999  
FXCA20 KWBC 021205  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
704 AM EST SAT DEC 02 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 02/06  
UTC: A MID LEVEL TROUGH NOW DOMINATES THE FLOW ACROSS THE WESTERN  
ATLANTIC TO THE NORTHERN CARIBBEAN ISLES. THIS PATTERN IS TO  
GENERALLY HOLD DURING THE NEXT THREE DAYS. THE MEANDERING TROUGH  
IS TO SUSTAIN THE GRADUAL EROSION OF TRADE WINDS CAP ACROSS THE  
FORECAST AREA. THIS IS EXPECTED TO 500/550 HPA DURING THE EARLY  
MORNING HOURS ON MONDAY. THE ELEVATED INVERSION WILL THEN PERSIST  
THROUGH WEDNESDAY MORNING. AS THE TRADE WINDS CAP RELAXES ITS  
FOOTHOLD ACROSS THE FORECAST AREA...PWAT CONTENT INCREASES TO 1.50  
INCHES LATER ON SUNDAY...PEAKING AT 1.75 INCHES ON  
MONDAY-WEDNESDAY.  
 
AT 250 HPA A SUBTROPICAL JET BOUNDS THE SOUTHERN FRINGES OF THIS  
TROUGH. ON TUESDAY...IN RESPONSE TO DIGGING HIGH AMPLITUDE TROUGH  
TO THE WEST...A RIDGE IS TO GRADUALLY BUILD ACROSS THE EASTERN USA  
TO THE WESTERN ATLANTIC. THIS...IN-TURN...WILL THEN DISPLACE THE  
WESTERN ATLANTIC TROUGH TO THE EAST-NORTHEAST. AS THE GRADIENT  
TIGHTENS...THE SUBTROPICAL JET MAXIMA ROUNDING THE BASE OF THE  
TROUGH IS FORECAST TO INTENSIFY WHILE MEANDERING EASTWARD. AS IT  
REPOSITIONS...PUERTO RICO AND THE VIRGIN ISLES WILL THEN LIE ON  
THE DIVERGENT RIGHT ENTRANCE REGION OF THIS JET.  
 
AT LOW LEVELS...A FRONTAL LOW NEAR 25N 63W EXTENDS A FRONT ACROSS  
THE BAHAMAS TO CUBA. OVER THE EASTERN CARIBBEAN THE LOW TO THE  
NORTH SUSTAINS AN INDUCED INVERTED TROUGH THAT EXTENDS JUST WEST  
OF THE ISLAND CHAIN. THIS TROUGH FAVORS A MOIST SOUTHEASTERLY FLOW  
ACROSS THE EASTERN CARIBBEAN. A PREFRONTAL SHEAR LINE NOW EXTENDS  
ACROSS THE NORTHERN LEEWARD/VIRGIN ISLES TO THE EASTERN CARIBBEAN.  
THIS FAVORS WIDELY ISOLATED CONVECTION. FRONTAL LOW WILL LIFTS TO  
THE EAST-NORTHEAST ON SUNDAY...DRIVING THE FRONT SOUTH TOWARDS THE  
NORTHERN CARIBBEAN. HOWEVER...BOUNDARY IS EXPECTED TO STALL TO THE  
NORTH OF THE GREATER ANTILLES ON SUNDAY. THE SHEAR LINE IS TO THEN  
BECOME ILL DEFINED. MEANDERING FRONT IS TO REMAIN NORTH OF THE  
FORECAST AREA THROUGH MONDAY. ON MONDAY TO TUESDAY...BROAD RIDGE  
ROLLS ACROSS THE EASTERN USA TO THE WESTERN ATLANTIC. THIS IS TO  
TIGHTEN THE PRESSURE GRADIENT WHILE FAVORING A COLD ADVECTIVE  
PATTERN. AS THE GRADIENT TIGHTENS THE BOUNDARY LAYER WINDS WILL  
INCREASE TO 20-25KT WHILE BACKING TO THE EAST-NORTHEAST. THIS IS  
TO ALSO SUSTAIN REGENERATION OF THE SHEAR LINE ACROSS THE  
NORTHEAST CARIBBEAN LATER ON MONDAY/EARLY TUESDAY MORNING. THIS  
WILL THEN LEAD TO A GRADUAL EROSION OF AVAILABLE WATER LATER IN  
THE WEEK.  
 
UNDER FAVORABLE UPPER LEVEL JET DYNAMICS...SURGING EASTERLY  
TRADES...A COOL ADVECTIVE PATTERN AND LOW LEVEL WIND CONVERGENCE  
ALONG THE SHEAR LINE WILL LIKELY RESULT IN AN ECHO TRAINING  
PATTERN. EXPECTING MOST INTENSE CONVECTION TO START DURING THE  
EARLY MORNING HOURS ON TUESDAY AND TO PERSIST THROUGH WEDNESDAY  
EVENING. IN THIS PATTERN STORM TOTAL AMOUNTS OF THREE TO FIVE  
INCHES WOULD BE HIGHLY PROBABLE...WITH MOST INTENSE AFFECTING  
NORTHEAST PUERTO RICO/SAN JUAN METRO AREA...WHILE ACROSS THE  
VIRGIN ISLES EXPECTING MAXIMA OF ONE TO THREE INCHES.  
 
DAVISON...WPC (USA)  
 

 
 
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