402  
FXCA20 KWBC 291153  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
652 AM EST FRI DEC 29 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 29/06  
UTC: A MID/UPPER LEVEL TROUGH EXTENDS OVER THE EASTERN USA TO THE  
WESTERN ATLANTIC...WITH AXIS PRESSING AGAINST A RIDGE TO THE SOUTH  
AS IT EXTENDS ACROSS THE CARIBBEAN BASIN. AT 250 HPA THIS WILL  
FAVOR GRADUAL INTENSIFICATION OF A JET MAXIMA ACROSS THE  
BAHAMAS-NORTH OF THE GREATER ANTILLES...WITH PEAK IN INTENSITY  
EARLY NEXT WEEK. AT 500 HPA THE RIDGE EXTENDS WEST-TO-EAST ACROSS  
THE GREATER ANTILLES. UNDER PRESSURE FROM THE TROUGH TO THE NORTH  
THE RIDGE WILL START TO WEAKEN DURING THE DAY TODAY WHILE  
GRADUALLY ERODING FROM THE WEST. ON SATURDAY THE RIDGE IS FORECAST  
TO COLLAPSE... WITH A WEAKER RIDGE TO ESTABLISH FARTHER TO THE  
SOUTH OVER NORTHERN SOUTH AMERICA. ALTHOUGH THE MID LEVEL RIDGE  
WEAKENS...UNDER THE INFLUENCE OF STRENGTHENING JET MAXIMA TO THE  
NORTH...AN ELEVATED TRADE WINDS CAP IS TO PERSIST OVER THE  
NORTHEAST CARIBBEAN ISLES...WITH INVERSION TO HOVER AROUND 600  
HPA. THIS IS HIGH ENOUGH TO ALLOW FOR AN INCREASE IN PRECIPITABLE  
WATER CONTENT ACROSS THE FORECAST AREA THROUGH THE SHORT RANGE  
PERIOD. EARLY IN THE CYCLE PLUME OF SHALLOW MOISTURE ADVECTS  
ACROSS THE FORECAST AREA...TO ONCE AGAIN FAVOR SCATTERED  
CONVECTION ACROSS INTERIOR PORTIONS OF PUERTO RICO. THIS IS DENSER  
THAN WHAT THE MODELS INITIALLY ANTICIPATED...SO INCREASING THE  
AMOUNTS OVER WESTERN PUERTO RICO TO NEARLY AN INCH DURING THE  
FIRST TWO PERIODS. A SECOND PLUME OF MOISTURE ENTERS THE FORECAST  
AREA LATER IN THE MORNING/EARLY SATURDAY AFTERNOON. MODELS ARE NOW  
SHOWING MOST ACTIVE CONVECTION WITH THIS FEATURE CLUSTERING ALONG  
THE BASE OF THE INVERTED TROUGH TO THE NORTH OF THE ISLANDS. AS A  
RESULT...IT SHOULD NOT BE AS INTENSE AS WHAT WE PREVIOUSLY  
FORECASTED. BUT NEVERTHELESS...AS DEEPER MOISTURE ADVECT ACROSS  
FORECAST AREA DIURNAL CONVECTION IS TO THEN CLUSTER ACROSS  
INTERIOR PORTIONS OF PUERTO RICO TO FAVOR LIGHT TO MODERATE  
RAINFALL AMOUNTS. TRAILING MOISTURE WILL SUSTAIN A SECOND ROUND OF  
CONVECTION ON SUNDAY.  
 
LATER DURING THE WEEKEND...AS THE MID LEVEL RIDGE OVER THE ISLANDS  
COLLAPSES...A TUTT OVER THE EASTERN TROPICAL ATLANTIC IS TO  
GRADUALLY PULL TO THE NORTHEAST. AS THE SOURCE OF EASTERLY WAVES  
MOVES AWAY...EXPECTING A DECREASE IN ACTIVITY EARLY NEXT  
WEEK...WITH A DRYER AIR MASS TO ENTRAIN ACROSS THE NORTHEAST  
CARIBBEAN ISLES ON MONDAY-TUESDAY. THIS WILL THEN LEAD TO FAIR  
WEATHER CONDITIONS ACROSS THE VIRGIN ISLES AND MOST OF PUERTO  
RICO.  
 
LONG RANGE FORECAST: DEEPENING POLAR TROUGH TO THE NORTH OF THE  
ISLANDS IS TO THEN INDUCED AN INVERTED/PREFRONTAL TROUGH ACROSS  
THE CENTRAL CARIBBEAN. IN RESPONSE TO THIS FEATURE THE WINDS OVER  
THE EASTERN CARIBBEAN ARE TO VEER TO THE SOUTH-SOUTHEAST. THIS IS  
TO THEN FAVOR A WARM/MOIST ADVECTIVE PATTERN. HOWEVER...IN  
CONTRAST TO PREVIOUS RUNS...MODELS ARE NOW SHOWING BEST MOISTURE  
CONVERGENCE WITH THIS FEATURE OVER HISPANIOLA. IN A SOUTHERLY FLOW  
HOT AND MUGGY CONDITIONS ARE TO THEN PREVAIL ACROSS PUERTO RICO  
WITH UNSEASONABLY WARM TEMPERATURES TO THE NORTH OF THE  
CORDILLERA. IN INTERACTION WITH SEA BREEZE CONVERGENCE...THIS IS  
TO THEN FAVOR MODERATE RAINFALL AMOUNTS OVER NORTHERN PORTIONS OF  
THE ISLAND IN EARLY AFTERNOON CONVECTION.  
 
DAVISON...WPC (USA)  
 

 
 
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