312  
FXCA20 KWBC 111132  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
631 AM EST THU JAN 11 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 11/06  
UTC: A 250 HPA HIGH OVER ISLA DE MARGARITA ANCHORS A CELL OF THE  
SUBEQUATORIAL RIDGE OVER THE CARIBBEAN TO THE WESTERN ATLANTIC. AT  
500 HPA...A HIGH TO THE NORTH OF THE LEEWARD/ VIRGIN ISLES EXTENDS  
A RIDGE TO THE SOUTH-SOUTHEAST INTO THE EASTERN CARIBBEAN...WHERE  
IT FAVORS AN ELEVATED CAP INVERSION. AT LOW LEVELS...BROAD RIDGE  
DOMINATES THE FLOW OVER THE ATLANTIC...SUSTAINING A TIGHT GRADIENT  
AND BRISK EAST-SOUTHEAST FLOW ACROSS THE ISLAND CHAIN INTO THE  
CARIBBEAN BASIN. THE BRISK EASTERLY FLOW FAVORS GENERATION OF  
STREAMERS ACROSS THE VIRGIN ISLES-EASTERN PUERTO RICO...  
TRIGGERING PASSING SHOWERS/LIGHT RAINFALL AMOUNTS. THIS IS ALSO  
ADVECTING A PLUME OF DEEPER TROPICAL MOISTURE ACROSS THE NORTHEAST  
CARIBBEAN ISLES THAT IS EXPECTED TO FEED CONVECTIVE DEVELOPMENT  
DURING THE DAY TODAY. MOST ACTIVE CLUSTERING OVER EASTERN PUERTO  
RICO DURING THE MORNING/EARLY AFTERNOON. A DRYER AIR MASS IS TO  
THEN ENTRAIN FROM THE EAST LATER THIS AFTERNOON...TO SUSTAIN A  
LULL IN ACTIVITY ON FRIDAY.  
 
MID LEVEL RIDGE TO THE NORTH IS TO THEN STEER A TUTT TO THE  
SOUTH-SOUTHWEST...WITH THE 250 HPA TROUGH MOVING OVER THE  
NORTHEAST CARIBBEAN EARLY ON SATURDAY MORNING. A JET MAXIMA  
ROUNDING THE BASE OF THE TROUGH IS TO BRIEFLY FAVOR AN UPPER  
DIVERGENT PATTERN ON SATURDAY MORNING/EARLY AFTERNOON. BUT AS THE  
JET MAXIMA PULLS AWAY AND THE TROUGH SETTLES TO THE EAST...AN  
UPPER CONVERGENT PATTERN QUICKLY RETURNS LATER ON SATURDAY  
EVENING. MODELS THEN AGREE ON RAPID EROSION OF AVAILABLE MOISTURE.  
AT LOW LEVELS...BROAD RIDGE ENVELOPING THE ATLANTIC IS TO PERSIST  
THROUGH SUNDAY MORNING. EMBEDDED IN THIS FLOW...MODELS AGREE ON A  
SECONDARY PLUME OF DEEPER TROPICAL MOISTURE TO ONCE AGAIN MOVE  
ACROSS THE ISLAND CHAIN TO THE NORTHEAST CARIBBEAN LATER ON FRIDAY  
EVENING...WITH PWAT CONTENT OF 40-45MM EXPECTED THROUGH SATURDAY  
EVENING. THIS IS TO FEED ANOTHER ROUND OF CONVECTION ACROSS THE  
FORECAST AREA DURING THE DAY ON SATURDAY. IN WEAK UPPER  
SUPPORT...MOST INTENSE IS TO CLUSTER ACROSS INTERIOR PORTIONS OF  
PUERTO RICO...WITH LOCALIZED MAXIMA OF ONE-TO-TWO INCHES POSSIBLE.  
DURING THAT PERIOD THE ECMWF GDI FORECAST IS SHOWING RISK OF  
THUNDERSTORMS FORMING OVER WESTERN PORTIONS OF THE ISLAND...WHILE  
THE GFS SHOWS PREDOMINANTLY RAIN SHOWERS. BUT WITH WEAK FORCING  
ALOFT WE CANNOT RULE OUT THE POSSIBILITY OF A FEW TSTMS.  
 
A DRYER AIR MASS IS TO THEN MOVE FROM THE EAST...WITH PWAT MINIMA  
TO PERSIST THROUGH EARLY NEXT WEEK. THIS WILL COINCIDE WITH  
WEAKENING OF LOW LEVEL RIDGE PATTERN...WITH EASTERLY TRADES  
DECREASING TO 10-15KT BY MIDMORNING ON SUNDAY. AS THE WINDS WEAKEN  
THE STREAMERS AFFECTING THE NORTHEAST CARIBBEAN ISLES WILL CEASE.  
IN THE ABSENCE OF MESO-SYNOPTIC FORCING...DIURNAL CONVECTIVE  
PATTERN WILL THEN RULE OUT ACROSS SAINT CROIX AND PUERTO RICO. BUT  
THESE ARE TO GENERALLY FAVOR LIGHT RAINFALL AMOUNTS IN SHORT  
LASTING SHALLOW CONVECTION.  
 
DAVISON...WPC (USA)  
 

 
 
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