993  
FXCA20 KWBC 131143  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
643 AM EST WED DEC 13 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 13/06  
UTC: BROAD POLAR TROUGH REMAINS THE DOMINANT FEATURE ACROSS THE  
EASTERN USA-WESTERN ATLANTIC. UNDER THE INFLUENCE OF A CLIPPER  
EXITING THE EASTERN USA EARLY THIS MORNING...THE MID/UPPER LEVEL  
TROUGH IS TO AMPLIFY ACROSS THE BAHAMAS-CUBA TO NORTHERN  
HISPANIOLA LATER THIS AFTERNOON. AS IT DEEPENS...THE TROUGH WILL  
PRESS AGAINST A WEAK CELL OF THE SUBEQUATORIAL RIDGE TO THE SOUTH.  
THIS IS TO FAVOR THE GRADUAL EROSION OF THE TRADE WINDS CAP BY  
MIDDAY TODAY...LIFTING TO AROUND 600 HPA EARLY THIS AFTERNOON.  
THIS WILL ALLOW FOR DEEPER MOISTURE TO POOL ACROSS THE FORECAST  
AREA. THIS PATTERN IS EXPECTED TO HOLD THROUGH MIDAFTERNOON ON  
THURSDAY. LATER IN THE DAY...AS A HIGH AMPLITUDE TROUGH DEEPENS  
ACROSS THE WESTERN USA TO MEXICO...A RIDGE IS TO RAPIDLY BUILD  
ACROSS THE BASIN/WESTERN ATLANTIC. THIS IS TO ONCE AGAIN  
STRENGTHEN THE TRADE WINDS CAP ACROSS THE FORECAST AREA...LEADING  
TO EROSION OF AVAILABLE MOISTURE AND A MORE STABLE AIR MASS.  
 
AT LOW LEVELS...AN ELONGATED FRONTAL BOUNDARY TRAILS ACROSS THE  
SOUTHEAST BAHAMAS/ TURKS AND CAICOS-EASTERN CUBA-JAMAICA TO THE  
CENTRAL CARIBBEAN. THE BROAD TROUGH PATTERN NORTH OF THE ISLANDS  
SUSTAINS A PREFRONTAL/INVERTED TROUGH THAT EXTENDS ALONG 70W TO  
HISPANIOLA...MEANWHILE FAVORING A MOIST SOUTHERLY FLOW ACROSS THE  
NORTHEAST CARIBBEAN ISLES. A SECONDARY FRONTAL BOUNDARY TO THE  
NORTH IS TAKING A BEELINE ACROSS THE WESTERN ATLANTIC-FLORIDA  
PENINSULA. THE LATTER IS FORECAST TO GRADUALLY REINFORCE THE  
FORMER...TO THEN MEANDER EAST INTO NORTHERN HISPANIOLA LATER THIS  
EVENING/EARLY MORNING HOURS ON THURSDAY. THIS...HOWEVER...WILL BE  
SHORT LIVED...WITH BOUNDARY WEAKENING AS IT RETROGRESSES TO THE  
BAHAMAS-CUBA ON FRIDAY WHILE LOSING ITS UPPER LEVEL SUPPORT. AS  
THE PATTERN TO THE NORTH AND WEST EVOLVES...THE LOW LEVEL WINDS  
OVER THE EASTERN CARIBBEAN ARE TO GRADUALLY BACK TO THE  
EAST-NORTHEAST LATER THIS EVENING. THIS FLOW IS TO GENERALLY  
HOLD...WITH A GRADUAL INCREASE IN WIND INTENSITY ON  
SATURDAY-SUNDAY AS A HIGH PRESSURE RIDGE BUILDS TO THE NORTH OF  
THE ISLANDS. AS THE WINDS BACK TO THE EAST-NORTHEAST...THIS IS TO  
ALSO FAVOR A COOL ADVECTIVE PATTERN ACROSS THE NORTHEAST CARIBBEAN  
ISLES...FORECAST TO STRENGTHEN ON SATURDAY-SUNDAY. THE STRONGER  
WINDS AND COOL ADVECTIVE PATTERN WILL THEN LEAD TO AN INCREASE IN  
TRADE WIND SHOWERS ACROSS THE FORECAST AREA.  
 
EARLY IN THE PERIOD...AS THE TRADE WINDS CAP RELAXES ITS FOOTHOLD  
ACROSS THE FORECAST AREA...EXPECTING ANOTHER ROUND OF SHOWERS TO  
AFFECT THE INTERIOR AND NORTHERN PORTIONS OF PUERTO RICO AS  
ENHANCED BY DIURNAL HEATING/SEA BREEZE CONVERGENCE. BUT THIS IS TO  
GENERALLY FAVOR LIGHT TO MODERATE RAINFALL AMOUNTS IN AFTERNOON  
CONVECTION. ON THURSDAY AND ONWARD...EARLY MORNING TRADE WIND  
SHOWERS ARE TO PESTER THE USVI-EASTERN/NORTHERN PORTIONS OF PUERTO  
RICO. THIS IS TO GRADUALLY BECOME BETTER ORGANIZED DURING THE  
WEEKEND...WHEN WE EXPECT A HIGHER CHANCE FOR MEASURABLE RAINFALL  
AMOUNTS. THE EASTERLY TRADES/DIURNAL HEATING ARE TO ALSO ENHANCE  
AFTERNOON CONVERGENCE ACROSS WESTERN PUERTO RICO...FAVORING LIGHT  
CONVECTION. FURTHERMORE...THE STRONGER WINDS DURING THE WEEKEND  
ARE TO ALSO FAVOR THE GENERATION OF STREAMERS ACROSS THE VIRGIN  
ISLES THAT ARE TO CLUSTER OVER EASTERN PUERTO RICO/EL YUNQUE  
RAINFOREST DURING THE AFTERNOON HOURS.  
 
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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