726  
FXCA20 KWBC 201143  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
642 AM EST WED DEC 20 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 20/06  
UTC: A MID LEVEL HIGH OVER WESTERN CUBA/NORTHWEST CARIBBEAN  
EXTENDS A RIDGE ACROSS THE BASIN-EASTERN USA TO THE WESTERN  
ATLANTIC. THIS SUSTAINS A STRONG TRADE WINDS CAP OVER THE  
NORTHEAST CARIBBEAN ISLES...WITH SHALLOW MOISTURE CONFINING TO  
LOWER LEVELS OF THE ATMOSPHERE. AT LOW LEVELS...THE UPPER LEVEL  
FEATURE MANIFESTS AS A CLOSED HIGH OVER THE NORTHWEST  
BAHAMAS...ANCHORING A RIDGE OVER THE WESTERN ATLANTIC/CARIBBEAN  
BASIN. OVER PUERTO RICO AND THE USVI THIS FAVORS A FRESH  
NORTHEASTERLY FLOW...WITH THE OCCASIONAL TRADE WIND SHOWER  
STREAMING ACROSS THE FORECAST AREA FROM TIME-TO-TIME. BUT FOR THE  
MOST PART THESE ARE TO ONLY TRIGGER LIGHT/TRACE RAINFALL AMOUNTS  
IN BRIEF RAIN SHOWERS.  
 
MID/UPPER LEVEL RIDGE PATTERN IS TO REMAIN NEARLY UNCHANGED DURING  
THE NEXT 72-84 HRS. AS IT HOLDS...STRONG TRADE WINDS CAP IS  
EXPECTED TO GENERALLY HOLD. THIS WILL CONTINUE INHIBITING  
ORGANIZED DEEP CONVECTION ACROSS THE FORECAST AREA THROUGH THE END  
OF THE WEEK. AT LOW LEVELS...THE RIDGE NORTH OF THE ISLANDS IS TO  
COLLAPSE LATER TODAY UNDER PRESSURE FROM POLAR FRONT EXITING THE  
EASTERN USA. AS THE RIDGE WEAKENS AND GRADIENT SLACKENS LIGHTER  
WINDS ARE TO ENVELOP THE NORTHEAST CARIBBEAN WHILE VEERING TO THE  
EAST. THE WEAKER WINDS WILL LEAD TO DECREASE IN TRADE WIND SHOWERS  
AFFECTING THE FORECAST AREA. ALTHOUGH THE FRONT WILL BE STRONG  
ENOUGH TO WEAKEN THE RIDGE...IT IS EXPECTED TO BOTTOM OUT FAR TO  
THE NORTH. ON ITS WAKE...A SECONDARY RIDGE WILL GRADUALLY ROLL  
FROM THE WEST...WITH A FRESH EAST-NORTHEAST BREEZE EXPECTED LATER  
IN THE WEEK.  
 
AS THE TRADES WEAKEN EARLY IN THE CYCLE...THIS WILL THEN ALLOW FOR  
DIURNAL/SEA BREEZE RELATED CONVECTION TO DEVELOP ACROSS PUERTO  
RICO AND PROBABLY SAINT CROIX...WHILE OVER THE NORTHERN VI FAIR  
WEATHER CONDITIONS ARE TO GENERALLY PREVAIL. OVER PUERTO RICO MOST  
ACTIVE IS EXPECTED ON WESTERN INTERIOR PORTIONS OF THE ISLAND IN  
AFTERNOON CONVECTION...BUT LACKING UPPER LEVEL SUPPORT...THIS IS  
TO ONLY TRIGGER LIGHT/LOCALLY MODERATE RAINFALL AMOUNTS.  
 
DURING THE WEEKEND...THE MODELS CONTINUE TO FORECAST A TUTT TO  
AMPLIFY TO THE SOUTH-SOUTHWEST INTO THE NORTHEAST CARIBBEAN.  
HOWEVER...IN A CHANGE FROM PREVIOUS FORECASTS...THE GFS IS NOW  
FORECASTING THE TUTT TO BOTTOM OUT OVER THE LEEWARD ISLES...WITH  
PUERTO RICO AND THE USVI TO REMAIN ON THE CONVERGENT/SUBSIDENT  
SIDE OF THE UPPER LEVEL CIRCULATION THROUGH EARLY NEXT WEEK. THE  
ECMWF AND UKMET INITIALLY AGREE WITH THIS EVOLUTION...BUT ON  
TUESDAY THEY FORECAST THE UPPER LEVEL TROUGH TO RETROGRESS ACROSS  
THE NORTHEAST CARIBBEAN/PUERTO RICO FORECAST AREA...WEAKENING THE  
TRADE WINDS CAP WHILE ALLOWING FOR DEEPER MOISTURE TO POOL ACROSS  
THE LEEWARD-VIRGIN ISLES-EASTERN PUERTO RICO. THIS...HOWEVER...IS  
AT A SLOWER PACE THAN WHAT THESE MODELS PREVIOUSLY SUGGESTED. SO  
CONDITIONS DURING THE CHRISTMAS BREAK MIGHT NOT BE AS OMINOUS AS  
WHAT THE MODELS PREVIOUSLY SUGGESTED.  
 
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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