Area Forecast Discussion

FXCA62 TJSJ 180811


Area Forecast Discussion

National Weather Service San Juan PR

411 AM AST Wed Oct 18 2017


A fast-moving tropical wave passed through the Caribbean south of

the U.S. Virgin Islands and Puerto Rico overnight. Moisture will

follow in southeast flow until this late afternoon when drier air

will move in at all but the lowest levels. Although several

patches of moisture move through Saturday and Sunday only minimal

shower activity is expected this tonight through Sunday in the

typical diurnal pattern of east southeast flow.

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

A tropical wave moved rapidly through the Caribbean Sea just

south of the local forecast waters overnight on Tuesday night at

about 23 knots. It was followed by a stream of moisture that

generated a few thunderstorms and north northwest-moving outflow

boundaries during the last 12 hours. This moisture is expected to

impact the U.S. Virgin Islands and Puerto Rico during the morning

hours. An upper level low over the eastern tip of the Dominican

Republic will drop south and reach Venezuela by Thursday and is

forecast to bring divergent flow aloft to enhance convective

activity over southeast Puerto Rico today. During the afternoon

much drier air moves into the area to cut the usual afternoon

convection short. Then, easterly winds ramp up through early

Thursday morning. The GFS shows precipitable water peaking at 12Z

today around 2.23 inches and falling to 1.06 inches as early as

19/18Z (Thursday afternoon). This invasion of drier air will mark

the end of the threat of flooding in the forecast area.

Thunderstorms will then become isolated Thursday and generally

limited to the interior and northwest portion of Puerto Rico. Low

level moisture will bring isolated showers during the overnight

and early morning hours to the windward coasts, but accumulations

will be minimal, if any, Thursday and Friday. The ECMWF is in

general agreement, but holds minor shower activity in the area

through Thursday instead of drying Puerto Rico out late Wednesday


Returning to the current situation, The satellite imagery shows

activity has diminished in intensity and coverage from Monday when

the upper level low was much stronger and more active, but the

MIMIC product suggests that rich moisture is pushing into the

forecast area from the southeast and may trigger thunderstorms

with very heavy rain during the day today before drier air moves

in. Thus the most successful forecast is expected to be a blend in

the timing from the GFS and ECMWF. Because of antecedent

conditions, the Flash Flood Watch will continue for Puerto Rico

through Wednesday evening (at 8 PM AST).

.LONG TERM...Saturday through Tuesday...

Low level moisture, mainly below the 850 mb level will continue

with relative humidities above 90 percent in an almost continual

fashion through the forecast period. This will produce shallow

showers with very little accumulations along the eastern and

southeastern foothills and slopes during the early morning hours

with a little extra help Saturday and Sunday from passing patches

of moisture at 700 mb. Isolated thunderstorms will be possible in

the west northwest portion of Puerto Rico during the afternoons,

but most showers will be find mid-level moisture of less than 10

percent that will be found from Friday through Monday

insurmountable, so any showers that do develop will be short-lived

and unable to deliver much rain. In addition, winds will not have

subsided much from the wind surge expected on Thursday and will

hinder convective development and not allow showers to remain very

long in any one place. Moisture begins to improve on Sunday and

will continue to do so through the following Thursday. An upper

level trough will cross through on Sunday. On Monday and Tuesday

the trough will deepen over the Anegada passage and the GFS is

bringing a 60 knot jet across the area from the northwest. As it

does so, divergence aloft will increase through Tuesday, and

making the best convection of the period Saturday through Tuesday

on Tuesday, even though moisture will still not be optimum or as

rich as it has been the last three days.


SHRA/TSRA are expected to begin early today in the southeast

portion of Puerto Rico and the waters southeast due to moisture

behind a fast moving tropical wave that swept through the

Caribbean Sea overnight. Expect mtn obscurations thru arnd 18/22Z

also ocnl MVFR conds at TJPS/TIST/TISX til arnd 18/18Z. SHRA/TSRA

will also dvlp aft 18/15Z nr TJMZ/TJBQ and psbly TJSJ. Most of

these sites will become VFR as drier air moves into the area

beginning arnd 18/20Z. This dry air and some increased winds has

already entered TNCM/TKPK and conds should remain VFR there thru

byd 19/10Z. Maximum winds will be east southeast 30 knots arnd

FL075 at 19/12Z.

.MARINE...The tropical wave and the moisture behind it will bring

scattered thunderstorms with gusty winds to the local waters

today. Drier air entering the area will be accompanied by a wind

surge which will bring 10 to 20 knot east southeast winds and

areas of seas in the exposed waters of 7 to 8 feet through Sunday.

The Atlantic waters behind Puerto Rico will not be impacted as



SJU 90 81 89 81 / 30 20 20 10

STT 84 82 83 82 / 30 20 20 10


PR...Flash Flood Watch through this evening for Central Interior-

Culebra-Eastern Interior-Mayaguez and Vicinity-North

Central-Northeast-Northwest-Ponce and Vicinity-San Juan and

Vicinity-Southeast-Southwest-Vieques-Western Interior.

Small Craft Advisory from 2 PM this afternoon to 8 PM AST

Thursday for Southeast.

VI...Small Craft Advisory until 8 PM AST Thursday for St.Thomas...St.

John...and Adjacent Islands.

High Rip Current Risk through this evening for St Croix.

AM...Flash Flood Watch through this evening for Coastal Waters OF

Southwestern Puerto Rico out 10 NM-Coastal Waters of

Southern Puerto Rico out 10 NM.

Small Craft Advisory from 2 PM this afternoon to 8 PM AST

Thursday for Coastal Waters of Southern Puerto Rico out 10


Small Craft Advisory until 8 PM AST Thursday 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 Southern USVI Vieques and

Eastern Puerto Rico out 10 NM-Mona Passage Southward to 17N.