271  
FXCA20 KWBC 051147  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
646 AM EST TUE DEC 05 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 05/06  
UTC: MID/UPPER LEVEL SHORT WAVE TROUGH MOVED ACROSS THE BAHAMAS  
EARLY THIS MORNING...AND AS IT STREAMS ACROSS THE WESTERN ATLANTIC  
IT IS RAPIDLY MERGING WITH BROAD TROUGH TO THE EAST. AS THEY  
MERGE...SUBTROPICAL JET MAXIMA NORTH OF THE FORECAST AREA IS  
FORECAST TO STRENGTHEN. DIVERGENCE ON ITS RIGHT ENTRANCE REGION IS  
TO THEN ENVELOP THE NORTHEAST CARIBBEAN ISLES THROUGH THURSDAY.  
BUILDING MID/UPPER LEVEL RIDGE WILL THEN BECOME THE DOMINANT UPPER  
FEATURE LATER IN THE WEEK.  
 
AT LOW LEVELS...A MEANDERING FRONT LIES NORTH OF THE ISLANDS WITH  
AXIS EXTENDING EAST-TO-WEST ALONG 21N/22N TO THE BAHAMAS-CUBA.  
HIGH PRESSURE RIDGE ROLLING ACROSS THE EASTERN USA TO THE WESTERN  
ATLANTIC EARLY THIS MORNING CONTINUES TO TIGHTEN THE PRESSURE  
GRADIENT. THIS...IN-TURN...FAVORS THE GRADUAL INTENSIFICATION OF  
THE EASTERLY TRADES WHILE TRIGGERING A PREFRONTAL SHEAR LINE  
ACROSS PUERTO RICO-VIRGIN ISLES. THE SHEAR LINE WILL CONTINUE TO  
THE LEEWARD ISLES EARLY THIS AFTERNOON...WITH BOUNDARY LAYER WINDS  
INCREASING TO 18-21KT. THE STRONGER WINDS WILL FAVOR GENERATION OF  
STREAMERS ACROSS THE LEEWARD/VIRGIN ISLES THAT ARE TO AFFECT  
EASTERN PUERTO RICO FROM TIME-TO-TIME. THIS IS TO ALSO FAVOR A  
COLD ADVECTIVE PATTERN...DRIVING THE MEANDERING FRONT SOUTH ACROSS  
THE VIRGIN ISLES TO EASTERN PUERTO RICO LATER THIS EVENING/EARLY  
WEDNESDAY MORNING. THE FRONT IS TO THEN REMAIN NEARLY STATIONARY  
WHILE GRADUALLY WEAKENING LATER IN THE WEEK. AS MOISTURE CONVERGES  
ALONG THIS BOUNDARY...PWAT CONTENT OF 1.50-2.0 INCHES IS EXPECTED  
TO LAST THROUGH MIDAFTERNOON HOURS ON THURSDAY. MODELS AGREE ON A  
DRYER AIR MASS TO RAPIDLY ENTRAIN FROM THE EAST STARTING ON  
THURSDAY EVENING...TO INTENSIFY THROUGH THE DAY ON FRIDAY.  
 
AS MOISTURE CONVERGES ALONG THE SURFACE FRONT...AND UNDER  
FAVORABLE UPPER LEVEL JET DYNAMICS...ENVIRONMENTAL CONDITIONS ARE  
TO REMAIN HIGHLY FAVORABLE FOR MODERATE TO HEAVY CONVECTION TO  
AFFECT THE FORECAST AREA DURING THE NEXT 48-72 HRS. GLOBAL MODEL  
GUIDANCE SHOWS THE MOST INTENSE CLUSTERING BETWEEN EASTERN PUERTO  
RICO AND THE VIRGIN ISLES...WITH TOTAL RAINFALL AMOUNTS OVER THE  
NEXT FOUR DAYS PEAKING BETWEEN 2-3 INCHES. HOWEVER...LOCALLY  
HIGHER AMOUNTS ARE HIGHLY PROBABLE DUE TO STRONGER TRADE WINDS  
FLOW AND TOPO FORCING ACROSS EASTERN PORTIONS OF PUERTO RICO.  
MODERATE RAINFALL AMOUNTS ARE TO ALSO AFFECT WESTERN INTERIOR  
PORTIONS OF PUERTO RICO IN AFTERNOON CONVECTION. A DRYING TREND IS  
EXPECTED LATER IN THE WEEK.  
 
DAVISON...WPC (USA)  
 

 
 
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