Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 170847

Area Forecast Discussion
National Weather Service San Juan PR
447 AM AST Tue Sep 17 2019

A tropical wave and its associated moisture will exit the area
through the morning giving way to a dry Saharan air mass.
Although some passing showers should not be ruled out across the
windward sections, hazy skies are expected to prevail. Locally
induced showers and thunderstorms will then form over the
northwest quadrant of Puerto Rico. Saharan dust particles will
persist through at least late Wednesday night.


.SHORT TERM...Today through Thursday...

A tropical wave and its associated moisture will exit the area this
afternoon giving way to a dry Saharan air mass. The Saharan air will
limit shower activity today through Wednesday, resulting in hazy
skies. Precipitable water values are expected to plummet from 2.0
to 1.5 inches this afternoon due to the drier air. In addition,
the drier air mass and east to southeast wind flow will cause
afternoon temperatures to soar throughout the region. The high
daytime temperatures combined with high relative humidity values
will result in three digit heat indices today through Wednesday.

Each day during the short-term period showers are possible during
the afternoon as a result of local and diurnal effects. However,
the dry Saharan air mass will cause showers that develop to be
short-lived due to the lack of upper-level support.

Thursday, tropical moisture from the Caribbean waters will lift
across Puerto Rico and adjacent islands. The increased moisture
will aid in the development of showers and isolated thunderstorms
during the afternoon. Precipitable water values are forecast to
climb over 2.0 inches, the added moisture plus increased
instability will cause showers and isolated thunderstorms to
linger. This could lead to urban and small stream flooding, and
ponding of water on roadways.

.LONG TERM...Friday through Tuesday...

A seasonal weather pattern it`s possible for Friday, with trade
wind showers over the windward sections during the morning,
followed by afternoon convection across mountains and western

The National Hurricane Center is giving 90 percent chance of
formation to Invest 97L, which both, GFS and ECMWF, put it as a
tropical cyclone moving off to the northeast of the islands
Saturday. If Invest 97L follows GFS and ECMWF suggested track, it
will pull plenty of tropical moisture across our County Warning
Area (CWA). Confidence is medium, as most of the model guidances
put it away from our CWA, off to the north and moving
northwestward. That said, this tropical disturbance will pool deep
tropical moisture which combined with local effects and diurnal
heating will aid in the development of showers and thunderstorms
through at least Sunday.

A dry trend with suspended Saharan dust particles on Monday and
early Tuesday. Another tropical wave is forecast to reach the
islands Tuesday afternoon into Wednesday.


VFR conds are currently prevailing across all terminal sites.
ISO/SCT SHRA are possible at the eastern terminals of PR/USVI
through 17/12Z. Brief MVFR conds are possible due to development
of VCTS/TSRA near terminals TJBQ/TJMZ by 17/18Z. This activity
will cease by 18/02Z then VFR conds expected to prevail. Sfc
winds calm to light and vrb, returning out of the E-ESE at 15 to
20 kts after 17/13z.


A series of swells are expected to affect the local area through
the forecast period. A northeasterly swell will continue to create
hazardous marine conditions with seas up to 10 feet across the
Atlantic and Caribbean Passages through Wednesday. Then, a
northwesterly swell generated by Hurricane Humberto will spread
across the local waters Thursday, and through early next week.
Small Craft and High Surf Advisories, as well as a High Rip
Current Risk are in effect due to these events. Please, refer to
the Coastal Hazards Message (CFWSJU) and Marine Weather Message
(MWWSJU) issued by the National Weather Service, San Juan office
for more information regarding these hazards.


SJU  92  79  93  79 /  40  20  30  30
STT  90  81  89  80 /  40  20  40  40


PR...High Rip Current Risk through late Thursday night for Culebra-
     North Central-Northeast-Northwest-San Juan and Vicinity-

     High Surf Advisory from 6 AM this morning to 6 AM AST Wednesday
     for Culebra-North Central-Northeast-Northwest-San Juan and

     High Rip Current Risk from 6 AM AST this morning through late
     Thursday night for Mayaguez and Vicinity-Vieques.

VI...High Rip Current Risk through late Thursday night for
     St.Thomas...St. John...and Adjacent Islands.

     High Rip Current Risk from 6 AM AST this morning through late
     Thursday night for St Croix.

AM...Small Craft Advisory from 8 AM this morning to 8 AM AST Friday
     for Caribbean Waters of Puerto Rico from 10 NM to 17N-
     Coastal Waters OF Northwestern Puerto Rico out 10 NM-
     Coastal Waters of Northern Puerto Rico out 10 NM-Mona
     Passage Southward to 17N.

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

     Small Craft Advisory until 8 AM AST Friday for Atlantic Waters
     of Puerto Rico and USVI from 10 NM to 19.5N-Coastal Waters
     of Northern USVI and Culebra out 10 NM.

     Small Craft Advisory until 2 AM AST Thursday for Anegada Passage
     Southward to 17N.

     High Rip Current Risk until 2 AM AST early this morning for
     Coastal Waters of Southern USVI Vieques and Eastern Puerto
     Rico out 10 NM.



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