170  
FXCA20 KWBC 221136  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
636 AM EST WED NOV 22 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM NOV 22/06  
UTC: A HIGH AMPLITUDE LONG WAVE TROUGH DOMINATES THE MID/UPPER  
LEVEL FLOW ACROSS THE EASTERN CARIBBEAN TO THE WESTERN ATLANTIC.  
AT 500 HPA THIS ANCHORS ON A CLOSED HIGH OVER THE NORTHWEST  
CARIBBEAN...MEANWHILE FAVORING A STRONG TRADE WINDS CAP ACROSS THE  
FORECAST AREA. THIS IS A SLOW TO EVOLVE PATTERN...WITH THE RIDGE  
TO SLOWLY FLATTEN LATER DURING THE WEEKEND AS THE TROUGH LIFTS  
OVER THIS AXIS.  
 
AT LOW LEVELS...A HIGH TO THE EAST-NORTHEAST OF BERMUDA ANCHORS A  
BROAD RIDGE OVER THE CENTRAL ATLANTIC. THIS FAVORS A TIGHT  
PRESSURE GRADIENT AND BRISK EASTERLY WINDS ACROSS THE ISLAND CHAIN  
TO THE EASTERN CARIBBEAN. UNDER PRESSURE FROM POLAR FRONTS EXITING  
THE EASTERN USA THE LOW LEVEL RIDGE IS TO WEAKEN EARLY IN THE  
CYCLE. AS THE GRADIENT SLACKENS...LIGHTER EASTERLY TRADES ARE TO  
THEN PERSIST DURING THE NEXT FOLLOWING DAYS. ALSO...UNDER THE  
INFLUENCE OF A WANING TUTT OVER THE CENTRAL TROPICAL ATLANTIC...A  
COUPLE OF TUTT INDUCED PERTURBATIONS ARE STREAMING TOWARDS THE  
NORTHEAST CARIBBEAN...WITH SATELLITE IMAGERY SHOWING A WELL  
DEFINED VORTEX ALIGNING ALONG 54W EARLY THIS MORNING. THESE EMBED  
IN A PLUME OF TROPICAL MOISTURE TO THE EAST OF THE LEEWARD  
ISLANDS. THE WAVES ARE TO STREAM ACROSS THE FORECAST AREA ON  
THURSDAY AND FRIDAY. BUT AS THEY MOVE UNDER THE MID LEVEL RIDGE  
ENVELOPING THE EASTERN CARIBBEAN...THEY WILL HAVE A HARD TIME  
OVERCOMING THE NEGATIVE INFLUENCE OF PERSISTENT MID LEVEL RIDGE  
PATTERN. THUS...EXPECTING SOME EROSION OF AVAILABLE MOISTURE AS  
THE WAVES ADVECT ACROSS THE FORECAST AREA.  
 
IN THIS CYCLE THE UKMET REVERSED TRENDS AND NOW JOINS THE ECMWF IN  
ONLY FORECASTING LIGHT RAINFALL AMOUNTS ON THURSDAY-FRIDAY...WHILE  
THE GFS CONTINUES TO FORECAST MODERATE CONVECTION. TAKING INTO  
ACCOUNT NEGATIVE INFLUENCE OF MID LEVEL RIDGE... EXPECTING MOST  
ACTIVE CONVECTION ON THURSDAY-FRIDAY TO CLUSTER ACROSS WESTERN  
INTERIOR PORTIONS OF PUERTO RICO. IN THIS AREA THE 24 HRS RAINFALL  
AMOUNTS ARE TO PEAK AT LESS THAN AN INCH. ACROSS THE VIRGIN  
ISLES/EASTERN PUERTO RICO ONLY EXPECTING LIGHT RAINFALL AMOUNTS  
DURING THE NEXT FEW DAYS. ON SATURDAY-SUNDAY EXPECTING FAIR  
WEATHER CONDITIONS TO PREVAIL AS A DRYER AIR MASS ENTRAINS FROM  
THE EAST.  
 
DAVISON...WPC (USA)  
 

 
 
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