Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 160849

Area Forecast Discussion
National Weather Service San Juan PR
Issued by National Weather Service Miami FL
449 AM AST Mon Oct 16 2017

The first pulse of moisture with the tropical wave has moved
north of Hispaniola. A second will cross through today bringing
another round of showers and thunderstorms. Moisture and
instability will continue through Wednesday, taper off on Thursday
and reach a minimum on Saturday.


.SHORT TERM...Today through Wednesday...
The plume of moisture that brought flooding on Sunday will
continue in southeast flow between high pressure over the eastern
Atlantic Ocean and the low pressure in the southern Caribbean.
Although high pressure will dominate the pattern aloft, flow is
light and generally divergent so that upper levels will support
continued convection. Most of the models support convection on
both the southeast coast and slopes of Puerto Rico and the
northwest section, as is typical in southeast flow. This pattern
is also expected to affect the upstream islands as well with some
scattered showers and thunderstorms. Conditions remain favorable
for convection Tuesday and Wednesday afternoons, with the Galvez-
Davison Index reporting even more favorable values Tuesday and
then Wednesday, mainly over the eastern two thirds of the island.
The southeast flow and very deep continuous moisture favors
flooding and mudslides today through Wednesday over much of Puerto
Rico with a focus on the southeastern third of the island. A
Flash Flood Watch is in effect for Puerto Rico today through
Wednesday evening.

.LONG TERM...Thursday through Monday...
The ECMWF and the GFS disagree on the exact timing of the arrival
of the drier air either Wednesday night or early Thursday evening
respectively, but do agree that it will be strong and dramatic.
Moisture will end no sooner than Wednesday night, but it is safe
to say that some convection will continue in lingering moisture on
Thursday in any case. Although widespread flooding is not
expected Thursday, local urban and small stream flooding cannot be
ruled out and the effects and danger will be enhanced by the
previous 3 days of heavy rains across the island. Friday will be a
transition to drier weather.

Precipitable water in the GFS drops to as little as 0.88 inches
on Saturday afternoon and this should be enough to generate only
isolated thunderstorms. Then the base of a 10 thousand foot band
of relative humidity of less than 10 percent is forecast to begin
just below the 700 mb level that should eliminate all but the most
robust convective parcels. Showers would also be short-lived.
Also northwest flow with anti-cyclonic curvature will persist
Friday through Sunday to enhance subsidence and often convergent
flow. The GFS does bring in several shots of divergence however
and therefore cannot rule out local thunderstorms in the west
during the afternoons and scattered light showers on the east
coast during the mornings. This point is confirmed by 850 mb
moisture above 80 percent over the weekend and into next week, in
spite of the weak moisture at 700 mb during the period.


Broken to overcast conditions will continue, although cigs will
generally be above FL090. Mtns will continue to be obscured. Aft
16/15Z SHRA/TSRA will dvlp, first along the slopes of SE PR and
then over the Cordillera Central and NW interior. Areas of
MVFR/IFR CIGS/VSBYS are likely in SHRA/TSRA. Tops may exceed 50
kft. Flow SE 15-30 kt SFC-FL100 bcmg westerly 5-20 kt abv FL310.


Marine conditions will improve slightly, but southeast flow will
keep the Atlantic and Caribbean waters rough and choppy. Small
craft advisory conditions will continue in parts of these waters
and the Anegada passage today and Tuesday and could return on
Thursday or Friday when winds increase again.


SJU  87  81  89  81 /  50  40  40  40
STT  84  83  84  82 /  40  40  30  50


PR...Flash Flood Watch from 8 AM AST this morning through Wednesday
     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.

VI...Small Craft Advisory until 6 PM AST this evening for
     St.Thomas...St. John...and Adjacent Islands.

AM...Flash Flood Watch from 8 AM AST this morning through Wednesday
     evening for Coastal Waters OF Southwestern Puerto Rico out
     10 NM-Coastal Waters of Southern Puerto Rico out 10 NM.

     Small Craft Advisory until 6 PM AST Tuesday for Caribbean Waters
     of Puerto Rico from 10 NM to 17N-Mona Passage Southward to

     Small Craft Advisory until 6 PM AST this evening for Coastal
     Waters of Southern USVI Vieques and Eastern Puerto Rico out
     10 NM.

     Small Craft Advisory until 2 AM AST Wednesday for Anegada
     Passage Southward to 17N-Atlantic Waters of Puerto Rico and
     USVI from 10 NM to 19.5N.



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