Prognostic Meteorological Discussion
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FXCA20 KWBC 181156
PMDCA

TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
656 AM EST SAT NOV 18 2017

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM NOV 18/06
UTC: A MID/UPPER LEVEL TROUGH OVER THE WESTERN CARIBBEAN IS
VENTING DEEP CONVECTION IN ASSOCIATION WITH MEANDERING LOW LEVEL
TROUGH TO THE EAST. THE TROUGH EXTENDS FROM HAITI/WINDWARD PASSAGE
TO THE GULF OF URABA/NORTHERN HONDURAS. THIS FAVORS A DEEP LAYER
SOUTHWESTERLY FLOW ACROSS THE FORECAST AREA...WITH BROAD AREA OF
PWAT CONTENT OF TWO INCHES ENVELOPING THE CENTRAL
CARIBBEAN-HISPANIOLA/PUERTO RICO. EMBEDDED IN THIS FLOW...THE
REMNANTS OF AN MCS THAT FORMED SOUTH-SOUTHEAST OF THE DOMINICAN
REPUBLIC ARE STREAMING ACROSS PUERTO RICO EARLY THIS
MORNING...FAVORING ORGANIZED HEAVY CONVECTION.

UPPER LEVEL TROUGH OVER THE WESTERN CARIBBEAN IS TO RAPIDLY LIFT
TO THE NORTHEAST EARLY IN THE CYCLE AS A HIGH AMPLITUDE POLAR
TROUGH DEEPENS ACROSS THE USA TO THE GULF OF MEXICO. AS THE TROUGH
WEAKENS...THE ENVIRONMENTAL CONDITIONS WILL NOT BE AS CONDUCIVE
FOR MCS TO CONTINUE FORMING OVER/SOUTH OF HISPANIOLA.
INSTEAD...DIURNAL CONVECTION WILL TAKE CENTER STAGE OVER PUERTO
RICO AND SAINT CROIX...WITH VEERING WINDS WITH HEIGHT TO RESULT IN
ENOUGH SHEAR FOR ORGANIZED DEEP CONVECTION TO DEVELOP. IN A DEEP
LAYER SOUTHWESTERLY FLOW...MOST INTENSE IS TO CLUSTER ACROSS THE
SAN JUAN METRO AREA DURING THE AFTERNOON HOURS...WITH PEAK IN
ACTIVITY EXPECTED BETWEEN 17-00 UTC. ON SUNDAY...AS THE LOW LEVEL
WINDS BACK TO THE SOUTHEAST...FOCUS OF THE HEAVY CONVECTION SHIFTS
TO WESTERN INTERIOR PORTIONS OF PUERTO RICO...WITH MODERATE TO
HEAVY CONVECTION EXPECTED ACROSS THE GUAJATACA BASIN. NOTE THAT
THIS IS MORE INTENSE AND LONGER LASTING THAN WHAT WE PREVIOUSLY
ANTICIPATED.

ON MONDAY AND ONWARD...MODELS SHOWING GRADUAL DRYING TREND AS
MID/UPPER LEVEL CELL OF THE SUBEQUATORIAL RIDGE STRENGTHENS ITS
FOOTHOLD ACROSS THE FORECAST AREA AND EASTERLY TRADES RETURN TO
THE FORECAST AREA. UNDER INFLUENCE OF THE MID LEVEL RIDGE  A
STRONG INVERSION TO SET AT 800/850 HPA LATER ON MONDAY/EARLY ON
TUESDAY MORNING...WITH LIGHT DIURNAL CONVECTION TO LIMIT TO EL
YUNQUE RAINFOREST AND WEST COAST OF PUERTO RICO. OTHERWISE...FAIR
WEATHER CONDITIONS ARE TO PREVAIL.

DAVISON...WPC (USA)

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