975  
FXCA20 KWBC 011149  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
649 AM EST FRI DEC 01 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 01/06  
UTC: A MID LEVEL SHORT WAVE TROUGH NORTH OF THE ISLANDS IS  
PROVIDING SUPPORT TO A DEEPENING LOW NEAR 26N 67W EARLY THIS  
MORNING. A FRONT EXTENDS FROM THIS LOW ACROSS THE CENTRAL BAHAMAS  
TO WESTERN CUBA. FURTHERMORE...THE DEEP LOW TO THE NORTH IS  
PHASING WITH AN EASTERLY WAVE OVER THE CARIBBEAN TO SUSTAIN AN  
INDUCED/INVERTED TROUGH WEST OF THE LESSER ANTILLES. IN RESPONSE  
TO THIS FEATURE...THE LOW LEVEL WINDS VEERED TO THE  
SOUTHEAST...WITH PLUME OF DEEP TROPICAL MOISTURE LIFTING ACROSS  
THE WINDWARD ISLES TO THE EASTERN CARIBBEAN.  
 
MID/UPPER LEVEL RIDGE OVER MEXICO AND THE GULF IS TO STEER SHORT  
WAVE PERTURBATIONS ON THE NORTHERN AND SOUTHERN STREAM FLOW ACROSS  
THE EASTERN USA TO THE WESTERN ATLANTIC. THESE ARE TO COMBINE IN  
SUPPORT OF A BROAD TROUGH THAT BOTTOMS OUT ACROSS THE GREATER  
ANTILLES DURING THE WEEKEND. THE DIGGING TROUGH CONTINUES TO PRESS  
AGAINST WANING CELL OF THE SUBEQUATORIAL RIDGE OVER THE EASTERN  
CARIBBEAN. UNDER PRESSURE...THE MODELS CONTINUE TO FORECAST  
GRADUAL EROSION OF THIS FEATURE DURING THE NEXT FEW DAYS...WITH  
TRADE WINDS CAP LIFTING ABOVE 550 HPA EARLY NEXT WEEK. THIS WILL  
ALLOW FOR DEEPER MOISTURE TO POOL ACROSS THE NORTHEAST CARIBBEAN  
ISLES DURING THE WEEKEND/EARLY NEXT WEEK...WITH PWAT CONTENT  
FORECAST TO PEAK AT 1.50-1.75 INCHES LATER ON SUNDAY/MONDAY  
MORNING. AT 250 HPA...A SUBTROPICAL JET MAXIMA IS TO ROUND THE  
BASE OF THE DEEPENING POLAR TROUGH. THE JET TENDS TO STRENGTHEN  
EARLY NEXT WEEK...WITH GFS FAVORING A SLIGHTLY FASTER RATE OF  
INTENSIFICATION THAN WHAT THE ECMWF SUGGESTS. THE JET ALOFT...AS  
IT MEANDERS NORTH OF THE FORECAST AREA...IS TO FAVOR AN UPPER  
DIVERGENT PATTERN THAT IS TO VENT CONVECTION.  
 
AT LOW LEVELS...THE FRONTAL LOW IS TO DEEPEN AS IT MEANDERS TO THE  
EAST-NORTHEAST EARLY IN THE CYCLE...WHILE SIMULTANEOUSLY A RIDGE  
BUILDS TO THE WEST. IN RESPONSE TO THE EVOLVING PATTERN A  
PREFRONTAL SHEAR LINE FORMS ACROSS EASTERN PUERTO RICO/MONA  
PASSAGE LATER THIS AFTERNOON...TO MEANDER EAST ACROSS PUERTO RICO  
TO THE VIRGIN ISLES BY DAYBREAK ON SATURDAY. THE SHEAR LINE MOVES  
TO THE NORTHERN LEEWARD ISLES LATER ON SATURDAY...WHILE A SAGGING  
SURFACE FRONT MOVES JUST NORTH OF THE FORECAST AREA EARLY ON  
SUNDAY MORNING. UNDER INFLUENCE OF THE SHEAR LINE/SURFACE FRONT  
AND POLAR RIDGE TO THE WEST THE LOW LEVEL WINDS BACK TO THE  
EAST-NORTHEAST DURING THE WEEKEND...WITH BOUNDARY LAYER WINDS OF  
10-15KT. ON MONDAY-TUESDAY...AS THE GRADIENT TIGHTENS...WINDS WILL  
INCREASE TO 15-20KT. THIS WILL THEN FAVOR A COOL ADVECTIVE PATTERN  
ACROSS THE NORTHEAST CARIBBEAN ISLES. THE COOL ADVECTION OVER THE  
WARM ATLANTIC WATERS TO THE NORTH WILL LIKELY FAVOR GENERATION OF  
SCATTERED CONVECTION. THESE ARE LIKELY TO CLUSTER ALONG AND SOUTH  
OF THE SURFACE FRONT.  
 
IN THIS CYCLE THE MODELS GENERALLY AGREE ON LIGHT RAINFALL AMOUNTS  
TO AFFECT THE FORECAST AREA DURING THE NEXT COUPLE OF DAYS...WITH  
SHEAR LINE CONVERGENCE LIMITING TO NORTHEAST PUERTO RICO/NORTHERN  
VIRGIN ISLES EARLY ON SATURDAY. ON MONDAY-TUESDAY...IN A COOL  
ADVECTIVE PATTERN...THE GFS AND THE EUROPEAN MODELS NOW AGREE ON A  
HIGHER RISK OF ORGANIZED CONVECTION. DURING THAT PERIOD THEY ARE  
SHOWING MOST ACTIVE CLUSTERING BETWEEN THE NORTHERN VI AND THE SAN  
JUAN METRO AREA.  
 
DAVISON...WPC (USA)  
 

 
 
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