720  
FXCA20 KWBC 141152  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
652 AM EST TUE NOV 14 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM NOV 14/06  
UTC: FORECAST AREA REMAINS ON THE CONVERGENT SIDE OF MID/UPPER  
LEVEL RIDGE ENVELOPING THE EASTERN CARIBBEAN. DIGGING POLAR TROUGH  
TO THE WEST IS TO PRESS AGAINST THIS AXIS...WITH MODELS AGREEING  
ON GRADUAL EROSION OF TRADE WINDS CAP BY MIDMORNING ON WEDNESDAY.  
ALTHOUGH THE TROUGH IS TO CONFINE TO THE WESTERN CARIBBEAN...SHORT  
WAVE VORTICES REVOLVING AROUND THE LONG WAVE AXIS ARE TO LIFT FROM  
TIME-TO-TIME ACROSS THE DOMINICAN REPUBLIC/MONA PASSAGE WHILE ALSO  
GRAZING NORTHWEST PORTIONS OF PUERTO RICO. THIS HAS THE POTENTIAL  
OF ENHANCING DIURNAL CONVECTION ACROSS THE ISLAND.  
 
AT LOW LEVELS...THE POLAR TROUGH WILL SUSTAIN A SURFACE FRONT THAT  
IS EXPECTED TO BOTTOM OUT FAR TO THE NORTH. BUT AS IT MEANDERS TO  
THE NORTH...THE UPPER LEVEL TROUGH IS TO SUSTAIN A LOW LEVEL  
TROUGH ACROSS WESTERN CUBA TO THE CENTRAL CARIBBEAN-SOUTHERN  
CENTRAL AMERICA. THE LOW/MID LEVEL WINDS WILL THEN RESPOND TO  
EVOLVING PATTERN TO THE WEST...WITH THE WINDS TO VEER TO THE  
SOUTH-SOUTHEAST LATER THIS AFTERNOON...AND TO THE SOUTH-SOUTHWEST  
DURING THE DAY ON WEDNESDAY. THIS WILL COINCIDE WITH WEAKENING OF  
THE TRADE WINDS CAP TO ALLOW FOR DEEPER MOISTURE TO POOL ACROSS  
THE FORECAST AREA. THE GFS AND ECMWF BOTH AGREE ON PWAT CONTENT OF  
AT LEAST TWO INCHES BY MIDMORNING ON WEDNESDAY. THE GFS GDI  
FORECAST NOW SHOWS MOST UNSTABLE ACROSS WESTERN INTERIOR/NORTHWEST  
PORTIONS OF THE ISLAND ON WEDNESDAY-SATURDAY...WITH SAN JUAN METRO  
AREA UNDER MARGINAL CONDITIONS FOR DEEP CONVECTION TO DEVELOP.  
 
ALTHOUGH THE MODELS GENERALLY AGREE ON PATTERN EVOLUTION...THE  
DISAGREE ON IMPACT THIS IS TO HAVE ACROSS THE NORTHERN CARIBBEAN  
ISLES. BOTH THE ECMWF AND UKMET SHOW MOST ACTIVE CONVECTION  
CLUSTERING ACROSS SOUTHERN HISPANIOLA...SHOWING POTENTIAL FOR AN  
MCS DURING THE EARLY MORNING HOURS ON THURSDAY. AS CONVECTION  
CLUSTERS TO THE WEST...THEY FORESEE UNFAVORABLE ENVIRONMENTAL  
CONDITIONS FOR CONVECTION TO DEVELOP ACROSS PUERTO RICO AND THE  
USVI...WITH THE ORGANIZED CONVECTION TO THE WEST SUPPRESSING THE  
DIURNAL PATTERN ACROSS THE FORECAST AREA. THE GFS DISAGREES...  
GENERALLY FAVORING DISORGANIZED CONVECTION TO THE WEST...WITH  
DIURNAL PATTERN ACROSS PUERTO RICO BECOMING BETTER ORGANIZED LATER  
IN THE WEEK. A PLURAL MAJORITY OF THE NCEP ENSEMBLE MEMBERS LEAN  
IN FAVOR OF THE WEAKER GFS SOLUTION. SO THE UNCERTAINTY IS HIGH  
BEYOND 48/60 HRS AND THE CONFIDENCE IN THE FORECAST INTO MEDIUM  
RANGE PERIOD IS LOW.  
 
DAVISON...WPC (USA)  
 

 
 
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