Area Forecast Discussion

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