Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 252045

Area Forecast Discussion
National Weather Service San Juan PR
445 PM AST Tue Jul 25 2017

.SYNOPSIS...Moisture from the east northeast brought showers and
scattered thunderstorms to the area today. Moisture will continue
into Wednesday when a tropical wave moves through the area and
triggers more showers and thunderstorms. Varied conditions will
bring areas of moisture with occasional dry slots that pass
through the area through early next week with a tropical wave on
Friday and a wind surge on Monday of next week.


.SHORT TERM...Tonight through Friday
Moisture that has been streaming through the area will continue
overnight, then a tropical wave, currently between Guadeloupe and
Saint Kitts will move through the area on Wednesday. This will
increase the shower and thunderstorms activity. The GFS showed a
lifted index of minus 8 today and has a similar solution for
tomorrow, Thursday and Friday. Therefore expect active
thunderstorms and more urban and small stream flooding for western
Puerto Rico. Beginning at 26/12Z and continuing through 27/00Z
winds become more southeasterly. This should spread the active
showers and thunderstorms into northwest Puerto Rico, but also
greatly increases the probability of a streamer of showers and
thunderstorms with heavy rain across the Greater San Juan
Metropolitan area including, but not limited to, Guaynabo,
Bayamon, Carolina and Trujillo Alto.

Although the GFS, and to a lesser extent, the ECMWF models show a
dry slot entering the forecast area across Saint Croix and the
northern U.S. Virgin Islands and eastern Puerto Rico between
26/18 and 27/06Z some scattered shower activity is expected to
persist due to the continued instability and low level moisture.
After about 27/04Z (early Thursday morning), moisture returns and
there is a good chance of showers around the USVI and over eastern
and southeastern Puerto Rico. This moisture will continue over the
area through at least 27/18Z to produce another round of very
active showers and thunderstorms with urban and small stream
flooding likely in local areas of western and northwestern Puerto
Rico as well and the interior of the island. Since winds do not
change much from the southeast flow of Wednesday, another San Juan
streamer cannot be ruled out.

Friday is very similar to Thursday except moisture is slightly
less abundant, but another weak tropical wave will come through.
So another round of showers and thunderstorms in much the same
areas is indicated.

.LONG TERM...Friday through Thursday
Although lower layers remain generally moist at 850 mb and below,
moisture levels at 700 and 600 mb dip considerably Saturday and
Sunday night especially over the eastern Caribbean and this will
interrupt the shower activity at times Friday through Monday. On
Monday the high pressure at the surface, which has remained
anchored in the central Atlantic the entire week before, will
begin to move west along 32 north and gradients will tighten. A
tropical wave will move through the area on Sunday night followed
by a wind surge that will boost 700 mb winds from 15 knots on
Sunday to 30 knots on Monday with a southeast tilt and sustain the
easterly trade winds at the surface during the week. Modest
moisture levels will continue the scattered showers over eastern
Puerto Rico and the U.S. Virgin Islands that will blossom into
thunderstorms in western and interior Puerto Rico during the


.AVIATION...VCSH across the local terminals through tonight.
SHRA/TSRA affecting PR could cause VCSH at TJSJ and VCTS at TJMZ and
TJPS with TEMPO TSRA at TJMZ. Weather to improve over PR after
25/23Z but VCSH may continue through tonight across the rest of the
local terminals. SHRA/TSRA is possible after 26/16Z in PR near
TJBQ, TJMZ, and TJSJ with mtn obscurations likely throughout the
forecast period.


.MARINE...Winds increasing during the end of the month Monday will
effect a rise in seas during the period. A wind surge arriving on
Monday will bring seas close to small craft advisory criteria,
and by Tuesday, they may be needed in both the outer Atlantic and
Caribbean waters. The risk of rip currents will remain low to
moderate until Monday.


SJU  79  90  81  90 /  60  60  30  40
STT  79  89  80  89 /  40  40  40  40




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