632  
FXCA20 KWBC 201134  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
633 AM EST MON NOV 20 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM NOV 20/06  
UTC: DIGGING MID/UPPER LEVEL TROUGH OVER THE GULF OF MEXICO IS TO  
FAVOR THE RAPID INTENSIFICATION OF THE SUBEQUATORIAL RIDGE ACROSS  
THE ISLAND CHAIN/EASTERN CARIBBEAN EARLY IN THE CYCLE. AS THE  
PATTERN EVOLVES...SHORT WAVE TROUGH OVER THE WESTERN CARIBBEAN IS  
TO FINALLY FILL WHILE SHEARING SHORT WAVE ENERGY ACROSS  
HISPANIOLA-WESTERN PUERTO RICO EARLY IN THE PERIOD. AT LOW  
LEVELS...AN ILL ORGANIZED LOW SOUTH OF HISPANIOLA IS ENHANCING LOW  
LEVEL MOISTURE CONVERGENCE ACROSS THE CENTRAL CARIBBEAN...WITH  
PLUME OF TROPICAL MOISTURE LIFTING ALONG THE WESTERN FRINGES OF  
THE SUBEQUATORIAL RIDGE TO THE FORECAST AREA. THIS FAVORS A HIGH  
PWAT CONTENT OF AROUND TWO INCHES. THIS WILL COMBINE WITH INFLOW  
OF MID LEVEL ENERGY TO SUSTAIN ANOTHER ROUND OF DEEP CONVECTION  
DURING THE DAY TODAY. IN LIGHT EASTERLY TRADES...WIDESPREAD  
CONVECTION IS HIGHLY PROBABLE DURING THE AFTERNOON HOURS.  
 
LATER THIS EVENING...UNDER INFLUENCE OF BUILDING SUBEQUATORIAL  
RIDGE...A STRONG TRADE WINDS CAP IS TO SETTLE ACROSS THE FORECAST  
AREA...WITH ENVIRONMENTAL CONDITIONS RAPIDLY GROWING HOSTILE TO  
ORGANIZED DEEP CONVECTION. THIS IS TO GENERALLY FAVOR FAIR WEATHER  
CONDITIONS ON TUESDAY-WEDNESDAY. DURING THAT PERIOD MEASURABLE  
AMOUNTS ARE TO CONFINE TO EL YUNQUE RAINFOREST AND WESTERN PUERTO  
RICO. THE MODELS START TO DISAGREE ON LOW LEVEL PATTERN EVOLUTION  
DURING THE MEDIUM RANGE PERIOD...WITH GFS SHOWING A WELL ORGANIZED  
TUTT INDUCED TROUGH IN THE EASTERLY TRADES TO STREAM ACROSS THE  
FORECAST AREA LATER ON THURSDAY. THE ECMWF AND UKMET...IN  
CONTRAST...SHOW STRONG NEGATIVE INTERACTION BETWEEN THIS FEATURE  
AND MID LEVEL RIDGE...AND AS A RESULT BOTH FAVOR A MUCH WEAKER  
PERTURBATION TO STREAM ACROSS THE FORECAST AREA. THE GFS AND ECMWF  
GDI FORECAST ALSO SUPPORT THE IDEA OF A WEAKER SYSTEM TO STREAM  
ACROSS THE FORECAST AREA. TAKING THIS INTO CONSIDERATION... OUR  
DAY 04 FORECAST SHOWS LESSER AMOUNTS THAN WHAT THE GFS SUGGESTS.  
 
DAVISON...WPC (USA)  
 

 
 
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