012  
FXCA20 KWBC 301154  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
653 AM EST SAT DEC 30 2017  
 
LAST NO AMENDS...NEXT FORECAST ON JANUARY 02...HAPPY NEW YEAR  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 30/06  
UTC: BROAD POLAR TROUGH REMAINS THE DOMINANT FEATURE ACROSS THE  
EASTERN USA TO THE WESTERN ATLANTIC. SOUTH OF THIS TROUGH...A  
WANING MID LEVEL RIDGE EXTENDS ACROSS HISPANIOLA-PUERTO  
RICO-LEEWARD ISLES. UNDER PRESSURE FROM POLAR TROUGH TO THE NORTH  
THE MID LEVEL RIDGE IS TO RAPIDLY WEAKEN DURING THE DAY  
TODAY...WITH REMNANTS OF THIS RIDGE RELOCATING TO THE SOUTHEAST  
CARIBBEAN-NORTHERN VENEZUELA LATER THIS EVENING. AS THE RIDGE  
WEAKENS AN ELEVATED TRADE WINDS CAP IS TO THEN PERSIST OVER THE  
NORTHEAST CARIBBEAN DURING THE NEXT FEW DAYS. ON TUESDAY-WEDNESDAY  
THE POLAR TROUGH IS TO AMPLIFY ACROSS THE BAHAMAS-CUBA TO THE  
WESTERN CARIBBEAN...GRADUALLY TIGHTENING THE GRADIENT WHILE  
SIMULTANEOUSLY FAVORING INTENSIFICATION OF A JET MAXIMA NORTH OF  
THE GREATER ANTILLES. ALSO...AS THE POLAR TROUGH DEEPENS...A RIDGE  
WILL THEN BUILD OVER THE SOUTHEAST CARIBBEAN/ISLAND CHAIN INTO THE  
CENTRAL ATLANTIC.  
 
AT LOW LEVELS...A TUTT INDUCED EASTERLY WAVE MOVES ACROSS THE  
FORECAST AREA DURING THE DAY TODAY...ADVECTING SHALLOW MOISTURE  
ACROSS THE VIRGIN ISLES EARLY THIS MORNING AND ACROSS PUERTO RICO  
THROUGH THE DAY. ALTHOUGH THE WAVE QUICKLY PULLS AWAY...A TRAILING  
MOIST TONGUE IS TO REMAIN ACROSS THE FORECAST AREA THROUGH AT  
LEAST MONDAY MORNING. UNDER INFLUENCE OF THIS FEATURE EXPECTING  
ANOTHER ROUND OF SHOWERS DURING THE AFTERNOON HOURS ON SATURDAY.  
LIGHT EASTERLY TRADES WILL ALLOW FOR SHOWERS TO LINGER AS THEY  
CLUSTER WHILE BUILDING ACROSS INTERIOR PORTIONS OF PUERTO RICO.  
OVER THE NEXT FEW DAYS...THE CLIMATE FORECAST SYSTEM SHOWS A  
PATTERN OF VELOCITY POTENTIAL ANOMALIES SUPPORTIVE OF FAVORABLE  
MJO CONDITIONS ACROSS THE FORECAST AREA. IN THIS PATTERN EXPECTING  
MODERATE RAINFALL AMOUNTS...WITH POTENTIAL FOR LOCALLY HEAVY  
CONVECTION ACROSS THE GUAJATACA BASIN TO PEAK AT 1-2 INCHES. ON  
SUNDAY...FOLLOWING WAVE PASSAGE...LOW LEVEL WINDS ARE TO VEER TO  
THE EAST-SOUTHEAST. THIS...IN-TURN...WILL FOCUS DIURNAL CONVECTION  
ACROSS NORTHWEST PORTIONS OF PUERTO RICO...BETWEEN  
BARCELONETA/ARECIBO AND AGUADA. BUT AS THE MOST ACTIVE CONVECTION  
IS TO CLUSTER FARTHER TO THE NORTH...ONLY EXPECTING RAINFALL  
AMOUNTS OF 0.25-0.50 OF AN INCH ACROSS NORTHWEST PUERTO RICO IN  
SCATTERED CONVECTION.  
 
THE GLOBAL MODELS THEN AGREE ON A DRYER AIR MASS TO RAPIDLY  
ENTRAIN ACROSS THE FORECAST AREA DURING THE DAY ON MONDAY...WITH  
THE EUROPEAN MODEL FAVORING A CONSIDERABLE DRYER AIR MASS THAN  
WHAT THE GFS SUGGESTS. DURING THIS PERIOD THE ECMWF HAS A PWAT  
MINIMA OF AROUND 0.50 OF AN INCH...MUCH LOWER THAN WHAT THE GFS IS  
FORECASTING. THIS WILL BE ACCOMPANIED BY A STRONGER TRADE WINDS  
CAP...TO GENERALLY FAVOR FAIR WEATHER CONDITIONS EXPECTED BETWEEN  
MONDAY AFTERNOON AND WEDNESDAY.  
 
DURING THE LONG RANGE PERIOD...THE DEEPENING POLAR TROUGH REACHES  
MAXIMUM AMPLITUDE BY MID WEEK...WITH ASSOCIATED FRONT PUSHING  
ACROSS THE BAHAMAS-CUBA TO NORTHERN HISPANIOLA EARLY ON THURSDAY.  
IN RESPONSE TO THE HIGH AMPLITUDE FRONT/POLAR TROUGH PATTERN TO  
THE NORTH OF THE ISLANDS...AN INDUCED/INVERTED TROUGH IS TO THEN  
EXTEND NORTH ACROSS THE CENTRAL CARIBBEAN. THIS IS TO THEN FAVOR A  
DEEP LAYER SOUTH-SOUTHEASTERLY FLOW ACROSS THE FORECAST AREA. THE  
ECMWF IS MORE AGGRESSIVE THAN THE GFS WITH THIS FEATURE...WITH  
WINDS VEERING NEARLY 24 HRS EARLIER THAN WHAT THE GFS PROJECTS.  
THIS IS TO FAVOR A MOIST ADVECTIVE PATTERN ACROSS THE FORECAST  
AREA...TO RESULT IN UNSEASONABLY WARM AND STUFFY CONDITIONS ACROSS  
THE FORECAST AREA. ALTHOUGH MOISTURE CONTENT IS TO INCREASE...BEST  
FORCING WITH THIS FEATURE IS EXPECTED ACROSS HISPANIOLA...WHERE IT  
IS TO SUSTAIN HEAVY CONVECTION.  
 
DAVISON...WPC (USA)  
 

 
 
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