432  
FXCA20 KWBC 221157  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
657 AM EST MON JAN 22 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 22/06  
UTC: FORECAST AREA LIES ON THE CONVERGENT SIDE OF A MID/UPPER  
LEVEL RIDGE THAT EXTENDS OVER THE EASTERN CARIBBEAN...WITH MID  
LEVEL AXIS CENTERING ON A CLOSED HIGH THAT MEANDERS BETWEEN PUERTO  
RICO AND THE VIRGIN ISLES. THE RIDGE FAVORS A FAIRLY STRONG TRADE  
WINDS CAP ACROSS THE FORECAST AREA. A SHORT WAVE TROUGH TO THE  
NORTHWEST IS STREAMING ACROSS THE BAHAMAS EARLY THIS MORNING. THE  
RIDGE IS TO STEER THIS VORTEX JUST NORTH OF PUERTO RICO LATER THIS  
EVENING...AND AS IT CLEARS THE FRINGES OF THE RIDGE ON TUESDAY IT  
IS TO THEN AMPLIFY ACROSS THE TROPICAL ATLANTIC. ON ITS WAKE...THE  
MID LEVEL RIDGE IS TO STRENGTHEN ITS FOOTHOLD ACROSS THE NORTHEAST  
CARIBBEAN. AN ELEVATED CAP INVERSION IS TO THEN PREVAIL THROUGH  
THE END OF THE WEEK.  
 
AT LOW LEVELS...BROAD RIDGE DOMINATES THE FLOW ACROSS THE WESTERN  
ATLANTIC INTO THE CARIBBEAN BASIN...FAVORING A TIGHT GRADIENT AND  
BRISK EASTERLY TRADES. THESE ARE STRONG ENOUGH TO FAVOR GENERATION  
OF STREAMERS ACROSS THE VIRGIN ISLES THAT AFFECT EASTERN PUERTO  
RICO FROM TIME-TO-TIME. ALSO AT LOW LEVELS...A SAGGING FRONT LIES  
TO THE NORTH OF PUERTO RICO AND HISPANIOLA. THIS BOUNDARY IS THE  
FOCUS OF LOW LEVEL MOISTURE CONVERGENCE...AND IN INTERACTION WITH  
THE MID LEVEL TROUGH IT SUSTAINS ORGANIZED DEEP CONVECTION ACROSS  
THE BAHAMAS EARLY THIS MORNING. SIMILAR PATTERN IS EXPECTED TO THE  
NORTH OF THE FORECAST AREA LATER TODAY AS THE MID LEVEL VORTEX  
STREAMS TO THE NORTH LATER THIS EVENING. BOUNDARY IS TO THEN  
DISSIPATE ON TUESDAY-WEDNESDAY. SIMULTANEOUSLY...A POLAR FRONT  
STREAMING ACROSS THE CENTRAL TO THE EASTERN USA ON TUESDAY IS TO  
THEN PRESS AGAINST THE LOW LEVEL RIDGE. THE RIDGE WILL YIELD TO  
THE POLAR TROUGH...WEAKENING AS IT RELOCATES TO THE CENTRAL  
ATLANTIC. AS THE PRESSURE GRADIENT SLACKENS LIGHTER EASTERLY  
TRADES ARE TO THEN PREVAIL THROUGH FRIDAY MORNING. AS THE WINDS  
WEAKENS...THE INCIDENCE OF STREAMERS FORMING OVER THE VIRGIN ISLES  
AND AFFECTING EASTERN PUERTO RICO IS EXPECTED TO DECREASE THROUGH  
AT LEAST FRIDAY MORNING. HOWEVER...THE LIGHTER WINDS WILL ALLOW  
FOR SHALLOW MOISTURE TO POOL ACROSS THE NORTHEAST CARIBBEAN...WITH  
PWAT CONTENT FORECAST TO PEAK AROUND 1.50 INCHES ON TUESDAY  
AFTERNOON THROUGH THURSDAY MORNING. THIS WILL THEN FEED DIURNAL  
CONVECTION BUILDING ACROSS THE FORECAST AREA...WITH THE NAM AND  
REGIONAL HRWRF SHOWING POTENTIAL FOR MODERATE TO HEAVY CONVECTION  
ACROSS WESTERN PUERTO RICO ON TUESDAY AND WEDNESDAY.  
 
LATER IN THE WEEK...POLAR FRONT IS TO MEANDER EAST ACROSS THE  
WESTERN ATLANTIC. THIS WILL SUSTAIN A PREFRONTAL SHEAR LINE THAT  
MOVES OVER PUERTO RICO LATER ON FRIDAY...WHERE IT IS TO MEANDER  
THROUGH AT LEAST SUNDAY AFTERNOON. AS THE SHEAR LINE ENTERS THE  
FORECAST AREA EXPECTING WINDS TO SURGE...WITH GFS SHOWING 850 HPA  
WINDS OF AROUND 30KT. THE STRONGER WINDS ARE TO PERSIST THROUGHOUT  
THE WEEKEND INTO EARLY NEXT WEEK. IN A COOL ADVECTIVE  
PATTERN...THE BRISK NORTHEASTERLY WINDS WILL FAVOR GENERATION OF  
TRADE WIND SHOWERS ACROSS THE FORECAST AREA...TO RESULT IN LIGHT  
RAINFALL AMOUNTS IN PASSING SHOWERS. THERE IS...HOWEVER...A SLIGHT  
RISK OF ECHO TRAINING ALONG THE SHEAR LINE.  
 
DAVISON/GALVEZ...WPC (USA)  
 

 
 
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