Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 260106
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
906 PM AST SUN SEP 25 2016

.UPDATE...Afternoon convection dissipated just before sunset.
At this time...satellite imagery shows mostly cloudy skies over
the western half of Puerto Rico while mostly clear skies prevail
elsewhere. No showers are affecting the islands...however isolated
to scattered showers continue across the Mona Passage and the
Caribbean Waters. The rest of tonight and overnight...little or no
rainfall is expected over Puerto Rico and the U.S. Virgin Islands.
Then..A few morning showers are possible across the U.S. Virgin
Islands and eastern Puerto Rico. On Monday afternoon...another
round of showers and thunderstorms is forecast to develop once
again across portions of the Cordillera Central and Western Puerto
Rico. Minor changes were introduced to the forecast package this
evening.

&&

.AVIATION...VFR conds expected across all TAF sites through 26/12z.
MVFR conds possible vcty TJMZ/TJBQ with brief MTN Top obscurations
near convection along interior PR between from 26/20Z-22z. Except
for local sea breeze variations, SFC wnds fm E at 5-10 kts overnight
and 10-15kts from 26/14z.

&&

.MARINE...No significant changes are expected through Tuesday as
seas will continue at 3-5 feet and winds of 10-15 knots. However...
marine conditions will likely deteriorate by midweek as a tropical
disturbance moves over the Eastern Caribbean.

&&

.PREV DISCUSSION... /issued 220 PM AST SUN SEP 25 2016/

SYNOPSIS...Moderate to gentle trade wind flow will continue
across the area from the east with mainly air-mass showers and a
few thunderstorms through Wednesday morning. The models are
developing a tropical cyclone Tuesday or Wednesday that will
enter the Caribbean on Wednesday and strengthen some distance
south of the forecast area. This will likely bring a surge of
shower and thunderstorm activity and stronger winds late
Wednesday through Monday.

At upper levels...Weak low pressure will develop over Puerto Rico
and the U.S. Virgin Islands. Beginning Tuesday...high pressure
will invade from the east and settle over Puerto Rico on
Thursday morning. High pressure will continue around the
periphery of the tropical cyclone and over the local area.

At mid levels...High pressure will continue from the coast of
Africa to Cuba until mid week. High pressure will continue north
and northeast of the area through the end of next week. The GFS
shows a tropical cyclone passing about 325 miles south of Puerto
Rico`s south coast on Thursday.

At lower levels...Weak high pressure just north of the area
combined with low pressure in the southwest Caribbean will
maintain gradients for gentle to moderate trade winds across the
area through mid week. The GFS and the ECMWF show a tropical
cyclone passing south of the area that will likely increase winds
somewhat and bring better shower activity Wednesday through
Sunday as it develops.

DISCUSSION...Showers formed in a lazy Y pattern across western
Puerto Rico and also in a sea breeze convergence and Yunque
streamer in northeast Puerto Rico. A streamer was also seen off
Saint Croix. Amounts have generally been less than three quarters of
an inch. Moisture remains relative steady through Wednesday with
stability decreasing slightly through Tuesday. This sets the
conditions for continued showers and isolated thunderstorms across
the same places as formed already today and a few early morning
showers Monday and Tuesday.

Models have become less consistent with the trajectory of the
tropical cyclone that both the GFS and the ECMWF develop Tuesday
or Wednesday. Interestingly most of the divergence of trajectory
occurs after Thursday when the system is already projected to be
beyond the local area. Currently models suggest that a tropical
storm will pass about 300-350 miles south of Puerto Rico`s
southern coast. However given the strong difference between
today`s solution and yesterday`s solution the exact distance from
the forecast area is somewhat uncertain. This means that there is
some possibility for elevated winds and increasing showers as
early as Wednesday and Wednesday night. We will continue to
monitor this situation.

AVIATION...VFR conds expected across the flying area and all TAF
sites during the prd. However, til 24/22z, Isold-sct SHRA/TSRA
development still possible along Central Interior and West PR, as
well on the windward side of the USVI where isold TSRA psbl. MVFR
conds possible vcty TJMZ/TJBQ with brief MTN Top obscurations near
convection along interior PR between til 25/22Z.  Except for local
sea breeze variations, SFC wnds fm E at 5-15 kts. L/lvl wnd fm E-NEe
10-15 knots BLO FL250.

MARINE...Winds and seas have been relatively tranquil and will
continue this way through Tuesday. On Wednesday winds will begin
to increase as high pressure builds against the projected passage
of a tropical cyclone. Regardless of the trajectory, small craft
advisories look possible as early as Wednesday night and almost
certain for Thursday and Thursday night and some time beyond.
Until the system forms however it will be difficult to say whether
the GFS projection of up to 14 foot seas in the Caribbean is
reasonable.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  79  89  79  90 /  10  30  30  30
STT  80  89  79  89 /  30  30  30  30

&&

.SJU Watches/Warnings/Advisories...
PR...None.
VI...None.
AM...None.
&&

$$

11/08



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