522  
FXCA20 KWBC 141131  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
631 AM EST THU DEC 14 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 14/06  
UTC: BROAD MID/UPPER LEVEL TROUGH DOMINATES THE FLOW ACROSS THE  
WESTERN ATLANTIC. SOUTH OF THIS AXIS...A FLAT WEST-TO-EAST RIDGE  
ENVELOPS THE BASIN/NORTHERN SOUTH AMERICA. ALTHOUGH WEAK...THE MID  
LEVEL RIDGE PATTERN SUSTAINS AN ELEVATED TRADE WINDS CAP ACROSS  
THE FORECAST AREA. AT LOW LEVELS...POLAR TROUGH TO THE NORTH  
SUSTAINS A SURFACE FRONT THAT EXTENDS FROM THE CENTRAL ATLANTIC TO  
JUST NORTH OF HISPANIOLA INTO THE CENTRAL CARIBBEAN. IN THIS  
PATTERN A LIGHT EASTERLY BREEZE DOMINATES THE NORTHEAST CARIBBEAN  
ISLES.  
 
DEEPENING TROUGH OVER WESTERN PORTIONS OF THE DOMAIN IS TO INDUCE  
THE NORTHWARD AMPLIFICATION OF A MID/UPPER LEVEL RIDGE ACROSS THE  
BASIN TO THE WESTERN ATLANTIC. THIS FEATURE IS TO QUICKLY BECOME  
THE DOMINANT FEATURE AS IT STRENGTHENS DURING THE  
WEEKEND...SUSTAINING A STRONG TRADE WINDS CAP THAT IS TO HOLD  
THROUGH THE MEDIUM RANGE PERIOD. AT LOW LEVELS THE RIDGE ALOFT IS  
TO SUSTAIN A HIGH PRESSURE RIDGE THAT IS TO GRADUALLY BUILD NORTH  
OF THE ISLANDS. AS THE RIDGE STRENGTHENS AND THE GRADIENT TIGHTENS  
THIS IS TO FAVOR GRADUAL INTENSIFICATION OF THE LOW LEVEL WINDS.  
AT 850 HPA THE WINDS INCREASE TO 20-25KT WHILE BACKING TO THE  
NORTHEAST ON SATURDAY...AND ON SUNDAY THEY ARE EXPECTED TO PEAK  
AROUND 30-35KT. THE STRONGER WINDS WILL BE ACCOMPANIED BY A COLD  
ADVECTIVE PATTERN OVER THE WARMER ATLANTIC WATERS NORTH OF THE  
ISLANDS...FAVORING SHALLOW CONVECTIVE INSTABILITY AND HIGHER  
INCIDENCE OF TRADE WIND SHOWERS ACROSS THE FORECAST AREA. ALSO  
EXPECTING THE OCCASIONAL PLUME OF SHALLOW MOISTURE TO ADVECT  
ACROSS THE FORECAST AREA FROM TIME-TO-TIME.  
 
FORECAST TREND REMAINS AS PREVIOUSLY INDICATED...WITH EARLY  
MORNING TRADE WIND SHOWERS PESTERING THE USVI-EASTERN/NORTHERN  
PORTIONS OF PUERTO RICO DURING THE NEXT COUPLE OF DAY. THIS  
GRADUALLY BECOMES BETTER ORGANIZED DURING THE WEEKEND AS THE TRADE  
WINDS SURGE ACROSS THE FORECAST AREA. ALSO...THE STRONGER WINDS  
DURING THE WEEKEND ARE TO FAVOR THE GENERATION OF STREAMERS ACROSS  
THE VIRGIN ISLES THAT ARE TO CLUSTER OVER EASTERN PUERTO RICO  
DURING THE AFTERNOON HOURS. THE EAST-NORTHEAST TRADES WILL COMBINE  
WITH DIURNAL HEATING TO ENHANCE AFTERNOON CONVERGENCE ACROSS  
WESTERN/SOUTHWESTERN PUERTO RICO...WHERE WE EXPECT LIGHT RAINFALL  
AMOUNTS IN A DIURNAL PATTERN.  
 
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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