346  
FXCA20 KWBC 121137  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
637 AM EST FRI JAN 12 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 11/06  
UTC: MID LEVEL RIDGE NORTH OF THE ISLANDS IS TO STEER A TUTT LOW  
ACROSS THE ATLANTIC TO JUST NORTH-NORTHEAST OF THE LEEWARD ISLES  
ON SATURDAY. THE BASE OF THE MID/UPPER LEVEL TROUGH IS TO EXTEND  
OVER THE NORTHEAST CARIBBEAN DURING THE AFTERNOON HOURS...WITH A  
JET MAXIMA ROUNDING THIS AXIS TO BRIEFLY FAVOR A DIVERGENT PATTERN  
ALOFT. THIS WILL COMBINE WITH WEAKENING OF THE TRADE WINDS CAP TO  
FAVOR DEEPER CONVECTIVE INSTABILITY ACROSS THE FORECAST AREA. THE  
CUT OFF LOW IS TO GRADUALLY WEAKEN THROUGH MONDAY MORNING...WITH  
THE MID LEVEL RIDGE TO ONCE AGAIN STRENGTHEN ITS FOOTHOLD ACROSS  
THE FORECAST AREA. THIS WILL BE ACCOMPANIED BY RAPID  
INTENSIFICATION OF THE TRADE WINDS CAP LATER DURING THE  
WEEKEND/EARLY NEXT WEEK.  
 
AT LOW LEVELS...BROAD RIDGE DOMINATES THE FLOW ACROSS THE  
ATLANTIC...FAVORING BRISK EAST-SOUTHEASTERLY FLOW ACROSS THE  
ISLAND CHAIN/EASTERN CARIBBEAN...WITH 850 HPA WINDS PEAKING AT  
20-25KT. THIS FAVORS GENERATION OF STREAMERS ACROSS THE LEEWARD  
ISLES THAT ARE TO AFFECT THE VIRGIN ISLES/EASTERN PUERTO RICO FROM  
TIME-TO-TIME. ALSO...EMBEDDED IN THIS FLOW...A MOIST PLUME ADVECTS  
ACROSS THE FORECAST AREA LATER THIS AFTERNOON. HIGHER PWAT CONTENT  
IS EXPECTED TO PERSIST THROUGH SATURDAY EVENING. MEANWHILE...THIS  
WILL FEED CONVECTIVE DEVELOPMENT DURING THE DAY...WITH MOST ACTIVE  
CLUSTERING OVER INTERIOR PORTIONS OF PUERTO RICO AS ENHANCED BY  
JET MAXIMA ALOFT AND DAYTIME HEATING. DURING THIS PERIOD EXPECTING  
POTENTIAL FOR MODERATE RAINFALL AMOUNTS.  
 
THE STRENGTHENING OF THE TRADE WINDS CAP LATER DURING THE WEEKEND  
WILL RESULT IN RAPID EROSION OF AVAILABLE MOISTURE ON SUNDAY.  
SIMULTANEOUSLY...UNDER PRESSURE FROM POLAR FRONT OVER THE WESTERN  
ATLANTIC...THE LOW LEVEL RIDGE IS TO ALSO CRUMBLE ON SUNDAY  
MORNING. AS THE GRADIENT SLACKENS THE 850 WINDS WILL DECREASE TO  
05-10KT WHILE SLOWLY BACKING TO THE EAST-NORTHEAST. THE WEAKER  
WINDS AND STRENGTHENING INVERSION ARE TO FAVOR FAIR WEATHER  
CONDITIONS ACROSS THE VIRGIN ISLES-EASTERN/NORTHERN PUERTO RICO.  
DIURNAL CONVECTION WILL LIMIT TO INTERIOR PORTIONS OF THE ISLAND  
DURING THE AFTERNOON HOURS...TO GENERALLY FAVOR LIGHT RAINFALL  
AMOUNTS IN BRIEF RAIN SHOWERS.  
 
DAVISON...WPC (USA)  
 

 
 
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