868  
FXCA20 KWBC 281140  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
639 AM EST THU DEC 28 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 28/06  
UTC: AT 250 HPA...BROAD UPPER LEVEL RIDGE DOMINATES THE FLOW  
ACROSS THE CARIBBEAN BASIN. AN ANTICYCLONICALLY CURVED SUBTROPICAL  
JET MAXIMA BOUNDS THE NORTHERN FRINGES OF THIS RIDGE WHILE  
EXTENDING OVER THE GREATER ANTILLES. UPPER LEVEL RIDGE IS TO HOLD  
ITS GROUNDS AS POLAR TROUGH PATTERN OVER THE EASTERN USA-WESTERN  
ATLANTIC STRENGTHENS DURING THE NEXT FEW DAYS. THIS IS TO TIGHTEN  
THE GRADIENT FAVORING GRADUAL INTENSIFICATION OF JET PATTERN  
ACROSS THE BAHAMAS-NORTH OF THE GREATER ANTILLES.  
 
AT MID LEVELS...A WEST-TO-EAST RIDGE EXTENDS FROM THE GULF OF  
MEXICO ACROSS THE NORTHERN CARIBBEAN ISLES...WHERE IT SUSTAINS AN  
ELEVATED CAP INVERSION. UNDER PRESSURE FROM DEEPENING TROUGH TO  
THE NORTH...THE RIDGE IS TO GRADUALLY WEAKEN FROM THE WEST...WITH  
AXIS TO COLLAPSE BY MIDDAY ON SATURDAY. ALTHOUGH THE RIDGE  
WEAKENS...THE TRADE WINDS CAP TENDS TO HOLD. HOWEVER...ON SATURDAY  
IT WEAKENS ENOUGH TO ALLOW FOR DEEPER MOISTURE TO POOL ACROSS THE  
FORECAST AREA. EARLY NEXT WEEK THE RIDGE TRIES TO BOUNCE BACK  
WHILE SLOWLY BUILDING FROM THE WEST. BUT IT IS GOING TO HAVE A  
HARD TIME OVERCOMING INFLUENCE OF BROAD POLAR TROUGH PATTERN  
ESTABLISHING OVER THE WESTERN ATLANTIC.  
 
AT LOW LEVELS...WANING TUTT EAST OF THE ISLANDS FAVORS THE  
GENERATION OF PERTURBATIONS IN THE EASTERLY TRADES. ONE IS TO PULL  
ACROSS THE FORECAST AREA DURING THE DAY TODAY...WITH ANOTHER TO  
FOLLOW DURING THE DAY ON SATURDAY. PWAT CONTENT WITH THESE  
PERTURBATIONS IS TO RANGE BETWEEN 1.25 TO 1.50 INCHES...WITH PEAK  
IN PWAT CONTENT EXPECTED DURING THE DAY TODAY. THIS WILL COMBINE  
WITH LIGHTER TRADE WINDS TO SUSTAIN SLOW MOVING SCATTERED  
CONVECTION ACROSS THE FORECAST AREA. MOST INTENSE EXPECTED OVER  
NORTHEAST PUERTO RICO THROUGHOUT THE DAY...AND WESTERN INTERIOR  
PORTIONS OF THE ISLAND IN AFTERNOON CONVECTION. TODAY AND SATURDAY  
EXPECTING 24 HRS RAINFALL AMOUNTS OF 0.75 TO 1.50 INCHES TO AFFECT  
WESTERN PUERTO RICO/GUAJATACA BASIN AND NORTHEAST PORTIONS OF  
PUERTO RICO.  
 
AS THE TUTT TO THE EAST WEAKENS EARLY DURING THE WEEKEND THE  
GENERATION OF EASTERLY WAVES IS TO ABATE...WITH MODELS AGREEING ON  
A DRYER AIR MASS TO ENTRAIN ACROSS THE FORECAST AREA LATER DURING  
THE WEEKEND/EARLY NEXT WEEK. FAIR WEATHER CONDITIONS ARE TO  
GENERALLY PREVAIL ON MONDAY-TUESDAY. ON WEDNESDAY...SURGING POLAR  
FRONT OVER THE WESTERN ATLANTIC WILL INDUCE A PREFRONTAL TROUGH  
ACROSS THE EASTERN CARIBBEAN. THIS WILL DRAW A PLUME OF DEEP  
TROPICAL MOISTURE ACROSS THE ISLAND CHAIN/EASTERN CARIBBEAN THAT  
IS TO CONVERGE OVER THE FORECAST AREA ON WEDNESDAY-THURSDAY.  
GLOBAL MODEL GUIDANCE ARE IN SURPRISINGLY CLOSE AGREEMENT WITH  
THIS FEATURE...SHOWING POTENTIAL FOR MODERATE TO HEAVY RAINFALL  
AMOUNTS.  
 
DAVISON...WPC (USA)  
 

 
 
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