240  
FXCA20 KWBC 111241  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
740 AM EST MON DEC 11 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 11/06  
UTC: AN UPPER RIDGE CENTERS OVER NORTHERN COLOMBIA AND EXTENDS AN  
AXIS TO THE NORTHEAST ACROSS HISPANIOLA AND PUERTO RICO INTO 28N  
61W. THE RIDGE ROBUST RIDGE IS ONE OF THE FACTORS LIMITING THE  
SOUTHEASTWARD PROGRESSION OF A STRONG UPPER TROUGH THAT HAS EXITED  
THE EASTERN COAST OF THE USA ON SUNDAY. AS THE TROUGH LIFTS TO THE  
NORTHEAST...THE ASSOCIATED SURFACE FRONT WILL STAY AWAY OF THE  
ISLANDS. AT MID-LEVELS...A RIDGE CENTERS OVER THE WINDWARD  
PASSAGE/HAITI...WHICH IS HELPING TO SUSTAIN THE TRADE WIND CAP IN  
THE REGION. AT PUERTO RICO AND THE VI...THE BASE OF THE CAP SITS  
AT 850 HPA. AT THE LOW-LEVELS...TRADE WINDS ARE FROM THE  
EAST-NORTHEAST AS 10-15KT. THIS HAS ESTABLISHED A COOL ADVECTIVE  
WIND PATTERN...AND IS LEADING TO VERY ISOLATED...SHORT  
LIVED...LIGHT RAIN SHOWERS ACROSS NORTHEASTERN PUERTO RICO AND THE  
NORTHERN VIRGIN ISLANDS. A LARGER COVERAGE OF PRECIPITATING  
SHOWERS EXTENDS TO THE SOUTH INCLUDING ST CROIX. THE LATTER IS IN  
ASSOCIATION WITH A REGION OF ENHANCED TRADE WIND CONVERGENCE.  
HERE...CONVECTION IS APPROACHING 700 HPA AND PRECIPITABLE WATER IS  
NEARING 40MM. OTHERWISE...FAIR CONDITIONS EXTEND OVER MOST OF THE  
FORECAST AREA.  
 
MODELS ARE FORECASTING THE MID-LEVEL RIDGE BUILDING INTO THE  
REGION AND ESTABLISHING OVER THE ISLANDS BY LATE WEDNESDAY. THIS  
WILL CONTINUE SUSTAINING THE TRADE WIND CAP AND LIMITING VERTICAL  
DEVELOPMENT OF ANY CONVECTION IN THE REGION. NO SIGNIFICANT  
PRECIPITATION IS EXPECTED THROUGH THE CYCLE. MOST OF THE  
VARIABILITY WILL OCCUR IN THE LOW-LEVELS...WHERE TRADES WILL  
CHANGE FROM 05-15KT EAST-NORTH EASTERLIES ON MONDAY...TO 05-10KT  
SOUTH EASTERLIES ON TUESDAY...TO WEAK SOUTHERLIES ON  
WEDNESDAY...AND BACK TO 05-10KT EASTERLIES ON THURSDAY. THIS WILL  
PLAY A ROLE IN THE DISTRIBUTION OF PRECIPITATION.  
 
INITIALLY...NORTHEASTERLY TRADES ON MONDAY AND EARLY TUESDAY WILL  
FAVOR ISOLATED...VERY LIGHT...AND SHORT-LIVED RAIN SHOWERS  
PESTERING NORTHEASTERN PUERTO RICO ON EARLY MORNING HOURS...TO  
PRODUCE TRACE RAINFALL AMOUNTS. DURING THE AFTERNOON...SHOWERS  
WILL DEVELOP IN SOUTHWESTERN PUERTO RICO WHERE THEY WILL PRODUCE  
ACCUMULATIONS GENERALLY UNDER 0.4 IN/DAY. DURING TUESDAY...TRADE  
WINDS WILL SHIFT TO SOUTH EASTERLIES AT 850 HPA...AND TO SOUTH  
WESTERLIES AT 700 HPA. THIS WILL FAVOR ENHANCED CONVERGENCE OF THE  
DIURNAL BREEZES ALONG THE CORDILLERA...AND A RESULTING ENHANCEMENT  
OF AFTERNOON SHOWERS ALONG THE CORDILLERA AND NORTHERN SLOPES.  
THESE SHOWERS WILL PRODUCE ACCUMULATIONS OF 0.5-0.8 IN/DAY...WITH  
THE POTENTIAL OF PRODUCING LIGHTER ACCUMULATIONS IN THE SAN JUAN  
METRO AREA DURING THE LATE AFTERNOON. DURING WEDNESDAY...AFTERNOON  
SHOWERS WILL CLUSTER MAINLY ALONG THE CENTRAL CORDILLERA/NORTHERN  
SLOPES...TO PRODUCE ACCUMULATIONS GENERALLY UNDER 0.4 INCHES/DAY.  
OTHERWISE...FAIR CONDITIONS ARE TO CONTINUE.  
 
GORDON...NMS (BELIZE)  
PECK...MS (JAMAICA)  
RIVAS...SENAMHI (PERU)  
GALVEZ...WPC (USA)  
 

 
 
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