339  
FXCA20 KWBC 221140  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
639 AM EST FRI DEC 22 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 22/06  
UTC: MID/UPPER LEVEL RIDGE REMAINS THE DOMINANT FEATURE ACROSS THE  
GULF OF MEXICO/SOUTHEAST USA INTO THE CARIBBEAN BASIN...WHERE IT  
SUSTAINS A TRADE WINDS CAP THAT IS INHIBITING ORGANIZED DEEP  
CONVECTION. THE RIDGE IS ALSO STEERING A TROUGH ACROSS BERMUDA  
EARLY THIS MORNING...WITH AXIS TO RAPIDLY AMPLIFY AS IT CLEARS THE  
EASTERN FRINGES OF THE MID LEVEL RIDGE PATTERN. ON SATURDAY THE  
TROUGH BOTTOMS OUT OVER THE NORTHEAST CARIBBEAN WHILE GRADUALLY  
EVOLVING INTO A CLOSED TUTT LOW TO THE NORTHEAST OF THE LEEWARD  
ISLES. A JET MAXIMA WILL BOUND THE SOUTHERN FRINGES OF THIS  
TROUGH... WITH AXIS ORIGINATING OVER THE SOUTHEAST CARIBBEAN.  
PUERTO RICO AND THE VIRGIN ISLES WILL LIE ON THE CONVERGENT SIDE  
OF THIS JET...WHICH IS TO HELP MAINTAIN THE TRADE WINDS CAP ACROSS  
THE NORTHEAST CARIBBEAN.  
 
UNDER PRESSURE FROM AMPLIFYING TROUGH OVER NORTH AMERICA...THE  
RIDGE OVER THE GULF/BASIN IS TO RAPIDLY FLATTEN DURING THE  
WEEKEND. AS THE RIDGE TO THE WEST COLLAPSES...THE TUTT WILL  
GRADUALLY WEAKEN AND START TO LIFT TO THE EAST-NORTHEAST.  
REMAINING MID LEVEL CIRCULATION IS TO THEN DECOUPLE FROM THE UPPER  
LEVEL AXIS...RETROGRESSING ACROSS THE LESSER ANTILLES TO THE  
EASTERN CARIBBEAN ON TUESDAY-WEDNESDAY. AS IT WEAKENS...IT IS TO  
SHEAR A COUPLE OF MID LEVEL VORTICES THAT ARE TO MEANDER ACROSS  
SAINT CROIX WHILE GRAZING THE CARIBBEAN COASTAL WATERS OF PUERTO  
RICO. THIS WILL BE ACCOMPANIED BY A PLUME OF TROPICAL  
MOISTURE...WITH PWAT CONTENT PEAKING BETWEEN 1.25-1.50 INCHES.  
NOTE THAT IN THIS RUN THE ECMWF IS NOT AS BULLISH AS IT HAS BEEN  
OVER THE LAST FEW CYCLES...FAVORING A SOLUTION CLOSER TO WHAT THE  
GFS HAS BEEN ADVERTISING. AS THE MODELS REACH A CONSENSUS...OUR  
CONFIDENCE IN THE FORECAST IS HIGHER THAN BEFORE.  
 
AT LOW LEVELS...LONG FETCH EAST-NORTHEASTERLY WINDS DOMINATE THE  
FLOW ACROSS THE NORTHEAST CARIBBEAN...WITH 850 HPA WINDS OF  
10-15KT. A HIGH MOVING FROM THE EASTERN USA TO THE WESTERN  
ATLANTIC DURING THE WEEKEND IS TO TIGHTEN THE GRADIENT WHILE  
FAVORING SLIGHTLY STRONGER WINDS ACROSS THE FORECAST AREA. THROUGH  
SUNDAY A LONG FETCH EASTERLY FLOW IS TO ESTABLISH WITH WINDS  
INCREASING TO 20-25KT. AS THE WINDS INTENSIFY...STREAMERS ARE  
LIKELY TO FORM OVER THE VIRGIN ISLES TO AFFECT EASTERN PUERTO RICO  
DURING THE LATE MORNING/AFTERNOON HOURS. BUT THESE ARE TO  
GENERALLY FAVOR LIGHT RAINFALL AMOUNTS IN PASSING SHOWERS.  
ALSO...UNDER THE INFLUENCE OF THE TUTT TO THE EAST...SHALLOW CLOUD  
CLUSTERS ARE TO FORM EAST OF THE ISLANDS. THESE WILL THEN EMBED IN  
THE LOW/MID LEVEL EASTERLY TRADES TO PESTER THE FORECAST AREA FROM  
TIME-TO-TIME. IT LOOKS LIKE THESE WILL BE THE MOST ACTIVE LATER ON  
SATURDAY-MONDAY MORNING. BUT AS THE TUTT WEAKENS THE ACTIVITY  
SHOULD WANE. MEANWHILE...THIS WILL FAVOR LIGHT TO MODERATE  
RAINFALL AMOUNTS ACROSS THE VIRGIN ISLES...WITH SLIGHTLY HIGHER  
AMOUNTS ACROSS LUQUILLO/EL YUNQUE. MOST ACTIVE CONVECTION IS TO  
AFFECT WESTERN INTERIOR PORTIONS OF PUERTO RICO ON SUNDAY...WITH  
24 HRS RAINFALL AMOUNTS PEAKING AT 0.50 TO 0.75 OF AN INCH. A  
SECOND ROUND ON MONDAY IS NOT GOING TO BE AS ACTIVE.  
 
DAVISON...WPC (USA)  
 

 
 
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