250  
FXCA20 KWBC 121149  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
649 AM EST TUE DEC 12 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 12/06  
UTC: BROAD POLAR TROUGH EXTENDS FROM THE CENTRAL USA TO THE  
WESTERN ATLANTIC...WITH AXIS BOTTOMING OUT NORTH OF 20N. ALSO AT  
MID LEVELS...A LOW AMPLITUDE RIDGE EXTENDS WEST-TO-EAST ACROSS THE  
CARIBBEAN BASIN...FAVORING A FAIRLY STRONG TRADE WINDS CAP ACROSS  
THE EASTERN CARIBBEAN ISLES. AT LOW LEVELS...THE TROUGH ALOFT  
SUSTAINS A SURFACE FRONT THAT STRETCHES ACROSS THE BAHAMAS-CUBA TO  
THE WESTERN CARIBBEAN. OVER THE EASTERN CARIBBEAN...IN RESPONSE TO  
THIS FEATURE...A LIGHT MEANDERING FLOW DOMINATES.  
 
GLOBAL MODELS FORECAST A CLIPPER TO STREAM ACROSS THE EASTERN USA  
LATER THIS AFTERNOON. AS IT ROUNDS THE BASE OF THE LONG WAVE  
TROUGH NORTH OF THE ISLANDS...IT IS TO PRESS AGAINST THE MID LEVEL  
RIDGE TO THE SOUTH. UNDER PRESSURE...THE RIDGE IS TO RELAX ITS  
FOOTHOLD ACROSS THE EASTERN CARIBBEAN...WITH STRONG TRADE WINDS  
CAP LIFTING ABOVE 700 HPA BY MIDDAY ON WEDNESDAY. DURING THAT  
PERIOD THE PWAT CONTENT IS TO PEAK AROUND 1.25 INCHES. THE  
SLIGHTLY MORE FAVORABLE ENVIRONMENTAL CONDITIONS...HOWEVER...WILL  
BE SHORT LIVED. ON THURSDAY-FRIDAY MID LEVEL RIDGE PATTERN BOUNCES  
BACK...STRENGTHENING THE TRADE WINDS CAP ACROSS THE CARIBBEAN  
WHILE SUSTAINING A DRYING TREND. AT LOW LEVELS...THE POLAR FRONT  
SLOWLY MOVES ACROSS THE TURKS AND CAICOS TO NORTHERN HISPANIOLA ON  
WEDNESDAY MORNING. BUT AS IT LOSES ITS UPPER LEVEL  
SUPPORT...FRONTAL BOUNDARY IS TO THEN GRADUALLY WEAKEN WHILE  
RETROGRESSING ACROSS THE BAHAMAS-CUBA LATER IN THE WEEK. EVOLVING  
LOW/MID LEVEL PATTERN TO THE WEST AND NORTH WILL HAVE AN INFLUENCE  
ON THE LOCAL FLOW PATTERN. EARLY THIS MORNING THE STEERING FLOW  
BECOME SLIGHTLY BETTER DEFINED AS THE WINDS VEER TO THE  
SOUTH-SOUTHEAST...SHIFTING TO THE SOUTH-SOUTHWEST LATER IN THE  
DAY/EARLY WEDNESDAY MORNING. ON THURSDAY-FRIDAY...AS A RIDGE  
GRADUALLY BUILDS TO THE NORTH...THE PREVAILING EASTERLY TRADES  
RETURN TO THE FORECAST AREA. STRONGER WINDS ARE EXPECTED BY THE  
END OF THE WEEK.  
 
AS THE WINDS VEER TO THE SOUTH...EXPECTING SLIGHT INCREASE IN  
DIURNAL CONVECTION ALONG THE CORDILLERA-NORTHERN PUERTO RICO  
DURING THE DAY TODAY. SLIGHTLY HIGHER AMOUNTS ARE EXPECTED ON  
WEDNESDAY AS THE MID LEVEL RIDGE PATTERN WEAKENS AND DEEPER  
MOISTURE POOLS ACROSS THE FORECAST AREA. ON THURSDAY AND ONWARD  
EXPECTING A LULL IN ACTIVITY...WITH EARLY MORNING TRADE WIND  
SHOWERS OVER THE VIRGIN ISLES-EASTERN PUERTO RICO TO ONLY TRIGGER  
LIGHT RAINFALL AMOUNTS. LIKEWISE ACROSS WESTERN INTERIOR PORTIONS  
OF PUERTO RICO IN AFTERNOON CONVECTION.  
 
GORDON...NMS (BELIZE)  
PECK...MS (JAMAICA)  
RIVAS...SENAMHI (PERU)  
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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