509  
FXCA20 KWBC 181154  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
653 AM EST THU JAN 18 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 18/06  
UTC: A MEANDERING MID LEVEL HIGH OVER PUERTO RICO-MONA PASSAGE  
ANCHORS A SHORT WAVE RIDGE ACROSS THE NORTHEAST CARIBBEAN ISLES.  
THIS FAVORS AN ELEVATED TRADE WINDS CAP THAT IS STRONG ENOUGH TO  
INHIBIT DEEP CONVECTION...BUT NOT STRONG ENOUGH TO SUPPRESS  
SHALLOW DEVELOPMENT. AT LOW LEVELS...IN A DISORGANIZED MEANDERING  
FLOW NORTH OF THE ISLANDS...LIGHT EASTERLY TRADES PREVAIL ACROSS  
THE FORECAST AREA...WITH BOUNDARY LAYER WINDS OF 05-10KT. EMBEDDED  
IN THIS FLOW...A PLUME OF SHALLOW MOISTURE IS PULLING ACROSS THE  
NORTHERN LEEWARD TO THE VIRGIN ISLES EARLY THIS MORNING. AT ITS  
CURRENT PACE IT SHOULD BE REACHING EASTERN PUERTO RICO AROUND  
15UTC TODAY...INTO WESTERN PORTIONS OF THE ISLAND BY 20-21UTC. THE  
PWAT CONTENT WITH THIS CLUSTER IS EXPECTED TO PEAK BETWEEN  
1.50-1.75 INCHES. IT LOOKS LIKE THE GFS HAS A BETTER HANDLE OF  
THIS PLUME OF MOISTURE THAN THE ECMWF. THE MOIST TONGUE IS TO  
REMAIN ACROSS THE FORECAST AREA THROUGH AT LEAST FRIDAY AFTERNOON.  
THE INFLOW OF LOW/MID LEVEL MOISTURE WILL FEED CONVECTION DURING  
THE DAY TODAY AND ON FRIDAY...WITH MOST INTENSE TO CONTINUE  
CLUSTERING OVER INTERIOR PORTIONS OF PUERTO RICO. THE GFS AND NAM  
ARE DOING BETTER THAN THE OTHER MODELS IN DEPICTING DIURNAL  
DEVELOPMENT ACROSS INTERIOR PORTIONS OF PUERTO RICO. RISK OF  
MODERATE TO HEAVY RAINFALL AMOUNTS WILL PERSIST OVER THE NEXT  
COUPLE OF DAYS.  
 
ALTHOUGH THE MID/UPPER LEVEL PATTERN IS TO REMAIN NEARLY UNCHANGED  
OVER THE NEXT SEVERAL DAYS...AT LOW LEVELS A POLAR FRONT OVER THE  
WESTERN ATLANTIC MOVES ACROSS BERMUDA EARLY THIS AFTERNOON.  
BOUNDARY TAKES A BEELINE ACROSS THE CENTRAL ATLANTIC ON  
FRIDAY...BUT AS IT LOSES ITS UPPER LEVEL SUPPORT IT IS EXPECTED TO  
BOTTOM OUT TO THE NORTH OF 22N EARLY ON SATURDAY MORNING. A POLAR  
RIDGE ROLLS BEHIND THIS AXIS...BUILDING OVER THE WESTERN ATLANTIC  
ON SATURDAY-SUNDAY. THIS IS TO FAVOR BRISK EAST-NORTHEASTERLY  
TRADES ACROSS THE FORECAST AREA...WITH 850 HPA WINDS EXPECTED TO  
PEAK AROUND 20KT. THIS WILL BE STRONG ENOUGH TO FAVOR GENERATION  
OF STREAMERS ACROSS THE NORTHERN LEEWARD/VIRGIN ISLES THAT ARE TO  
AFFECT EASTERN PORTIONS OF PUERTO RICO FROM TIME-TO-TIME...WITH  
MOST ACTIVE DURING THE MORNING HOURS. THE SURGING TRADES WILL BE  
ACCOMPANIED BY A SLIGHTLY STRONGER CAP INVERSION...WITH PWAT  
CONTENT DROPPING TO ABOUT 1.0-1.25 INCHES DURING THE WEEKEND-EARLY  
NEXT WEEK. AS A RESULT DIURNAL ACTIVITY ACROSS INTERIOR PORTIONS  
OF PUERTO RICO IS TO BECOME MORE SPARSE.  
 
DAVISON/GALVEZ...WPC (USA)  
 

 
 
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