641  
FXCA20 KWBC 271138  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
637 AM EST MON NOV 27 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO FROM NOV 26/06 UTC: GOOD  
AGREEMENT AMONG GLOBAL MODEL GUIDANCE THROUGH THE MEDIUM RANGE  
PERIOD...WITH MODELS NOW AGREEING ON THE AMPLITUDE OF MID LEVEL  
TROUGH ENTERING THE FORECAST AREA LATER IN THE WEEK.  
 
FORECAST AREA REMAINS UNDER INFLUENCE OF BROAD CELL OF THE  
SUBEQUATORIAL RIDGE...WITH UPPER LEVEL AXIS EXTENDING NORTH ACROSS  
THE EASTERN CARIBBEAN TO THE WESTERN ATLANTIC. AS IT MEANDERS OVER  
THE EASTERN CARIBBEAN ISLES...THE MID LEVEL RIDGE FAVORS A STRONG  
TRADE WINDS CAP THAT IS INHIBITING ORGANIZED DEEP CONVECTION. THIS  
PATTERN HOLDS NEARLY UNCHANGED THROUGH TUESDAY EVENING. ON  
WEDNESDAY THE SUBEQUATORIAL RIDGE STARTS TO FLATTEN AS A TROUGH TO  
THE WEST LIFTS OVER THIS AXIS. IN THIS PROCESS THE TROUGH IS TO  
FILL AND THE RIDGE IS EXPECTED TO CRUMBLE LATER IN THE DAY. AS IT  
WEAKENS...THE TRADE WINDS CAP RELAXES ITS FOOTHOLD ACROSS THE  
FORECAST AREA...WITH AN ELEVATED INVERSION TO THEN REMAIN LATER IN  
THE WEEK. DURING THAT PERIOD THE INVERSION CAP APPEARS STRONG  
ENOUGH TO DELAY ONSET OF DEEP CONVECTION...BUT NOT STRONG ENOUGH  
TO SUPPRESS IT. SO CONDITIONS SHOULD BECOME SOMEWHAT MORE  
FAVORABLE FOR DEEP CONVECTION LATER IN THE WEEK.  
 
AT LOW LEVELS...LIGHT EASTERLY TRADES ARE TO PREVAIL THROUGH  
MIDWEEK...WITH LIGHT AND VARIABLE WINDS EXPECTED TOWARDS THE END  
OF THE WEEK/WEEKEND. EMBEDDED IN THIS FLOW...WEAK PERTURBATIONS IN  
THE EASTERLY TRADES ARE TO STREAM ACROSS THE FORECAST AREA FROM  
TIME-TO-TIME. THESE ARE TO GRADUALLY DRAW A PLUME OF TROPICAL  
MOISTURE ACROSS THE LEEWARD ISLES TO THE VIRGIN ISLANDS LATER ON  
TUESDAY...AND OVER PUERTO RICO DURING THE MORNING HOURS ON  
WEDNESDAY. AS THE TRADE WINDS CAP LIFTS...PWAT CONTENT IS TO PEAK  
AT 1.50-1.75 INCHES BY MIDWEEK AND ONWARD. ALTHOUGH PWAT CONTENT  
INCREASES...FROM TIME-TO-TIME PWAT MINIMA ARE TO ALSO ADVECT  
ACROSS THE FORECAST AREA...FRACTURING THE PLUME OF TROPICAL  
MOISTURE INTO SEGMENTS.  
 
IN THIS PATTERN...AS A STRONG TRADE WINDS CAP HOLDS ON  
MONDAY-TUESDAY...EXPECTING FAIR WEATHER CONDITIONS TO PREVAIL  
ACROSS THE ENTIRE FORECAST AREA. SHOWERS...IF ANY...WILL GENERALLY  
LIMIT TO EL YUNQUE RAINFOREST/EASTERN PORTIONS OF PUERTO RICO IN  
MORNING AND AFTERNOON CONVECTION. MEASURABLE  
AMOUNTS...HOWEVER...ARE TO LIMIT TO WESTERN PUERTO RICO IN  
AFTERNOON CONVECTION. ON WEDNESDAY...WHEN HIGHER PWAT CONTENT  
ENVELOPS THE ENTIRE FORECAST AREA...EXPECTING A GRADUAL INCREASE  
IN DAYTIME CONVECTION ACROSS THE FORECAST AREA...WITH MOST INTENSE  
CLUSTERING ACROSS PUERTO RICO. A LULL IN ACTIVITY ON THURSDAY WILL  
BE FOLLOWED BY A SOMEWHAT MORE ACTIVE PATTERN ON FRIDAY AS THE  
NEXT PLUME OF MOISTURE STREAMS ACROSS THE FORECAST AREA.  
 
DAVISON...WPC (USA)  
 

 
 
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