694  
FXCA20 KWBC 021130  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
630 AM EST TUE JAN 02 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 02/06  
UTC: BROAD POLAR TROUGH DOMINATES THE FLOW ACROSS NORTH  
AMERICA-THE GULF OF MEXICO...WHILE FARTHER EAST A MID LEVEL HIGH  
OVER THE CENTRAL ATLANTIC EXTENDS A RIDGE TO THE SOUTHWEST ACROSS  
THE ISLAND CHAIN TO THE EASTERN CARIBBEAN. THIS FAVORS A TRADE  
WINDS CAP ACROSS THE EASTERN CARIBBEAN ISLES...WHERE IT IS  
INHIBITING ORGANIZED DEEP CONVECTION. AT LOW LEVELS...BROAD POLAR  
TROUGH DOMINATES THE EASTERN USA/WESTERN ATLANTIC...WITH POLAR  
FRONT EXTENDING ACROSS THE NORTHWEST BAHAMAS-STRAITS OF FLORIDA TO  
SOUTHERN MEXICO...WHILE FARTHER EAST LIGHT EASTERLY TRADES PREVAIL  
ACROSS THE NORTHEAST CARIBBEAN.  
 
BROAD POLAR TROUGH IS TO PERSIST OVER THE EASTERN USA-WESTERN  
ATLANTIC THROUGH THE END OF THE WEEK. AS IT HOLDS...SHORT WAVE  
PERTURBATIONS ARE TO ROUND THE BASE OF THE LONG WAVE  
AXIS...LIFTING ACROSS CUBA-THE BAHAMAS WHILE PRESSING AGAINST THE  
MEANDERING RIDGE OVER THE EASTERN CARIBBEAN/CENTRAL ATLANTIC.  
THROUGH THE END OF THE WEEK THE RIDGE WILL WEAKEN ENOUGH AND THE  
CAP WILL LIFT TO ALLOW FOR AN INCREASE IN PWAT CONTENT LATER ON  
THURSDAY THROUGH SATURDAY. BEST MOISTURE  
CONVERGENCE...HOWEVER...IS EXPECTED ALONG THE SURFACE FRONT AS IT  
MEANDERS ACROSS THE BAHAMAS-CUBA TO HISPANIOLA ON  
THURSDAY-SATURDAY. AS IT APPROACHES THE ISLANDS... THE LOW LEVEL  
WINDS ARE TO BRIEFLY VEER TO THE SOUTHEAST ON WEDNESDAY AFTERNOON.  
THIS WILL ENHANCE MOISTURE TRANSPORT ACROSS THE FORECAST AREA. BUT  
LATER ON THURSDAY THE PREVAILING EASTERLY WINDS RETURN...WITH A  
DRYER AIR MASS TO ENTRAIN FROM THE EAST LATER DURING THE WEEKEND.  
 
AS A PLUME OF TROPICAL MOISTURE ADVECTS ACROSS THE FORECAST AREA  
ON THURSDAY THIS IS TO FAVOR AN INCREASE IN DIURNAL CONVECTION  
ACROSS PUERTO RICO. BUT LACKING UPPER LEVEL SUPPORT ONLY EXPECTING  
LIGHT TO MODERATE RAINFALL AMOUNTS. MODELS THEN DISAGREE ON HOW  
PATTERN IS GOING TO UNFOLD. ON FRIDAY-SATURDAY THE EUROPEAN MODELS  
AGREE ON ORGANIZED DEEP CONVECTION TO BUILD ALONG THE SURFACE  
FRONT OVER HISPANIOLA. DUE TO THE STRONG UPWARD VERTICAL MOTION TO  
THE WEST...THE ECMWF-UKMET ONLY SHOW LIGHT CONVECTION ACROSS  
PUERTO RICO AND THE VIRGIN ISLES. THE  
GFS...IN-CONTRAST...DOWNPLAYS THE POTENTIAL FOR ORGANIZED  
CONVECTION TO THE WEST... WHICH IN-TURN ALLOWS FOR DEEPER  
CONVECTION TO DEVELOP ACROSS PUERTO RICO AND THE VIRGIN ISLES. THE  
BULLISH GFS SOLUTION IS NOT SUPPORTED BY THE GLOBAL ENSEMBLES. IN  
A HIGHLY AMPLIFIED LONG WAVE PATTERN WITH FAVORABLE UPPER LEVEL  
JET SUPPORT...WE ARE MORE INCLINED TO BELIEVE THE EUROPEAN  
SOLUTION OVER THE GFS GUIDANCE.  
 
DAVISON...WPC (USA)  
 

 
 
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