA62 TJSJ 110943
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
543 AM AST Wed Dec 11 2019

.SYNOPSIS...

Showers will increase across the region over the
next few days as moisture works in from the east, with a slight
chance of thunderstorms for today. Breezy easterly trade winds
continue for the next several days, driving hazardous seas and a
high risk for rip currents. A trend to drier weather will ensue
for next week.

&&


.SHORT TERM...Today through Friday...

A cut-off low has formed in the passing upper level trough and it
will hold about 400 miles east northeast of San Juan through
tonight before receding northeastward. Upper level winds will be
northwest and subside through the period. 500 mb temperatures will
be around minus 8 to minus 9 degrees today to create a slight
chance of thunderstorms. Chances will decrease on Thursday and
Friday.

At the surface, high pressure of around 1042 mb will be
just west of the Azores today. It will continue to drift east and
fade as another high moves out of the eastern United States and
into the western Atlantic north of 40 degrees north latitude
Thursday and Friday. This will drive surface winds over the local
waters of 15 to 20 knots that will last beyond Friday. Pockets of
moisture are interspersed in the flow over the western tropical
Atlantic with areas of drier air between. One of those pockets is
moving across the Leeward Islands this morning. Although
precipitable water values are modest, they will increase through
Friday, and the lowest levels of the atmosphere will remain quite
moist bringing periods of showers to most of the forecast area--
but especially the windward slopes of higher terrain in Puerto
Rico.

Moisture is rich below 850 mb throughout the period and well
beyond, but little is seen above 500 mb. Moisture between 850 mb
and 500 mb will increase Thursday and Friday and will generate
better showers. Winds will keep these moving quickly so excessive
rainfall amounts are not expected. Rainfall amounts over the next
3 days should not exceed 1.5 to 3.0 inches even in the wettest
areas.

&&


.LONG TERM...Saturday through Thursday...

A fairly wet weather pattern continues into the weekend as
moisture gets pulled up from the southeast into our region. For
the weekend, Sunday looks to be the wetter of the two days. Under
the direction of the forecasted wind flow, southern and eastern
Puerto Rico will see the bulk of the shower activity, with a
second maximum in rainfall for northwestern Puerto Rico in the
afternoon under the convergent windflow around the island. The
atmosphere looks to be more stable for the weekend compared to the
short term period, so the thunderstorm threat will be minimal.
Low level winds are expected to slow down a little bit, but still
have decent speed to keep showers moving along. Thus the threat
for local flooding should be rather low.

As has been discussed over the past few days, a strong area of
high pressure in the Atlantic has been driving stronger than
normal easterly trade winds across the region. These winds are
expected to subside a bit as this high moves off to the east over
the weekend. However, another area of high pressure will move out
over the Atlantic for Monday, and this will once again enhance the
low level wind flow. This will impact marine conditions once
again, and will keep fairly breezy winds in the forecast. Along
with that area of high pressure, drier and more stable air is
expected to move in for next week. Long range models are even
hinting at some Saharan dust that could work in for the middle of
next week. Shower activity is of course possible, but these
showers will be less prevalent than what we have seen over the
past few days.

&&


.AVIATION...

VFR conditions are expected to prevail across all terminals. Sct
SHRA will continue over the area and spread into SW PR aft 11/15Z.
Hir trrn will be obscured. Isold TSRA are psbl. Sfc winds
increasing to 10-20 kt with gusts to 26 kt on nrn coasts. Max
winds WNW 50-60 knots btwn FL430-480 and diminishing during the
day.

&&


.MARINE...

Hazardous seas continue across the region. As such, a
Small Craft Advisory is in effect for all the local and outer
waters except for the southern nearshore waters of Puerto Rico.
Seas will range from 6 to 10 feet with seas occasionally reaching
11 feet. Seas are forecast to remain choppy into the weekend.
Though the hazardous conditions will subside a bit for the
beginning of next week, it appears another area of high pressure
to our north will drive windy conditions producing more rough seas
for next week. There is a high risk of rip currents for the
northern beaches of Puerto Rico and the eastern beaches of
Culebra. In addition, there is a high risk of rip currents for the
northeastern beach of the U.S. Virgin Islands, and the
northwestern beach of Saint Thomas.

&&



.PRELIMINARY POINT TEMPS/POPS...
SJU  85  76  84  74 /  80  80  80  80
STT  87  76  85  74 /  60  60  50  70

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Saturday morning for Culebra-North
     Central-Northeast-Northwest-San Juan and Vicinity-Southeast.

     High Rip Current Risk through late Thursday night for Mayaguez
     and Vicinity.

VI...High Rip Current Risk through Saturday morning for St Croix-
     St.Thomas...St. John...and Adjacent Islands.

AM...Small Craft Advisory until 8 AM AST Saturday for Anegada Passage
     Southward to 17N-Atlantic Waters of Puerto Rico and USVI
     from 10 NM to 19.5N-Caribbean Waters of Puerto Rico from 10
     NM to 17N-Coastal Waters OF Northwestern Puerto Rico out 10
     NM-Coastal Waters of Northern Puerto Rico out 10 NM-Coastal
     Waters of Northern USVI and Culebra out 10 NM-Coastal
     Waters of Southern USVI Vieques and Eastern Puerto Rico out
     10 NM-Mona Passage Southward to 17N.

&&

$$

SHORT TERM...WS
LONG TERM....MB



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