400  
FXCA20 KWBC 041146  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
645 AM EST MON DEC 04 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 04/06  
UTC: BROAD MID/UPPER LEVEL TROUGH REMAINS TO THE NORTH OF THE  
ISLANDS. THIS PROVIDES SUPPORT TO A SURFACE FRONT THAT EXTENDS  
EAST-TO-WEST ALONG 21N/22N TO THE BAHAMAS/THE TURKS...WITH DEEP  
CONVECTION CLUSTERING ALONG THIS FRONT. OVER THE EASTERN  
CARIBBEAN...POLAR TROUGH TO THE NORTH FAVORS AN INDUCED/INVERTED  
TROUGH OVER THE BASIN. THE TROUGH SUSTAINS A MOIST SOUTHEASTERLY  
FLOW ACROSS THE ISLAND CHAIN TO THE NORTHEAST CARIBBEAN ISLES.  
THIS IS FEEDING CONVECTIVE DEVELOPMENT ACROSS THE VIRGIN ISLES  
EARLY THIS MORNING.  
 
CONSISTENT WITH PREVIOUS FORECAST...A 500/250 HPA SHORT WAVE  
TROUGH ON THE SOUTHERN STREAM EXITS FLORIDA LATER THIS AFTERNOON.  
EARLY ON TUESDAY THIS IS TO QUICKLY MOVE NORTH OF THE FORECAST  
AREA...MEANWHILE FAVORING INTENSIFICATION OF A SUBTROPICAL JET  
MAXIMA NORTH OF PUERTO RICO/VIRGIN ISLES. DIVERGENCE ON ITS RIGHT  
ENTRANCE REGION IS TO THEN VENT CONVECTIVE DEVELOPMENT ON  
TUESDAY-THURSDAY. AT LOW LEVELS...A RIDGE ROLLING ACROSS THE  
EASTERN USA TO THE WESTERN ATLANTIC LATER TODAY IS TO TIGHTEN THE  
PRESSURE GRADIENT. AS THE GRADIENT TIGHTENS A PREFRONTAL SHEAR  
LINE IS TO THEN FORM ACROSS THE FORECAST AREA LATER THIS  
EVENING/EARLY TUESDAY MORNING. THIS IS TO MEANDER SOUTH ACROSS THE  
VIRGIN ISLES LATER ON TUESDAY MORNING...TO SETTLE OVER THE LEEWARD  
ISLES/EASTERN CARIBBEAN LATER IN THE DAY. AS THE SHEAR LINE  
APPROACHES...THE LOW LEVEL WINDS ARE TO BACK TO THE EAST WHILE  
INCREASING TO 15-20KT. THE MODELS THEN AGREE IN BREEZY CONDITIONS  
TO PERSIST THROUGH THE END OF THE WEEK. THIS WILL FAVOR A COLD  
ADVECTIVE PATTERN...WITH WANING SURFACE FRONT SAGGING ACROSS THE  
NORTHERN VIRGIN ISLES TO EASTERN PUERTO RICO EARLY ON WEDNESDAY  
MORNING. A DRYER AIR MASS IS TO THEN ENTRAIN FROM THE EAST ON  
FRIDAY-SATURDAY.  
 
MEANWHILE...THE EVOLVING ENVIRONMENTAL CONDITIONS ARE TO FAVOR  
SCATTERED CONVECTION ACROSS THE FORECAST AREA...WITH MOST INTENSE  
EXPECTED BETWEEN THE VIRGIN ISLES AND NORTHEASTERN PUERTO RICO IN  
ASSOCIATION WITH THE SHEAR LINE AND THE SAGGING FRONT. THESE ARE  
TO ENHANCE LOW LEVEL CONVERGENCE AS THEY APPROACH THE FORECAST  
AREA ON TUESDAY AND WEDNESDAY. FURTHERMORE...THE PERSISTENT BRISK  
EASTERLY WINDS ARE TO SUSTAIN GENERATION OF TRADE WIND SHOWERS AS  
A COOLER AIR MASS ADVECTS OVER THE WARMER WATERS TO THE NORTH.  
THIS PATTERN IS TO GENERALLY HOLD THROUGH THE END OF THE  
WORKWEEK...WITH STORM TOTAL AMOUNTS PEAKING BETWEEN 2-4 INCHES.  
 
DAVISON...WPC (USA)  
 

 
 
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