322  
FXCA20 KWBC 301155  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
654 AM EST THU NOV 30 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM NOV 30/06  
UTC: AT MID LEVELS...THE FORECAST AREA REMAINS UNDER THE INFLUENCE  
OF WANING CELL OF THE SUBEQUATORIAL RIDGE. THIS SUSTAINS A STRONG  
TRADE WINDS INVERSION THAT IS CAPPING MOISTURE TO THE BOUNDARY  
LAYER. THIS PATTERN IS EXPECTED TO HOLD DURING THE NEXT COUPLE OF  
DAYS...MEANWHILE FAVORING FAIR WEATHER CONDITIONS ACROSS THE  
ENTIRE FORECAST AREA. SHOWERS...IF ANY...WILL BE BRIEF AND ARE TO  
CONFINE TO THE CORDILLERA AND WESTERN INTERIOR PORTIONS PUERTO  
RICO.  
 
NORTH OF THIS RIDGE...A MID/UPPER LEVEL SHORT WAVE TROUGH IS  
STREAMING ACROSS THE WESTERN ATLANTIC...WITH A SECONDARY VORTEX TO  
FOLLOW DURING THE WEEKEND. THE INFLOW OF SHORT WAVE ENERGY WILL  
LEAD TO GRADUAL EROSION OF SUBEQUATORIAL RIDGE PATTERN ACROSS THE  
EASTERN CARIBBEAN LATER DURING THE WEEKEND...WITH INVERSION  
LIFTING ABOVE 600 HPA EARLY NEXT WEEK. AT LOW LEVELS THESE  
VORTICES ARE TO INTERACT WITH MEANDERING FRONT NORTH OF THE  
ISLANDS. THE FIRST IS INDUCING CYCLOGENESIS TO THE EAST OF THE  
BAHAMAS EARLY THIS MORNING. THE LOW LEVEL CYCLONIC CIRCULATION IS  
FORECAST TO GRADUALLY DEEPEN AS IT MEANDERS EAST...WITH SURFACE  
LOW CENTERING BETWEEN PUERTO RICO AND BERMUDA LATER THIS  
AFTERNOON. AS THE LOW LEVEL CYCLONE BECOMES BETTER ORGANIZED...IT  
IS TO DRIVE A SURFACE FRONT SOUTH...WITH BOUNDARY MOVING JUST  
NORTH OF PUERTO RICO-VIRGIN ISLES ON SUNDAY. A PREFRONTAL SHEAR  
LINE IS TO PRECEDE THIS BOUNDARY...TO STREAM ACROSS THE FORECAST  
AREA LATER ON FRIDAY-SATURDAY. ALSO AT LOW LEVELS...AS THE LOW  
DEEPENS TO THE NORTH...IT IS TO INDUCE AN INVERTED TROUGH ACROSS  
THE EASTERN CARIBBEAN EARLY IN THE CYCLE. IN RESPONSE TO THIS  
FEATURE THE LOW LEVEL WINDS ARE TO VEER TO THE SOUTHEAST. THIS IS  
TO THEN ADVECT DEEP TROPICAL MOISTURE ACROSS THE WINDWARD  
ISLES...REACHING THE VIRGIN ISLES/EASTERN PUERTO RICO EARLY ON  
SATURDAY MORNING...WITH PWAT CONTENT OF 1.50-1.75 INCHES. THE  
INCREASE IN MOISTURE WILL COINCIDE WITH SHEAR LINE PASSAGE ACROSS  
THE FORECAST AREA ON SATURDAY. THE HIGHER PWAT CONTENT IS EXPECTED  
TO PERSIST THROUGH EARLY NEXT WEEK AS TRADE WINDS CAP RELAXES ITS  
FOOTHOLD OVER THE FORECAST AREA.  
 
ALTHOUGH FOLLOWING A SIMILAR EVOLUTION...MODELS DIVERGE ON IMPACT  
THIS IS GOING TO HAVE ACROSS THE FORECAST AREA. THE ECMWF AND  
UKMET FAVOR A DRYER SOLUTION...SHOWING LITTLE TO NO CONVECTION IN  
ASSOCIATION WITH THE PREFRONTAL SHEAR LINE. THE GFS FAVORS A  
WETTER SOLUTION...WITH SCATTERED SHOWERS AFFECTING  
CENTRAL/INTERIOR PORTIONS OF PUERTO RICO ON SATURDAY-SUNDAY. THIS  
IS CLOSER TO THE NORMAL EVOLUTION OF A SHEAR LINE/FRONTAL  
BOUNDARY APPROACHING THE FORECAST AREA. AS A RESULT...THE GFS  
SOLUTION IS FAVORED OVER THE EUROPEAN MODELS.  
 
DAVISON...WPC (USA)  
 

 
 
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