597  
FXCA20 KWBC 061127  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
627 AM EST WED DEC 06 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 06/06  
UTC: DEEPENING POLAR TROUGH OVER NORTH AMERICA WILL INDUCE THE  
GRADUAL NORTHWARD AMPLIFICATION OF A MID/UPPER LEVEL RIDGE FROM  
THE EASTERN CARIBBEAN TO THE WESTERN ATLANTIC. AS THE RIDGE  
BUILDS...A TROUGH TO THE EAST RELOCATES TO THE CENTRAL ATLANTIC  
EARLY ON FRIDAY MORNING. EARLY IN THE CYCLE...AT 250 HPA...A  
SUBTROPICAL JET MAXIMA SOUTH OF THIS TROUGH WILL LIE TO THE NORTH  
OF PUERTO RICO/VIRGIN ISLES. THIS IS TO ALSO PULL AWAY AS THE  
RIDGE BUILDS TO THE WEST. MEANWHILE...THE JET MAXIMA IS TO FAVOR  
AN UPPER DIVERGENT PATTERN THAT IS TO VENT DEEP CONVECTION OVER  
THE NEXT COUPLE OF DAYS. BY MIDMORNING ON FRIDAY...AS THE RIDGE  
PATTERN STRENGTHENS OVER THE WESTERN ATLANTIC...A STRONG  
SUBSIDENCE INVERSION IS TO ESTABLISH ACROSS THE FORECAST AREA.  
THIS IS TO THEN FAVOR RAPID EROSION OF AVAILABLE WATER FROM THE  
EAST.  
 
AT LOW LEVELS...A BROAD RIDGE DOMINATES THE FLOW ACROSS THE  
ATLANTIC TO THE WEST OF 45W. THIS SUSTAINS A TIGHT PRESSURE  
GRADIENT WITH BRISK EASTERLY TRADES ACROSS THE NORTHEAST CARIBBEAN  
ISLES. FURTHERMORE...AN ELONGATED FRONT BOUNDS THE SOUTHERN  
FRINGES OF THE RIDGE...WITH BOUNDARY MEANDERING OVER THE VIRGIN  
ISLES-NORTHEAST PUERTO RICO EARLY THIS MORNING. THE MEANDERING LOW  
LEVEL RIDGE SUSTAINS COLD ADVECTION ACROSS THE WARMER WATERS NORTH  
OF THE ISLANDS. THIS IN-TURN SUSTAINS A CONVECTIVELY UNSTABLE AIR  
MASS AND GENERATION OF RAIN SHOWERS THAT AFFECT THE NORTHEAST  
CARIBBEAN FROM TIME-TO-TIME. THE FRONT IS FORECAST TO REMAIN  
NEARLY STATIONARY WHILE GRADUALLY FRONTOLIZING DURING THE DAY ON  
FRIDAY. THE SATELLITE DERIVED PWAT ANALYSIS SHOWS MOISTURE  
CLUSTERING ALONG THE SURFACE FRONT...WITH PWAT CONTENT OF  
1.25-1.50 INCHES ALONG THIS BOUNDARY...WHILE TO THE SOUTH OF 15N  
THE PWAT PEAKS AT TWO INCHES. A POLAR FRONT EXITING THE EASTERN  
USA WILL PRESS AGAINST THE LOW LEVEL RIDGE. THE RIDGE IS TO  
INITIALLY HOLD ITS GROUND...BUT LATER ON THURSDAY IT WILL START TO  
YIELD. AS THE HIGH RELOCATES...THE PRESSURE GRADIENT WILL SLOWLY  
SLACKEN DURING THE WEEKEND. BRISK EASTERLY TRADES ARE FORECAST TO  
PEAK AT 18-21KT THROUGH AT LEAST FRIDAY AFTERNOON. LATER DURING  
THE WEEKEND THE BOUNDARY LAYER WINDS WILL DECREASE TO 10-15KT. AS  
THE FRONT WEAKENS AND THE TRADES SLACKEN THE COLD ADVECTIVE  
PATTERN OVER THE WESTERN ATLANTIC IS TO ALSO WANE.  
 
IN THIS PATTERN...MODELS CONTINUE TO AGREE ON SCATTERED SHOWERS TO  
AFFECT THE ENTIRE FORECAST AREA DURING THE NEXT TWO DAYS. UNDER  
INFLUENCE OF THE MEANDERING SURFACE FRONT...MOST ACTIVE IS  
EXPECTED BETWEEN THE VIRGIN ISLES-NORTHEASTERN PUERTO RICO. PEAK  
IN CONVECTION IS EXPECTED TODAY...TO THEN GRADUALLY DECREASE ON  
THURSDAY-FRIDAY AS THE SURFACE FRONT WEAKENS AND A DRYER AIR MASS  
ENTRAINS FROM THE EAST. LATER IN THE WEEK...SHOWERS WILL LIMIT TO  
STREAMERS AFFECTING THE VIRGIN ISLES FROM TIME TO TIME AND LIGHT  
DIURNAL CONVECTION OVER WESTERN PUERTO RICO.  
 
PECK...MS (JAMAICA)  
RIVAS...SENAMHI (PERU)  
DAVISON...WPC (USA)  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page