719  
FXCA20 KWBC 291128  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
627 AM EST MON JAN 29 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 28/06  
UTC: LITTLE HAS CHANGED OVER THE LAST 24 HRS...WITH BROAD  
MID/UPPER LEVEL RIDGE DOMINATING THE FLOW OVER THE WESTERN-CENTRAL  
ATLANTIC/EASTERN CARIBBEAN. THE RIDGE SUSTAINS AN ELEVATED TRADE  
WINDS CAP THAT CONTINUES TO INHIBIT DEEP CONVECTION.  
IT...HOWEVER...IS NOT STRONG ENOUGH TO SUPPRESS SHALLOW  
DEVELOPMENT. AT LOW LEVELS...A CLOSED HIGH TO THE NORTHEAST OF  
BERMUDA ANCHORS A MASSIVE POLAR RIDGE OVER THE WESTERN ATLANTIC...  
WITH AXIS ENVELOPING AREA TO THE WEST OF 50W. THIS RIDGE FAVORS A  
TIGHT PRESSURE GRADIENT AND STRONG BOUNDARY LAYER WINDS ACROSS THE  
ISLAND CHAIN TO THE EASTERN CARIBBEAN. THE STRONG WINDS FAVOR  
GENERATION OF TRADE WIND SHOWERS THAT CONTINUE TO PESTER THE USVI  
AND PUERTO RICO FROM TIME-TO-TIME. THESE SUSTAIN LIGHT RAINFALL  
AMOUNTS IN PASSING SHOWERS. THE STRONG WINDS ALSO FAVOR GENERATION  
OF STREAMERS ACROSS THE LEEWARD/VIRGIN ISLES DURING THE DAYTIME  
HOURS...WITH CELLS CONVERGING ACROSS EASTERN PORTIONS OF PUERTO  
RICO/EL YUNQUE RAINFOREST.  
 
CONSISTENT WITH PREVIOUS FORECAST...BROAD RIDGE OVER THE WESTERN  
ATLANTIC IS TO YIELD TO POLAR TROUGH EXITING THE EASTERN USA.  
UNDER PRESSURE...THE RIDGE WILL WEAKEN TO THE NORTH OF 20N...WHILE  
ITS REMNANTS ESTABLISH A FOOTHOLD ACROSS THE CARIBBEAN BASIN. AS A  
RESULT...TRADE WINDS CAP IS FORECAST TO STRENGTHEN THROUGH  
WEDNESDAY MORNING...WITH INVERSION HEIGHT FORECAST TO DROP TO  
AROUND 750-800 HPA. THE GFS AND ECMWF ARE SLOWLY REACHING AN  
AGREEMENT ON THIS...NOW FORECASTING THE STRONG INVERSION TO HOLD  
THROUGH THE END OF THE WEEK. THIS WILL FAVOR EROSION OF AVAILABLE  
MOISTURE WITH A PWAT MINIMA EXPECTED ON WEDNESDAY TO FRIDAY  
MORNING. AT LOW LEVELS...POLAR FRONT EXITS THE EASTERN USA EARLY  
IN THE CYCLE. THIS IS TO THEN PRESS AGAINST RIDGE PATTERN OVER THE  
WESTERN NORTH ATLANTIC...WITH RIDGE FORECAST TO GRADUALLY CRUMBLE  
ON TUESDAY. AS THE PRESSURE GRADIENT SLACKENS THE STRONG EASTERLY  
TRADES ENVELOPING THE EASTERN CARIBBEAN ISLES ARE TO GRADUALLY  
SUBSIDE...WITH THE MODELS FORECASTING BOUNDARY LAYER WINDS OF  
05-10KT EARLY ON WEDNESDAY MORNING. SIMULTANEOUSLY...POLAR RIDGE  
BUILDS ACROSS THE EASTERN USA TO THE WESTERN ATLANTIC...DRIVING  
THE SURFACE FRONT EAST AND SOUTH. THE FRONTAL BOUNDARY IS TO THEN  
BOTTOM OUT AROUND 23N/24N ON THURSDAY MORNING. UNDER INFLUENCE OF  
THIS FEATURE THE 850 HPA WINDS ARE TO BACK TO THE NORTH-NORTHEAST  
ON THURSDAY MORNING...WITH WINDS OF 20-25KT. BOUNDARY LAYER WINDS  
ARE FORECAST TO PEAK AT 15-20KT. THIS IS TO FAVOR A COOL ADVECTIVE  
PATTERN AND GENERATION OF TRADE WINDS SHOWERS. HOWEVER...AS A  
STRONGER CAP INVERSION HOLDS ACROSS THE FORECAST AREA...THESE WILL  
NOT BE AS INTENSE AS THE ONES OF LATE.  
 
STRONG EASTERLY TRADES DURING THE NEXT 24-36 HRS ARE TO CONTINUE  
FAVORING GENERATION OF TRADE WIND SHOWERS ACROSS THE FORECAST  
AREA. THIS WILL COMBINE WITH STREAMERS FORMING OVER THE VIRGIN  
ISLES DURING THE DAY TO FAVOR LIGHT TO MODERATE RAINFALL AMOUNTS  
ACROSS EASTERN PUERTO RICO/EL YUNQUE. OVER CENTRAL-WESTERN PUERTO  
RICO THIS IS TO ONLY FAVOR LIGHT RAINFALL AMOUNTS IN PASSING  
SHOWERS. AS THE WINDS START TO WEAKEN ON TUESDAY...THE INCIDENCE  
OF TRADE WIND SHOWERS WILL DECREASE... LEADING TO LIGHTER AMOUNTS  
DURING THE DAY. ON WEDNESDAY...AS A STRONG TRADE WINDS CAP SETTLES  
ACROSS THE FORECAST AREA...EXPECTING FAIR WEATHER CONDITIONS.  
RAINFALL...IF ANY...WILL LIMIT TO THE CORDILLERA/EL YUNQUE IN  
MID/LATE AFTERNOON CONVECTION. LATER IN THE WEEK SCATTERED SHOWERS  
ARE TO ONCE AGAIN AFFECT THE FORECAST AREA AS THE TRADES  
STRENGTHENS AND A COOL ADVECTIVE PATTERN ESTABLISHES.  
 
DAVISON/GALVEZ...WPC (USA)  
 

 
 
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