Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 240158

Area Forecast Discussion
National Weather Service San Juan PR
958 PM AST Wed Aug 23 2017

.UPDATE...No change in expectations regarding synoptic
developments. Minor adjustments for increased confidence in
showers and thunderstorm probabilities in areas discussed below.
Also some minor adjustments to temperatures--but still expect
warmer temperatures on the north coast with the winds veering to
the southeast Thursday afternoon.


.AVIATION...VFR conditions expected all TAF sites, except brief
MVFR possible at TKPK and TNCM as weak moisture-bearing trof moves
through from southeast. This same trof will pass thru the fcst
area aft 24/15Z bringing SHRA and aft 24/16Z TSRA--mainly to
northwestern and interior PR but spreading north across some
coastal areas. Psbl streamer off El Yunque many bring MVFR/TSRA
to TSJU btwn 24/17-21z. Mtn obscurations to increase aft 24/15Z.
Winds alf E 8-12 kt bcmg ESE by 24/15Z but returning aft 25/00Z.
Winds at all levels less than 18 kts thru 25/00Z.


.MARINE...Tranquil marine conditions with seas less than 5 feet
next several days. No small craft advisories expected through the
end of the month.


.PREV DISCUSSION... /issued 420 PM AST Wed Aug 23 2017/

A TUTT low off to the northeast of the islands, over the Atlantic
Ocean, is forecast to sink into the region Thursday into the
upcoming week. Saharan air particles are expected to reach the
islands by Friday. In addition, a surface high pressure across the
Central Atlantic will induce a light to moderate trade wind flow
across the region.

The available moisture combined with the diurnal heating
induced showers and thunderstorms mainly to the west of mainland
Puerto Rico. Also, showers developed downwind from El Yunque into
the San Juan Metro Area without any significant impact. Showers
and thunderstorms are expected to vanish by sunset.

By Thursday, a TUTT low is forecast to sink over the region,
moisture is expected to increase and winds will veer from the
southeast. As a result, Hi-Res models continue to indicate
showers and thunderstorms across the northern half of Puerto Rico
with the heaviest activity across the Northwest Municipalities.
Meanwhile, fair weather conditions are expected across the U.S.
Virgin Islands with partly cloudy skies and some showers
developing downwind from the islands. Model guidance is
indicating the arrival of more Saharan dust particles by Friday,
which will result in hazy skies across the region.

LONG TERM...Saturday through Wednesday...Previous Discussion...

Upper level low will retrograde westward as upper level ridge
builds across the region. Moisture will increase somewhat by
Sunday as a tropical wave passes mainly south of the region.
Patches of low level moisture embedded in the trade winds will
affect the region Monday and Tuesday. A much drier air with
possible Saharan dust is forecast by computer models to approach
the region Wednesday next week.

VFR conditions expected across most of the TAF sites.
TJBQ and TJMZ may experience MVFR/IFR conds at times with the
afternoon TSRA thru 22Z. East winds of 10-15 knots will prevail
below FL150.

Tranquil marine conditions are expected to prevail through the
forecast period. Model guidance are suggesting easterly winds at
less than 18 knots and seas of 5 feet or less through much of the
forecast period.


SJU  80  92  80  88 /  20  40  40  40
STT  81  89  81  90 /  20  30  30  30




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