511  
FXCA20 KWBC 171144  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
643 AM EST FRI NOV 17 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM NOV 17/06  
UTC: UPPER LEVEL TROUGH OVER THE WESTERN ATLANTIC-WESTERN  
CARIBBEAN BOTTOMS OUT OVER NORTHERN HONDURAS EARLY THIS AFTERNOON.  
AXIS IS TO THEN GRADUALLY WEAKEN DURING THE WEEKEND WHILE  
GRADUALLY SHEARING SHORT WAVE ENERGY OVER A BUILDING CELL OF THE  
SUBEQUATORIAL RIDGE TO THE EAST. AMPLIFYING POLAR TROUGH OVER THE  
USA/GULF OF MEXICO EARLY NEXT WEEK WILL THEN FAVOR FURTHER  
INTENSIFICATION OF MID/UPPER LEVEL SUBEQUATORIAL RIDGE PATTERN  
ACROSS THE EASTERN CARIBBEAN. THE BUILDING RIDGE IS TO THEN  
SUSTAIN A TRADE WINDS CAP...EXPECTED TO SLOWLY STRENGTHEN ITS  
FOOTHOLD ACROSS THE NORTHEAST CARIBBEAN ISLES ON MONDAY-TUESDAY.  
 
AT LOW LEVELS...A TROUGH EXTENDS TO THE SOUTH-SOUTHWEST ACROSS THE  
CENTRAL CARIBBEAN. THE TROUGH FAVORS A DEEP LAYER SOUTHWESTERLY  
FLOW ACROSS THE CENTRAL-NORTHEAST CARIBBEAN...WITH DEEP TROPICAL  
MOISTURE POOLING ACROSS HISPANIOLA-PUERTO RICO TO THE VIRGIN  
ISLES. THIS PATTERN HOLDS THROUGH MIDDAY ON SATURDAY...WITH WINDS  
BACKING TO THE SOUTH-SOUTHEAST LATER IN THE DAY. ON MONDAY TO  
TUESDAY PREVAILING EASTERLY TRADES RETURN TO THE EASTERN  
CARIBBEAN. AS THE WINDS BACK TO THE SOUTHEAST...IT WILL CUTOFF THE  
SUPPLY OF DEEP TROPICAL MOISTURE...WITH GLOBAL MODEL GUIDANCE  
AGREEING ON GRADUAL EROSION OF AVAILABLE WATER EARLY NEXT WEEK AS  
MID LEVEL RIDGE STRENGTHENS. THE GFS GDI FORECAST RESPONDS IN  
KIND...SHOWING A CONVECTIVELY UNSTABLE AIR MASS ENVELOPING THE  
FORECAST AREA THROUGH MIDAFTERNOON ON SUNDAY. DURING THAT PERIOD  
ENVIRONMENTAL CONDITIONS APPEAR HIGHLY FAVORABLE FOR GENERATION OF  
THUNDERSTORMS (NIGHTTIME AND DAYTIME CBS). ON MONDAY TO TUESDAY A  
CONVECTIVELY STABLE AIR MASS IS TO THEN ENVELOP THE NORTHEAST  
CARIBBEAN...WITH MOSTLY FAIR WEATHER CONDITIONS.  
 
IN THIS PATTERN...THE UKMET AND ECMWF CONTINUE TO SHOW MOST ACTIVE  
CONVECTION CLUSTERING OVER CARIBBEAN COASTAL WATERS OF THE  
DOMINICAN REPUBLIC DURING THE NEXT 48-60 HRS...WITH STRONG  
CONVECTION TO THE WEST HAVING A NEGATIVE IMPACT ON DIURNAL WEATHER  
PATTERN ACROSS PUERTO RICO AND THE VIRGIN ISLES. ALTHOUGH THE GFS  
SHOWS CONVECTION FORMING SOUTH OF THE DOMINICAN REPUBLIC...IT  
CONTINUES TO DOWNPLAY THE POTENTIAL FOR ORGANIZED HEAVY  
CONVECTION. IN RESPONSE TO THE PERCEIVED WEAKER CONVECTION TO THE  
WEST...THE GFS SHOWS A MORE ACTIVE DIURNAL PATTERN ACROSS PUERTO  
RICO THAN WHAT THE OTHER MODELS SUGGEST. THIS SHOULD BE MORE OF AN  
ISSUE DURING THE DAY TODAY...BUT NOT AS STRONG ON SATURDAY-SUNDAY  
WHEN THE UPPER TROUGH STARTS TO WEAKEN. SO EXPECTING CONVECTION  
ACROSS PUERTO RICO AND THE VIRGIN ISLES TO PEAK ON SATURDAY...WITH  
LESSER AMOUNTS ON SUNDAY AND EVEN LESS ON MONDAY TO TUESDAY.  
 
DAVISON...WPC (USA)  
 

 
 
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