Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 100140 AAA

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
940 PM AST Sun Dec 9 2018

.UPDATE...With ridging aloft and dry air in place, there is no
change in current thinking.


.AVIATION...No changes--VFR will continue.


.MARINE...The high risk of rip currents has ended--moderate risk
continues in most areas.


.PREV DISCUSSION... /issued 339 PM AST Sun Dec 9 2018/

SYNOPSIS...700 mb ridge centered just north of the ne Caribbean
slowly drifting eastward. Surface high centered over the central
North Atlantic with ridge extending west-southwest to southern
Bahamas and Hispaniola. Short wave trough and associated frontal
system exiting U.S. eastern seaboard over the Atlantic well north
of the area.

SHORT TERM...Tonight through Tuesday Night...700 mb ridge
mentioned above...and associated subsidence...will continue to
limit shower activity at least through Monday. However, this ridge
should drift eastward and so subsidence should begin to diminish
by Monday night. At the same time...weak disturbances/pocket of
moisture moving westward around the base of the surface ridge will
begin to reach the local area by the later part of this period.
Less subsidence coupled with higher moisture content will result
in an increased chance for showers...especially over USVI and
eastern PR due to increased low level convergence over those
areas. Decided to keep relatively low chances for rain from
Tonight through Monday night but increase them to mention
scattered showers over USVI/eastern PR Tuesday night.

LONG TERM...Wednesday through Sunday...Weak disturbances/pocket
of moisture will continue to move west around the base of the
surface Atlantic ridge with these affecting the islands
intermittently through Wednesday night. At the same time...the
short wave trough/frontal system mentioned above will move across
the Northwest Atlantic and push the 700 mb ridge eastward.
However, most energy with this short wave will move northeastward
and so the surface high pressure across the central Atlantic
should remain in place. Prevailing east flow around the base of
this ridge will continue to bring periods of passing nighttime and
early morning showers over portions of the islands. Most
models...especially GFS...continue to indicate that the frontal
boundary will not reach the eastern Caribbean and instead will
become stationary and weaken well north of the region. A transient
surface high pressure system moving rapidly behind the frontal
boundary will build eastward resulting in the tightening of the
pressure gradient across the northeast Caribbean...not only
increasing the surface winds, but also the frequency of patches of
moisture across the islands.

AVIATION...VFR conds are expected for the next 24 hours. Some SHRA
activity this evening could cause VCSH at TJSJ, TIST, and TISX
after 10/00Z. Winds will be mainly from the east at around 10KT to
15KT through 09/22Z, becoming light after that time.

MARINE...Small craft operators should continue to exercise
caution across most of the offshore waters through Monday night
mostly due to seas up to 6 feet. Seas may briefly subside below 6
feet Tuesday through Wednesday, but will increase again possibly
to small craft advisory criteria by Thursday as the pressure
gradient tightens across the northeast Caribbean.

A high risk of rip currents remains in effect for the northern
beaches of Puerto Rico, and Culebra through 6 pm this evening. The
risk is expected to diminish after that time to a moderate level
for local beaches through Tuesday. Please refer to the latest Surf
Zone Forecast issued (SRFSJU) issued by WFO San Juan Puerto Rico.


SJU  73  87  74  85 /  20  20  20  30
STT  76  86  75  85 /  20  20  20  30




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