410  
FXCA20 KWBC 251151  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
650 AM EST THU JAN 25 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 25/06  
UTC: MID LEVEL RIDGE CONTINUES TO DOMINATE THE WESTERN  
ATLANTIC-EASTERN CARIBBEAN ISLES...WITH AXIS ANCHORING ON A 500  
HPA HIGH TO THE NORTH OF PUERTO RICO/VIRGIN ISLES. THIS FAVORS AN  
ELEVATED CAP INVERSION...THAT IS TRAPPING MOISTURE BELOW 650 HPA.  
AT LOW LEVELS...BROAD RIDGE CONTINUES TO YIELD TO POLAR TROUGH  
STREAMING ACROSS THE WESTERN ATLANTIC. ASSOCIATED SURFACE FRONT  
EXTENDED ACROSS THE NORTHWEST BAHAMAS-FLORIDA KEYS TO THE GULF OF  
MEXICO EARLY THIS MORNING. FURTHERMORE...BUILDING POLAR RIDGE TO  
THE WEST TRIGGERED A PREFRONTAL SHEAR LINE THAT EXTENDS ACROSS THE  
CENTRAL BAHAMAS-WESTERN CUBA TO THE GULF OF HONDURAS. BRISK  
EASTERLY TRADES PERSIST ACROSS THE NORTHEAST CARIBBEAN...WITH  
SHALLOW MOISTURE POOLING ACROSS THE FORECAST AREA SUSTAINING A  
PWAT CONTENT OF 1.25-1.50 INCHES.  
 
CONSISTENT WITH PREVIOUS FORECAST...MID LEVEL HIGH TO THE NORTH OF  
PUERTO RICO IS TO COLLAPSE EARLY IN THE CYCLE WHILE A SECONDARY  
HIGH FORMS OVER THE NORTHWEST BAHAMAS LATER ON FRIDAY. ALTHOUGH  
THE HIGH WEAKENS THE MID LEVEL RIDGE AND ASSOCIATED TRADE WINDS  
CAP ARE EXPECTED TO PERSIST THROUGHOUT THE WEEKEND-EARLY NEXT  
WEEK. THE RIDGE WILL BE STRONG ENOUGH TO INHIBIT ORGANIZED DEEP  
CONVECTION BUT NOT STRONG ENOUGH TO SUPPRESS SHALLOW DEVELOPMENT.  
AT LOW LEVELS...POLAR FRONT WILL GRADUALLY SLIDE UNDER THE MID  
LEVEL RIDGE...ALIGNING ALONG 22N/23N ON FRIDAY-SATURDAY. THE  
PREFRONTAL SHEAR LINE MOVES ACROSS HISPANIOLA LATER THIS  
EVENING/EARLY ON FRIDAY MORNING. THIS IS TO THEN MAKE LANDFALL  
OVER PUERTO RICO DURING MAX HEATING ON FRIDAY AFTERNOON. THE SHEAR  
LINE IS TO THEN LINGER BETWEEN EASTERN PUERTO RICO AND THE VIRGIN  
ISLES THROUGH MIDMORNING ON SATURDAY...AND LATER IN THE DAY  
MEANDERS OVER THE LEEWARD ISLANDS. AHEAD OF THE SHEAR LINE...FRESH  
EASTERLY TRADES WILL PREVAIL...WITH 850 HPA WINDS OF 10-20KT.  
EARLY ON FRIDAY THE WINDS WILL BACK TO THE NORTHEAST...AND THROUGH  
THE DAY THEY ARE TO GRADUALLY INCREASE TO 25-30KT. THIS WILL  
SUSTAIN A COLD ADVECTIVE PATTERN ACROSS THE WARMER ATLANTIC WATERS  
NORTH OF THE ISLANDS TO ENHANCE SHALLOW CONVECTIVE  
INSTABILITY...WITH RISK OF ECHO TRAINING OVER THE VIRGIN  
ISLES-EASTERN PUERTO RICO EARLY ON SATURDAY MORNING. THE STRONG  
BOUNDARY LAYER WINDS ARE TO PERSIST THROUGHOUT THE WEEKEND/EARLY  
NEXT WEEK WHILE GRADUALLY VEERING TO THE EAST ON SATURDAY EVENING.  
DURING THIS PERIOD MODELS SHOWING PEAK WIND INTENSITY OF 30-35KT.  
 
SHALLOW MOISTURE POOLING ACROSS THE FORECAST AREA WILL FEED  
CONVECTIVE DEVELOPMENT DURING THE DAY TODAY. MOST INTENSE IS  
FORECAST TO CLUSTER OVER INTERIOR PORTIONS OF PUERTO RICO IN  
AFTERNOON CONVECTION WHERE WE EXPECT LIGHT TO MODERATE RAINFALL  
AMOUNTS. LATER THIS EVENING A DRYER AIR MASS IS TO ENTRAIN FROM  
THE EAST AHEAD OF THE SHEAR LINE. BUT THROUGH THE DAY ON  
FRIDAY...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 STRONGER 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 AND MEANDERING SHEAR LINE TO LIKELY FAVOR  
HEAVY RAINFALL AMOUNTS ACROSS THE VIRGIN ISLES AND EASTERN PUERTO  
RICO/SAN JUAN METRO AREA. THE ECMWF GDI FORECAST IS SHOWING A  
HIGHER RISK THAN WHAT THE GFS IS FORECASTING. CONSIDERING SLIGHTLY  
ABOVE NORMAL SEA SURFACE TEMPERATURES AND COLD ADVECTIVE PATTERN  
WE ARE LEANING TOWARDS THE EUROPEAN SOLUTION.  
 
DAVISON/GALVEZ...WPC (USA)  
 

 
 
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