Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 210857

Area Forecast Discussion
National Weather Service San Juan PR
457 AM AST Tue Nov 21 2017

.SYNOPSIS...Drier air will reduce showers and thunderstorms today,
but moisture will return Thursday and Friday. Another break over
the weekend will occur before moisture returns from the south on
Monday. Cold fronts are still being restrained from reaching the
area by local trade winds during the next 7 to 10 days.


.SHORT TERM...Today through Thursday...

Mostly clear skies prevailed across the forecast area overnight and
early this morning. A few passing showers were observed across the
local waters but none over land areas. Coastal temperatures were in
the mid to upper 70s under east southeast winds at 10 mph or

Although a mid to upper level ridge will continue to build across
the forecast area today and hold through much of the week, easterly
winds will continue to bring patches of low level moisture at

The GFS is showing a drop in precipitable water to as
little as 1.1 inches tonight. Environmental conditions are
rapidly becoming hostile for organized deep convection under the
building ridge, but there will still be enough low level moisture
to assure shower activity, particularly late Wednesday into
Thursday. In fact, based on the latest guidance, today through
Wednesday afternoon is the driest period with precipitable water
increasing to near two inches Wednesday night into Thursday.
Therefore, expect a fair and generally stable weather pattern
today and Wednesday with morning passing showers across USVI and
east PR followed by some afternoon showers across west Puerto
Rico. The chance for shower activity will increase Wednesday night
into Thursday as low level moisture increases across the eastern

The flash flood watch in Northwest Puerto Rico is for the
downstream portion of Rio Guajataca below the lake. Recent rains
have raised the lake levels several feet and local authorities
want to maintain vigilance regarding the unstable condition of
the dam.

.LONG TERM...Friday through Wednesday... High pressure that forms
just north of the Mona channel Wednesday night will bring in
moisture on Thursday from the northeast that will linger until
early Saturday morning. This and the easterly wave that will move
through on Friday will continue the good chances for both night
and early morning showers in eastern Puerto Rico and the U.S.
Virgin Islands and showers and isolated thunderstorms in western
Puerto Rico in the afternoon. A break in the moisture will occur
over the weekend. As a cold front sweeps down through the last of
the Bahama islands on Monday a new surge of tropical moisture
invades the area from the Caribbean Sea to the south. Lower levels
will then see wet and dry periods during the first part of next
week and the front will lift to the north mid week next week before
reaching the area.

The ridge of high pressure at upper levels will continue just
east of the area and keep convection limited. The deep long wave
trough invading the western Atlantic on Sunday will weaken in the
process of flattening that ridge in its passage through the area
on Wednesday of next week. Although good moisture will be
available during the week next week it will not be as strong as
has occurred in the last few days.


.AVIATION...Mostly VFR conds are expected to prevail at all TAF
sites throughout the day. Brief MVFR conds will remain possible in
and around JMZ/JBQ due to SHRA btw 21/16-22z. ESE winds at around 10
knots with some sea breeze variation to continue.


.MARINE...East northeast winds behind a front dipping toward the
tropical Atlantic today will drive swell into the outer Atlantic
waters late Wednesday. seas will approach small craft advisory
conditions then. Small craft advisories are not expected through
the middle of next week after that.


SJU  87  77  88  77 /  20  20  30  30
STT  86  77  85  76 /  20  30  30  50


PR...Flash Flood Watch until 2 PM AST this afternoon for Northwest-
     Western Interior.



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