631  
FXCA20 KWBC 241136  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
636 AM EST WED JAN 24 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 24/06  
UTC: A 500 HPA HIGH TO THE NORTH OF THE TURKS AND CAICOS ANCHORS A  
RIDGE OVER THE WESTERN ATLANTIC/NORTHERN CARIBBEAN ISLES. FORECAST  
AREA REMAINS ON THE CONVERGENT SIDE OF THIS RIDGE. THIS FAVORS AN  
ELEVATED TRADE WINDS CAP THAT IS INHIBITING DEEP CONVECTION...BUT  
IT IS NOT STRONG ENOUGH TO SUPPRESS SHALLOW CONVECTIVE  
DEVELOPMENT. OVER THE NEXT COUPLE OF DAYS THE HIGH COLLAPSES AS A  
SECONDARY HIGH FORMS OVER THE NORTHWEST BAHAMAS. HOWEVER...THE  
RIDGE AND ASSOCIATED INVERSION ARE TO GENERALLY PERSIST WELL  
THROUGH THE WEEKEND/EARLY NEXT WEEK.  
 
AT LOW LEVELS...POLAR FRONT EXTENDS OVER THE WESTERN ATLANTIC.  
THIS IS PRESSING AGAINST A BROAD CELL OF THE SUBTROPICAL RIDGE TO  
THE EAST. UNDER PRESSURE...THE RIDGE WILL RELOCATE TO THE CENTRAL  
ATLANTIC WHILE YIELDING TO THE POLAR FRONT. BOUNDARY BOTTOMS OUT  
NORTH OF THE ISLANDS ALONG 22N/23N LATER ON SATURDAY.  
SIMULTANEOUSLY...UNDER THE INFLUENCE OF A POLAR RIDGE TO THE  
WEST...A PREFRONTAL SHEAR LINE FORMS AHEAD OF THE SURFACE FRONT  
EARLY THIS MORNING. THE SHEAR LINE REACHES THE DOMINICAN REPUBLIC  
EARLY ON FRIDAY...AND MOVES OVER PUERTO RICO/VIRGIN ISLES LATER IN  
THE DAY. IT IS TO THEN MEANDER OVER THE FORECAST AREA THROUGH  
EARLY MORNING ON SATURDAY...MOVING OVER THE LEEWARD ISLANDS BY  
MIDDAY ON SATURDAY. AHEAD OF THIS FEATURE...FRESH EASTERLY TRADES  
WILL PREVAIL...WITH 850 HPA WINDS OF 10-15KT. EARLY ON FRIDAY THE  
WINDS WILL BACK TO THE NORTHEAST...AND THROUGH THE DAY THEY ARE TO  
GRADUALLY INCREASE. THIS WILL SUSTAIN A COLD ADVECTIVE PATTERN  
ACROSS THE FORECAST AREA THAT IS TO ENHANCE SHALLOW CONVECTIVE  
INSTABILITY. BY THE END OF THE DAY EXPECTING WINDS OF 25-30KT TO  
ENVELOP THE FORECAST AREA. THESE ARE TO PERSIST THROUGHOUT THE  
WEEKEND/EARLY NEXT WEEK WHILE GRADUALLY VEERING TO THE EAST ON  
SATURDAY AFTERNOON.  
 
LIGHTER WINDS EARLY IN THE CYCLE WILL ALLOW FOR SHALLOW MOISTURE  
TO POOL ACROSS THE FORECAST AREA...WITH PWAT CONTENT OF 1.25-1.50  
INCHES EXPECTED THROUGH THURSDAY AFTERNOON. THIS WILL FEED DIURNAL  
CONVECTION ACROSS THE FORECAST AREA...WITH MOST INTENSE CLUSTERING  
ACROSS INTERIOR PORTIONS OF PUERTO RICO IN AFTERNOON CONVECTION.  
LATER ON THURSDAY A DRYER AIR MASS IS TO ENTRAIN FROM THE EAST  
AHEAD OF THE SHEAR LINE. BUT THROUGH THE DAY...AS THE WINDS SURGE  
AND A COLD ADVECTIVE PATTERN ESTABLISHES ACROSS THE FORECAST  
AREA...EXPECTING A MORE SEASONAL PATTERN OF TRADE WIND SHOWERS TO  
AFFECT PUERTO RICO AND THE VIRGIN ISLES. THE BRISK EASTERLY TRADES  
ARE TO ALSO FAVOR GENERATION OF STREAMERS THAT ARE TO CONVERGE  
ACROSS VIEQUES...CULEBRA AND EASTERN PORTIONS OF PUERTO RICO. THIS  
WILL COMBINE WITH PERSISTENT COOL ADVECTIVE PATTERN TO LIKELY  
FAVOR MODERATE RAINFALL AMOUNTS.  
 
DAVISON/GALVEZ...WPC (USA)  
 

 
 
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