870  
FXCA20 KWBC 191149  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
649 AM EST TUE DEC 19 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 19/06  
UTC: A MID LEVEL HIGH OVER CUBA ANCHORS A RIDGE THAT EXTENDS FROM  
THE GULF OF MEXICO TO THE WESTERN ATLANTIC/CARIBBEAN BASIN. THIS  
RIDGE SUSTAINS A STRONG TRADE WINDS CAP THAT IS INHIBITING DEEP  
CONVECTION. THIS PATTERN IS TO GENERALLY HOLD THROUGH THE END OF  
THE WEEK. AT LOW LEVELS...UNDER INFLUENCE OF A RIDGE OVER THE  
WESTERN ATLANTIC...BRISK NORTHEASTERLY TRADES DOMINATE THE FLOW  
ACROSS THE EASTERN CARIBBEAN. EMBEDDED IN THIS FLOW...PATCHY CLOUD  
CLUSTERS CONTINUE TO ADVECT ACROSS THE FORECAST AREA FROM  
TIME-TO-TIME...FAVORING PERIODS OF SCATTERED SHALLOW CONVECTION.  
HOWEVER...UNDER THE PRESSURE OF A POLAR FRONT EXITING THE EASTERN  
USA ON WEDNESDAY...THE LOW LEVEL RIDGE IS FORECAST TO WEAKEN LATER  
IN THE DAY. AS THE GRADIENT SLACKENS EXPECTING LIGHTER  
EAST-NORTHEASTERLY TRADES TO ENVELOP THE FORECAST AREA THROUGH  
FRIDAY MORNING. AS THE TRADES WEAKENS...INCIDENCE OF TRADE WIND  
SHOWERS SHOULD BECOME MORE SPARSE AND LESS INTENSE. BUT DIURNAL  
CONVECTION ACROSS INTERIOR PORTIONS OF PUERTO RICO IS EXPECTED TO  
BECOME MORE ACTIVE AS ENHANCED BY SEA BREEZE CONVERGENCE.  
OVERALL...EXPECTING LIGHT/LOCALLY MODERATE RAINFALL AMOUNTS DURING  
THE NEXT FOUR-TO-FIVE DAYS...WITH MOST ACTIVE OVER THE VIRGIN  
ISLES OVER THE NEXT DAY OR SO.  
 
LATER DURING THE WEEKEND...THE MID/UPPER LEVEL RIDGE IS TO STEER A  
DIGGING TUTT ACROSS THE CENTRAL ATLANTIC. THE TROUGH REACHES  
MAXIMUM AMPLITUDE ON SUNDAY-MONDAY WHEN IT IS TO EXTEND TO THE  
SOUTHWEST TO THE EASTERN CARIBBEAN. A CLOSED LOW WILL FORM ALONG  
THIS AXIS EARLY NEXT WEEK...TO RETROGRESS ACROSS THE FORECAST AREA  
ON MONDAY-TUESDAY. THE NORTHEAST CARIBBEAN IS TO INITIALLY LIE ON  
THE CONVERGENT SIDE OF THE UPPER LEVEL TROUGH. BUT AS THE TROUGH  
MEANDERS WEST...THE ENVIRONMENTAL CONDITIONS/JET DYNAMICS ARE TO  
BECOME MORE FAVORABLE AND CONDUCIVE FOR DEEP CONVECTION TO DEVELOP  
ON MONDAY AND TUESDAY. AT LOW LEVELS THE TROUGH ALOFT IS TO INDUCE  
AN INVERTED TROUGH IN THE EASTERLY TRADES. THIS WILL ENHANCE  
MOISTURE TRANSPORT/CONVERGENCE ACROSS THE EASTERN CARIBBEAN THAT  
IS TO HELP SUSTAIN WIDESPREAD CONVECTION ACROSS THE ENTIRE  
FORECAST AREA. EARLY NEXT WEEK THE CFS MODEL IS SHOWING A BROAD  
AREA OF NEGATIVE VELOCITY POTENTIAL ANOMALIES ENVELOPING CENTRAL  
AMERICA AND THE WESTERN CARIBBEAN...WITH WEAKER ANOMALIES AND  
SOMEWHAT FAVORABLE MJO CONDITIONS OVER THE NORTHEAST CARIBBEAN  
ISLES. THIS MIGHT BE ENOUGH TO TILT THE SCALE IN FAVOR OF HIGHER  
RAINFALL AMOUNTS THAN WHAT THE MODELS CURRENTLY SUGGEST.  
 
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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