112  
FXCA20 KWBC 051150  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
649 AM EST FRI JAN 05 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 05/06  
UTC: BROAD POLAR TROUGH EXTENDS OVER THE EASTERN USA-WESTERN  
ATLANTIC TO THE GULF OF MEXICO/NORTHWEST CARIBBEAN. THE MID/UPPER  
LEVEL TROUGH IS PRESSING AGAINST A RIDGE TO THE EAST. THE LATTER  
EXTENDS TO THE NORTHEAST FROM A 250 HPA HIGH OVER NORTHERN  
VENEZUELA/ABC ISLES...ACROSS THE FORECAST AREA/LEEWARD ISLES INTO  
THE CENTRAL ATLANTIC. AT LOW LEVELS...POLAR FRONT EXTENDS ACROSS  
THE TURKS AND CAICOS-WINDWARD PASSAGE TO THE WESTERN  
CARIBBEAN/NORTHERN CENTRAL AMERICA. A PREFRONTAL SHEAR LINE LINES  
ACROSS HISPANIOLA TO THE SOUTHERN CARIBBEAN/WESTERN PANAMA...WITH  
AN INDUCED/PREFRONTAL TROUGH TO THE EAST FAVORING A SOUTHEASTERLY  
FLOW ACROSS THE NORTHEAST CARIBBEAN. IN THIS PATTERN AN ELEVATED  
CAP INVERSION ENVELOPS THE FORECAST AREA...WITH SOUTHEASTERLY  
WINDS ADVECTING A PLUME OF TROPICAL MOISTURE ACROSS THE  
LEEWARD/VIRGIN ISLES EARLY THIS MORNING. THIS REACHES THE SAN JUAN  
METRO AREA EARLY THIS AFTERNOON.  
 
MID/UPPER LEVEL TROUGH IS TO REMAIN ACROSS THE EASTERN USA-WESTERN  
ATLANTIC THROUGH SUNDAY MORNING. LATER IN THE DAY IT IS TO THEN  
PULL TO THE NORTHEAST AS THE SUBEQUATORIAL RIDGE TO THE SOUTH  
GRADUALLY BUILDS ACROSS THE CARIBBEAN BASIN/GREATER ANTILLES. AT  
500 HPA THIS WILL ALLOW FOR THE MID LEVEL RIDGE PATTERN TO ALSO  
STRENGTHEN...WITH AXIS TO ANCHOR ON A CLOSED HIGH TO THE NORTH OF  
PUERTO RICO. THE BUILDING RIDGE WILL THEN STRENGTHEN THE TRADE  
WINDS CAP ACROSS THE NORTHEAST CARIBBEAN...LEADING TO RAPID  
EROSION OF AVAILABLE MOISTURE DURING THE DAY ON SUNDAY-MONDAY. AT  
LOW LEVELS...SURFACE FRONT IS TO MEANDER OVER  
HAITI-JAMAICA-NORTHERN CENTRAL AMERICA DURING THE NEXT TWO  
DAYS...WITH PREFRONTAL TROUGH OVER THE DOMINICAN REPUBLIC TO  
CONTINUE FAVORING A MOIST SOUTHEASTERLY FLOW ACROSS THE FORECAST  
AREA THROUGH SUNDAY MORNING. ALSO AT 700 HPA...A SHORT WAVE  
PERTURBATION EAST OF THE LEEWARD ISLES EMBEDS IN THIS FLOW...WITH  
THE GFS SHOWING THE LOW/MID LEVEL VORTEX PULLING ACROSS THE VIRGIN  
ISLES TO PUERTO RICO ON SATURDAY. ON SUNDAY...AS THE UPPER LEVEL  
TROUGH PULLS AWAY AND SURFACE FRONT LOSES UPPER LEVEL  
SUPPORT...THE LOW LEVEL TROUGH WILL FILL AND PREVAILING EASTERLY  
TRADES RETURN TO THE FORECAST AREA. THE FRONT WILL THEN MEANDER  
AWAY...WEAKENING WHILE RETROGRESSING TO CUBA AND THE BAHAMAS.  
 
THE MOIST SOUTHEASTERLY FLOW WILL FEED CONVECTIVE DEVELOPMENT  
DURING THE NEXT COUPLE OF DAYS. THIS IS TO FAVOR LIGHT CONVECTION  
ALONG THE CORDILLERA CENTRAL IN LATE MORNING/EARLY AFTERNOON  
CONVECTION...WITH MOST ACTIVE CLUSTERING OVER NORTHWEST PUERTO  
RICO DURING THE AFTERNOON HOURS. ALTHOUGH MID LEVEL PERTURBATION  
CROSSES THE FORECAST AREA ON SATURDAY...HIGHER RESOLUTION MODEL  
GUIDANCE AGREE ON A HIGHER THREAT OF HEAVY CONVECTION ACROSS  
NORTHWEST PUERTO RICO DURING THE MID/LATE AFTERNOON HOURS TODAY.  
THIS IS REFLECTED IN OUR QPF CHARTS FOR THE DAY. LATER DURING THE  
WEEKEND/EARLY NEXT WEEK...AS A DRYER AIR MASS ENTRAINS FROM THE  
EAST...FAIR WEATHER CONDITIONS ARE TO PREVAIL.  
 
DAVISON...WPC (USA)  
 

 
 
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