Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 222311 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
711 PM AST Sat Jul 22 2017

.UPDATE...Have amended grids and zones to reflect a better chance
of rain for Saint Thomas and Saint John, and later eastern Puerto
Rico. Thunderstorms are moving toward the northern U.S. Virgin
Islands and, although the tops have come down from over 50 kft,
they are still expected to bear significant rains.

&&

.AVIATION...SHRA and psbl TSRA accompanying the tropical wave
will begin to impact TIST before 23/00z with MVFR conds and mtn
obscurations. Winds at the sfc will continue from the east at
about 10-15KT with higher gusts in SHRA/TSRA, Overnight weather
will likely cause VCSH across the local terminals except for TJMZ.
Limited MVFR and widespread mtn obscurations will likely continue
through at least 23/16Z for PR while diminishing aft 23/12Z for
TNCM/TKPK. Winds alf easterly 10-20 kt up thru FL320 becmg
easterly at all levels by 24/00z. Winds strongest blo FL100.

&&

.MARINE...No changes to previous discussion. Thunderstorms may
create locally higher winds and seas.

&&

.PREV DISCUSSION... /issued 308 PM AST Sat Jul 22 2017/

SYNOPSIS...A tropical wave with good moisture will move through
the area early tonight followed by drier air Sunday and Monday.
Moisture wrapping around a high pressure will reach the area on
Tuesday. A second tropical wave will follow on Thursday and better
moisture will follow it. Moisture tapers off slightly over the
following weekend.

SHORT TERM...Tonight through Tuesday
Winds shifted shortly after noon from a prevailing northeast flow
to an east southeast flow suggesting that the tropical wave or a
weaker trough may have passed through the area. The MIMIC product
shows the best moisture hanging back at about 61.5 degrees west as
of 22/13Z and present calculations suggest that it should cross
Saint Thomas at 23/05Z and San Juan at about 23/09Z--or earlier,
since there are indications that it is accelerating somewhat.
Currently tops have exceeded 50kft in western Puerto Rico, but
convection has been weaker in eastern Puerto Rico. Better rainfall
is expected overnight but it will likely be confined to the
southeast third of Puerto Rico with scattered showers around the
U.S. Virgin Islands.

With east to east southeast flow and a definite drop in
columnar moisture the first half of Sunday, maximum temperatures
should again broach the 90s on most of the north coast. Limited
moisture to counteract this heating on the south coast may also
allow high temperatures to resist the sea breezes and be
predominantly in the lower 90s. Monday will continue this trend.

Early Tuesday morning moisture arrives from the northeast that had
traveled around a high pressure north of the area and by Tuesday
afternoon precipitable water will be close to 2 inches again. This
will begin a wetter trend.

LONG TERM...Wednesday through Saturday
Areas of moisture with a few dry slots move through the area
through Saturday on mostly easterly low level flow that will
sustain our seasonal summer pattern of showers and thunderstorms
each day. First around the U.S. Virgin Islands in the early
mornings and into eastern Puerto Rico, and then in the western and
interior portions of the Puerto Rico during the afternoons. No
strong features are currently seen in the models through the end
of July, but seasonal showers and thunderstorms often do produce
urban and small stream flooding in the afternoons.

AVIATION...SHRA/TSRA expected to continue over PR this afternoon
until around 22/22Z. This will cause VCSH or VCTS near the
terminals in PR. VCSH is expected for the remaining local
terminals as SHRA move in with the tropical wave that is bringing
plenty of moisture. Winds will continue from the east at about
10-15KT with higher gusts this afternoon, then decrease overnight
to around 10KT. Overnight weather will likely cause VCSH across
the local terminals except for TJMZ Limited MVFR and mtn
obscurations will likely continue through at least 23/16Z. Winds
alf easterly 10-20 kt up thru FL320 becmg easterly at all levels
by 24/00z. Winds strongest blo FL100.

MARINE...Seasonally tranquil seas of 5 feet or less are expected
in most of the local waters through the end of July. Small craft
advisories are not expected to be issued this month under the
current forecast. But winds up to 20 knots are possible in exposed
waters and small craft will need to exercise caution.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  79  90  81  90 /  60  50  20  20
STT  80  89  81  90 /  70  60  20  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM...DS
LONG TERM....WS



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