004  
FXCA20 KWBC 201126  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
726 AM EDT FRI OCT 20 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE VI FROM OCT  
20/00UTC: FAIR AND WINDY CONDITIONS ARE TO PERSIST INTO EARLY NEXT  
WEEK. ANY PRECIPITATION WILL BE LIGHT AND ISOLATED...GENERALLY IN  
THE FORM OF FAST MOVING TRADE WIND SHOWERS.  
 
AT UPPER LEVELS...A LARGE TROUGH SITS OVER THE TROPICAL ATLANTIC.  
THIS TROUGH CENTERS ON A LOW AT 21N 42W...AND EXTENDS AN AXIS INTO  
THE CENTRAL LESSER ANTILLES. THE UPPER TROUGH IS INTERACTING WITH  
A TROPICAL WAVE THAT EXTENDS NEAR 40W. TO THE WEST OF THE  
LOW...HOWEVER...AN UPPER CONVERGENT PATTERN IS DOMINATING.  
ALSO...AN UPPER TROUGH EXTENDS OFF THE EAST COAST OF THE UNITED  
STATES AND IS PROPAGATING TO THE SOUTHEAST. AT LOW-LEVELS...A  
SIGNIFICANT FEATURE IS A READE WIND SURGE THAT CROSSED THE ISLANDS  
LATE ON WEDNESDAY...AND NOW EXTENDS ACROSS CENTRAL HISPANIOLA. THE  
AIR MASS TO THE EAST OF THE LEADING EDGE OF THE SURGE IS  
CHARACTERIZED BY A SUBSIDENCE CAP SITTING AT 900 HPA...AND BY A  
VERY DRY AIR MASS THAT SITS OVER THE CAP AND EXTENDS INTO THE  
MID/UPPER TROPOSPHERE. TRADE WINDS ARE FAST...WITH SPEEDS BETWEEN  
20 AND 30KT. THIS IS LIMITING ACTIVITY TO VERY ISOLATED AND  
RAPIDLY MOVING TRADE WIND SHOWERS.  
 
MODELS ARE SHOWING THIS PATTERN PERSISTING INTO EARLY NEXT WEEK.  
THE UPPER TROUGH EXITING THE EAST COAST OF THE UNITED STATES IS  
FORECAST TO CONTINUE PROPAGATING TO THE SOUTHEAST TO SIT JUST TO  
THE NORTH OF THE ISLANDS BY SUNDAY. YET...THE LOW AND MID-LEVELS  
WILL BE TOO DRY TO SUSTAIN ANY SIGNIFICANT PRECIPITATION. FROM  
SUNDAY TO TUESDAY...THE UPPER TROUGH WILL SIT JUST TO THE NORTH OF  
THE LEEWARD ISLANDS NEAR 24N 63W. IT APPEARS THAT THE POSITIVE  
INTERACTION BETWEEN THE UPPER TROUGH AND THE MOIST PLUME  
ASSOCIATED WITH THE TROPICAL WAVE AND THE ITCZ WILL HAPPEN TO THE  
SOUTH OF THE FRENCH ANTILLES WHILE AN UPPER CONVERGENT PATTERN  
STRENGTHENS OVER THE ISLANDS. THIS WILL CONTINUE SUSTAINING  
CURRENT FAIR WEATHER PATTERN INTO EARLY NEXT WEEK.  
 
IN TERMS OF THE DISTRIBUTION OF PRECIPITATION...EXPECTING LARGEST  
AMOUNTS IN EASTERN PUERTO RICO IN THE FORM OF LIGHT AND  
SHORT-LIVED TRADE WIND SHOWERS. THESE ARE TO PRODUCE ACCUMULATIONS  
OF 0.2-0.3 INCHES/DAY. SIMILAR AMOUNTS ARE EXPECTED IN AFTERNOON  
SHOWERS IN EXTREME WESTERN PUERTO RICO. EASTERLY TRADES OF 15-20KT  
ARE TO PERSIST THROUGH LATE FRIDAY...TO THEN DECREASE TO 10-15KT.  
 
GUY...NMS (BELIZE)  
PECK...MS (JAMAICA)  
GALVEZ...WPC (USA)  

 
 
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