290  
FXCA20 KWBC 181148  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
648 AM EST MON DEC 18 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 18/06  
UTC: THE FORECAST AREA REMAINS ON THE CONVERGENT SIDE OF A  
MID/UPPER LEVEL RIDGE ENVELOPING THE CARIBBEAN BASIN-WESTERN  
ATLANTIC. THIS FAVORS A CONVERGENT/SUBSIDENT PATTERN THAT IS  
INHIBITING ORGANIZED DEEP CONVECTION ACROSS THE NORTHEAST  
CARIBBEAN ISLES. THIS PATTERN IS TO GENERALLY HOLD DURING THE NEXT  
FEW DAYS.  
 
AT LOW LEVELS...BROAD RIDGE OVER THE WESTERN ATLANTIC FAVORS BRISK  
NORTHEASTERLY WINDS...WITH CONFLUENCE OF THE FLOW ALONG A SHEAR  
LINE THAT MEANDERS OVER THE NORTHERN LEEWARD-VIRGIN ISLES/EASTERN  
PUERTO RICO. THIS FAVORS GENERATION OF TRADE WIND SHOWERS THAT  
AFFECT THE FORECAST AREA FROM TIME-TO-TIME. THIS TENDS TO PERSIST  
DURING THE NEXT 48-72 HRS...WITH LIGHTER WINDS BY MIDWEEK TO  
RESULT IN DECREASE IN INCIDENCE OF TRADE WIND SHOWERS.  
OTHERWISE...EXPECTING LITTLE TO NO CHANGE IN CURRENT WEATHER  
CONDITIONS.  
 
THIS PATTERN WILL RESULT IN LIGHT TO MODERATE RAINFALL AMOUNTS  
ACROSS THE ENTIRE FORECAST AREA DURING THE NEXT FEW DAYS...WITH  
MOST ACTIVE EXPECTED ALONG THE SHEAR LINE OVER THE VIRGIN  
ISLES-NORTHEAST PUERTO RICO. THE NORTHEASTERLY TRADES ARE TO ALSO  
FAVOR A SECONDARY MAXIMA OVER SOUTHWEST PUERTO RICO.  
 
LONG RANGE FORECAST...AS THE RIDGE MEANDERS TO THE NORTH AND  
WEST...IT IS TO STEER A DEEP TUTT ACROSS THE CENTRAL ATLANTIC TO  
THE NORTHEAST CARIBBEAN LATER DURING THE WEEKEND. OVER THE LAST  
FEW DAYS THE GFS AND ECMWF HAVE COME TO A CONSENSUS ON HOW THIS  
FEATURE IS GOING TO EVOLVE...WITH UPPER LEVEL TROUGH ENTERING THE  
NORTHEAST CARIBBEAN ON SATURDAY-SUNDAY. THEY NOW SHOW BEST  
DYNAMICAL FORCING WITH THIS FEATURE ACROSS THE LEEWARD/FRENCH  
ISLES...AND POSSIBLY AS FAR WEST AS THE VIRGIN ISLES. THE TUTT IS  
TO THEN FAVOR THE GRADUAL EROSION OF THE TRADE WINDS CAP FROM THE  
EAST EARLY NEXT WEEK...WITH MODELS SHOWING PLUME OF DEEPER  
MOISTURE ACROSS THE NORTHEAST CARIBBEAN. AT LOW LEVELS...THE  
TROUGH ALOFT IS TO ALSO INDUCE AN INVERTED TROUGH IN THE EASTERLY  
TRADES THAT IS TO ENHANCE LOW LEVEL CONVERGENCE. IN RESPONSE TO  
EVOLVING PATTERN...THE MODELS AGREE ON SCATTERED CONVECTION  
BUILDING BETWEEN EASTERN PUERTO RICO AND THE LEEWARD ISLES LATER  
DURING THE WEEKEND/EARLY NEXT WEEK...WITH POTENTIAL FOR MODERATE  
RAINFALL AMOUNTS AS ENHANCED BY DIURNAL HEATING.  
 
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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