389  
FXCA20 KWBC 211128  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
627 AM EST TUE NOV 21 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM NOV 21/06  
UTC: AT 500/250 HPA...CELL OF THE SUBEQUATORIAL RIDGE IS QUICKLY  
BECOMING THE DOMINANT FEATURE ACROSS THE EASTERN CARIBBEAN...WITH  
AXIS TO CENTER AT 500 HPA ON A CLOSED HIGH OVER PUERTO RICO LATER  
ON WEDNESDAY. THIS PATTERN IS FORECAST TO HOLD NEARLY UNCHANGED  
THROUGH SUNDAY EVENING. AS IT BUILDS ACROSS THE EASTERN  
CARIBBEAN...THE MID LEVEL RIDGE FAVORS A STRONG CAP INVERSION THAT  
IS TO SUSTAIN A SHARP DROP IN AVAILABLE WATER DURING THE DAY  
TODAY...WITH A PWAT MINIMA AND DRY/STABLE CONDITIONS TO PERSIST  
THROUGH AT LEAST THURSDAY AFTERNOON.  
 
AT LOW LEVELS...BROAD POLAR RIDGE OVER THE WESTERN NORTH ATLANTIC  
FAVORS A TIGHT GRADIENT AND BRISK EASTERLY TRADES ACROSS THE  
EASTERN CARIBBEAN ISLES. LOW LEVEL RIDGE PATTERN IS EXPECTED TO  
WEAKEN ON WEDNESDAY...WITH LIGHTER WINDS TO THEN ENVELOP THE  
FORECAST AREA LATER IN THE DAY. EMBEDDED IN THIS FLOW...A TUTT  
INDUCED PERTURBATION IN THE EASTERLY TRADES MOVES ACROSS THE  
ISLAND CHAIN TO THE VIRGIN ISLES LATER ON THURSDAY...REACHING  
EASTERN PUERTO RICO BY FIRST LIGHT ON FRIDAY. THIS WILL BE  
ACCOMPANIED BY A SHALLOW MOIST TONGUE THAT IS FORECAST TO ADVECT  
FROM THE EAST. THIS...HOWEVER...WILL BE SHORT LASTING...WITH A  
DRYER AIR MASS TO ONCE AGAIN SETTLE ACROSS THE NORTHEAST CARIBBEAN  
ISLES ON SATURDAY-SUNDAY.  
 
ALTHOUGH IN AGREEMENT ON HOW PATTERN EVOLVES THROUGH THE MEDIUM  
RANGE PERIOD...THE GLOBAL MODELS CONTINUE TO DISAGREE ON ITS  
IMPACT ACROSS THE FORECAST AREA...WITH THE ECMWF CONSISTENTLY  
FORECASTING TRACE RAINFALL AMOUNTS WITH THIS WAVE. THE UKMET  
REVERSED TRENDS AND NOW JOINS THE GFS IN FORECASTING LIGHT TO  
MODERATE CONVECTION IN ASSOCIATION WITH THIS FEATURE LATER ON  
THURSDAY-FRIDAY. ALTHOUGH THERE IS POTENTIAL FOR SHOWERS TO  
DEVELOP...UNDER NEGATIVE INFLUENCE OF MID LEVEL HIGH/RIDGE...THE  
ENVIRONMENTAL CONDITIONS WILL REMAIN HOSTILE FOR ORGANIZED DEEP  
CONVECTION TO DEVELOP. AS A RESULT...EXPECTING SLIGHTLY WETTER  
CONDITIONS THAN WHAT THE ECMWF SUGGESTS...BUT NOT AS WET AS  
FORECAST BY THE GFS.  
 
DAVISON...WPC (USA)  
 

 
 
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