650  
FXCA20 KWBC 211101  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
700 AM EDT SAT OCT 21 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE VI FROM OCT 21/06  
UTC: FAIR AND WINDY CONDITIONS ARE TO PREVAIL DURING THE NEXT  
SEVERAL DAYS. A DEEP AND ELONGATED UPPER TROUGH HAS EXISTED THE  
UNITED STATES EAST COAST AND NOW EXTENDS AN AXIS ALONG 33N 60W  
INTO THE SOUTHEASTERN BAHAMAS AT 24N 73W. THE UPPER TROUGH IS  
PROPAGATING TO THE SOUTH WEST TOWARDS THE ISLANDS. THIS TROUGH IS  
PRESSING INTO AN UPPER RIDGE THAT EXTENDS AN AXIS FROM THE  
NORTHERN ANTILLES TO THE NORTHEAST ALONG 20N 60W...AND INTO 32N  
43W. AT LOW-LEVELS...A TRADE WIND SURGE HAS CROSSED THE EASTERN  
AND CENTRAL CARIBBEAN FROM EAST TO WEST AND NOW EXTENDS AN AXIS  
ACROSS WESTERN HAITI/TURKS AND CAICOS. THE AIR MASS BEHIND THE  
SURGE...NOW SITTING OVER PUERTO RICO AND THE VI...IS CHARACTERIZED  
BY A TRADE WIND CAP NEAR 900 HPA...AND BY A VERY DRY TROPOSPHERE  
OVER THE INVERSION. WINDS ARE FROM THE EAST AT 15-20KT. THIS IS  
SUSTAINING ISOLATED...LIGHT AND RAPIDLY-MOVING TRADE WIND SHOWERS.  
DEEP-LAYER MOISTURE LIES FAR SOUTH ALONG THE EXTREME SOUTHERN  
CARIBBEAN...WHERE IT IS FORECAST TO REMAIN THROUGH THE NEXT  
SEVERAL DAYS.  
 
MODELS SHOW THE UPPER TROUGH NEAR THE BAHAMAS PROPAGATING  
SOUTHEASTWARD. THE BASE OF THE UPPER TROUGH IS FORECAST TO REACH  
PUERTO RICO BY SUNDAY MORNING. YET...THE MID-TROPOSPHERE WILL  
CONTINUE BEING TOO DRY TO SUSTAIN HEAVY RAINFALL...AND MODELS ARE  
AGREEING IN THUNDERSTORMS DEVELOPMENT CONSTRAINING TO REGIONS TO  
THE NORTH OF 21-22N. THE TROUGH IS THEN FORECAST TO SLOW DOWN ITS  
PROPAGATION...AND BY MONDAY EVENING IT IS FORECAST TO EXTEND AN  
AXIS ACROSS THE LEEWARD ISLANDS INTO NORTHEASTERN VENEZUELA. THIS  
WILL PLACE PUERTO RICO AND THE VI UNDER THE UPPER CONVERGENT  
PATTERN OF THE TROUGH...WHICH WILL CONTINUE SUSTAINING THE TRADE  
WIND CAP AND LIMITING THUNDERSTORM DEVELOPMENT. POSITIVE  
INTERACTION BETWEEN THE TROUGH AND THE MOISTURE PLUME IN THE  
SOUTHEASTERN CARIBBEAN IS FORECAST TO LIMIT TO LATITUDES TO THE  
SOUTH OF 15N...AND TO THE EAST OF 65W. THIS WILL SUSTAIN A WET  
SPELL IN THE FRENCH ISLES AND LESSER ANTILLES DURING  
MONDAY-WEDNESDAY...WHILE PUERTO RICO AND THE VI CONTINUE  
EXPERIENCING FAIR CONDITIONS.  
 
IN TERMS OF THE PRECIPITATION PATTERNS AND  
ACCUMULATIONS...EXPECTING ISOLATED BUT NUMEROUS LIGHT TRADE WIND  
SHOWERS DOTTING THE VIRGIN ISLANDS AND EASTERN PUERTO RICO  
ESPECIALLY DURING LATE NIGHT AND EARLY MORNING HOURS. THIS WILL  
LEAD TO DAILY ACCUMULATIONS IN THE ORDER OF 0.2 INCHES/DAY...AND  
0.5 INCHES/DAY IN REGIONS OF ENHANCED OROGRAPHIC UPLIFT IN EASTERN  
SLOPES OF EASTERN PUERTO RICO. DURING THE AFTERNOON...SHOWERS ARE  
TO CONCENTRATE IN WESTERN PUERTO RICO. THERE IS A CHANGE FOR  
ISOLATED THUNDERSTORMS ON SUNDAY AND MONDAY IN THE WESTERN  
CORDILLERA...YET ANY DEEP CONVECTION WILL BE SHORT LIVED AND MOVE  
RAPIDLY WESTWARD...WHICH WILL LIMIT MAXIMUM ACCUMULATIONS ALSO TO  
0.3-0.5 INCHES.  
 
GUY...NMS (BELIZE)  
PECK...MS (JAMAICA)  
GALVEZ...WPC (USA)  

 
 
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