Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 261835

Area Forecast Discussion
National Weather Service San Juan PR
235 PM AST MON SEP 26 2016

.SYNOPSIS...Gentle to moderate trade winds will bring a band of
showers across the area this afternoon and tonight. A weak wave
will bring increasing showers on Tuesday. On Wednesday the
approach of a developing low will cause drying. The low...likely
becoming a tropical still forecast to pass 300-350
miles south of Puerto Rico`s south coast on Thursday, turn north
on Friday and cross the Dominican Republic on Sunday
night...bringing rain and windy conditions to Puerto Rico and the
U.S Virgin Islands during much of this period. Models have changed
each run so far, so the forecast is offered with only moderate
confidence contingent on the actual formation and trajectory of
the low which was 950 miles east southeast of the Windward Islands
at 26/15Z.

At upper levels...A weak low over Puerto Rico this morning will
retreat to the northwest as high pressure takes its place on
Thursday and Friday. An upper level low associated with a tropical
cyclone, will pass south of the area Thursday and Thursday night.
On Friday night the low is expected to turn north and cross the
Dominican Republic Sunday night with high pressure over the local
area just to the east.

At mid levels...High pressure extends from the Central Atlantic at
20 degrees north latitude across the Windward Passage. It shifts
to the Central Atlantic Tuesday through Thursday as a tropical
cyclone makes its way through the southern Caribbean south of the
local area. The cyclone crosses through the southeastern
Caribbean and north across the Dominican Republic on Sunday
according to most models and then toward Bermuda.

At lower levels...Weak gradients across the area will begin to
tighten when a tropical cyclone crosses the Windward Islands and
High pressure in the north Atlantic extends southwest through the
Bahama Islands. Wet and windy conditions will persist until mid
week next week after the tropical cyclone exits the Dominican
Republic for the Western Atlantic Ocean.


.DISCUSSION...Showers have begun to form over interior Puerto Rico
near Adjuntas and Maricao and have crossed the northern U.S.
Virgin Islands from the east around midday. The band of shower
over Saint Thomas and Saint John appear to be a filament of
moisture that broke off a band of convergence left over from the
recently departed Karl and this band is expected to continue
across Puerto Rico during the early evening hours. Another band of
moisture crosses over the local area Tuesday and Tuesday evening.

Low pressure is currently 950 miles east southeast of the Windward
Islands and the hurricane center is giving it a 70 percent chance
of developing into a tropical storm by Wednesday. On Wednesday it
is expected to cross the windward islands at about 13 degrees
north, pushing significantly drier air over the local forecast
area. The current forecast drives it almost due west at 13 degrees
north through Saturday night. This would cause it to pass about
300-350 miles south of Puerto Rico`s south coast on Thursday.
Beyond this, models diverge considerably, with some crossing the
Dominican Republic on Sunday and Sunday night and a few driving
toward and across Cuba, one calling for a reversal of track back
under Haiti and one even crossing the southwest corner of Puerto
Rico. The most probable trajectory has been described above.

This most probable scenario would bring increasing winds and seas
beginning Wednesday and Wednesday night, increasing winds and
rains Thursday through Monday or Tuesday. With the current
trajectory, widespread flooding could be possible across Puerto
Rico with storm force winds by Sunday. Other scenarios could bring
worse conditions, with only a few leaving Puerto Rico and the U.S.
Virgin Islands unscathed. Preparations for this reasonable worst
case scenario would be advisable, but it would be too early to
put these plans in to action just yet.


.AVIATION...SHRA will continue to affect TIST through at least
26/20Z. MVFR conds possible at TJMZ with MTN Top obscurations near
convection along interior and west PR between from 26/18Z-22Z. Low
level winds mainly E 5-15 KTS with sea breeze variations. Mostly VFR
cond with some passing light showers after 26/22Z.


.MARINE...Models are continuing to indicate increasing seas
Wednesday with seas in the southern part of our Caribbean waters
reaching 7 feet as early as 29/03Z and peaking at 15 feet at
30/00Z, reaching a minimum again of 9 feet at 01/21Z and returning
to 13 feet by 03/00z. Actual seas with the current trajectory
should be a few feet lower than this with most models in
agreement as to timing. If the trajectory turns north sooner than
expected condition could be much worse. Current guidance places
maximum seas at 29 feet at buoy 42059 at 30/06Z. Would expect
small craft advisories for most of the local waters by Wednesday


SJU  77  91  77  91 /  30  30  20  20
STT  78  89  78  89 /  30  30  30  30


.SJU Watches/Warnings/Advisories...


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