854  
FXCA20 KWBC 261148  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
648 AM EST TUE DEC 26 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM 26/06  
UTC: AT UPPER LEVELS...A LOW AMPLITUDE RIDGE EXTENDS OVER THE  
CARIBBEAN BASIN. AN ANTICYCLONICALLY CURVED SUBTROPICAL JET BOUNDS  
THE NORTHERN FRINGES OF THIS RIDGE...EXTENDING OVER  
HISPANIOLA-PUERTO RICO-NORTHERN LEEWARD ISLES...WHERE IT FAVORS AN  
UPPER CONVERGENT PATTERN. THIS PATTERN IS EXPECTED TO HOLD THROUGH  
THE MEDIUM RANGE PERIOD.  
 
AT MID LEVELS...A RIDGE EXTENDS EAST FROM THE GULF OF MEXICO TO  
JUST NORTH OF THE GREATER ANTILLES. REMNANTS OF A TUTT LIE TO THE  
SOUTH...EXTENDING WEST ACROSS THE FRENCH/WINDWARD ISLES TO THE  
EASTERN CARIBBEAN. UNDER INFLUENCE OF THE RIDGE TO THE NORTH THE  
TUTT IS FORECAST TO STRETCH INTO THE CENTRAL CARIBBEAN AS THE WEEK  
WEARS ON. THIS PATTERN PERSISTS THROUGH THE END OF THE WEEK...THEN  
QUICKLY UNRAVELS LATER DURING THE WEEKEND UNDER PRESSURE FROM  
AMPLIFYING TROUGH OVER THE WESTERN ATLANTIC.  
 
MEANWHILE...THE MEANDERING MID LEVEL TUTT IS TO CONTINUE FAVORING  
GENERATION OF EASTERLY WAVES IN THE LOW/MID LEVEL EASTERLY TRADES.  
THE GLOBAL MODELS ARE IN SURPRISINGLY CLOSE AGREEMENT ON THESE  
PERTURBATIONS...WITH THE FIRST ONE TO STREAM ACROSS THE FORECAST  
AREA DURING THE DAY TODAY. A STRONGER ONE FOLLOWS...PULLING ACROSS  
THE VIRGIN ISLES-PUERTO RICO ON WEDNESDAY-THURSDAY...WHILE A THIRD  
IS FORECAST ON SATURDAY. THESE WILL SUSTAIN A LOW LEVEL CONVERGENT  
PATTERN THAT WILL ALLOW PWAT CONTENT TO INCREASE ACROSS THE  
FORECAST AREA. HOWEVER...PERSISTENT UPPER LEVEL INVERSION WILL CAP  
MOISTURE TO ATMOSPHERE BELOW 600 HPA...WITH MODERATE CU TO  
POPULATE THE FORECAST AREA.  
 
THESE PERTURBATIONS...AS THEY STREAM ACROSS THE EASTERN  
CARIBBEAN...ARE TO FAVOR SCATTERED CONVECTION ACROSS THE ENTIRE  
FORECAST AREA. THE UKMET AND ECMWF GENERALLY AGREEING ON MOSTLY  
LIGHT RAINFALL AMOUNTS IN SHALLOW CONVECTION. GFS APPEARS SOMEWHAT  
MORE BULLISH THAN THESE OTHER MODELS...IN PARTICULAR WITH THE  
SECOND WAVE ON WEDNESDAY-THURSDAY. PWAT CONTENT WITH THIS WAVE IS  
TO PEAK AROUND 1.50 INCHES...ENOUGH TO SUSTAIN MODERATE RAINFALL  
AMOUNTS. CONSIDERING THE INFLUENCE OF TERRAIN AND DIURNAL HEATING  
ON CONVECTIVE DEVELOPMENT...WE ARE SIDING WITH THE WETTER GFS  
SOLUTION. ALSO AT LOW LEVELS...BRISK EAST-NORTHEASTERLY TRADES  
WILL FAVOR GENERATION OF STREAMERS ACROSS THE VIRGIN ISLES THAT  
ARE TO AFFECT PUERTO RICO FROM TIME-TO-TIME. BUT THESE WILL  
GENERALLY FAVOR LIGHT RAINFALL AMOUNTS IN PASSING RAIN SHOWERS.  
 
GORDON...NMS (BELIZE)  
PECK...MS (JAMAICA)  
DAVISON...WPC (USA)  
 

 
 
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