646  
FXCA20 KWBC 211147  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
647 AM EST THU DEC 21 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 21/06  
UTC: BROAD MID/UPPER LEVEL RIDGE CONTINUES TO DOMINATE THE FLOW  
ACROSS THE CARIBBEAN BASIN...WHERE IT SUSTAINS A LARGE SCALE  
SUBSIDENCE PATTERN THAT IS INHIBITING ORGANIZED DEEP CONVECTION.  
UNDER INFLUENCE OF THIS RIDGE...A TUTT IS TO AMPLIFY ACROSS THE  
CENTRAL ATLANTIC LATER ON FRIDAY...TO EVOLVE INTO A CLOSED LOW TO  
THE EAST-NORTHEAST OF THE LEEWARD ISLES BY MIDMORNING ON SATURDAY.  
THIS IS TO THEN EXTEND A SHORT WAVE TROUGH TO THE WEST-SOUTHEAST  
ACROSS THE LEEWARD ISLES TO THE NORTHEAST CARIBBEAN. THE UPPER  
TROUGH IS TO THEN GRADUALLY WEAKEN/NEARLY DISSIPATE BY MIDWEEK.  
ALTHOUGH THE TROUGH AMPLIFIES ACROSS THE NORTHEAST CARIBBEAN...THE  
FORECAST AREA IS TO GENERALLY REMAIN ON THE CONVERGENT/SUBSIDENT  
SIDE OF THIS FEATURE THROUGH CHRISTMAS DAY.  
 
AT LOW LEVELS...A HIGH NORTH OF THE ISLANDS SUSTAINS A TIGHT  
PRESSURE GRADIENT AND BRISK NORTHEASTERLY TRADES ACROSS THE  
FORECAST AREA. CLUSTERS OF SHALLOW CONVECTION EMBED IN THIS  
FLOW...STREAMING ACROSS PUERTO RICO AND THE VIRGIN ISLES FROM TIME  
TO TIME WHILE TRIGGERING MOSTLY LIGHT RAINFALL AMOUNTS. UNDER  
PRESSURE FROM POLAR FRONT STREAMING TO THE NORTH...THE LOW LEVEL  
RIDGE IS TO COLLAPSE EARLY IN THE CYCLE. AS THE GRADIENT  
SLACKENS...LIGHTER WINDS ARE TO THEN ENVELOP THE FORECAST AREA  
DURING THE DAY TODAY AND FRIDAY. THE LIGHTER WINDS WILL ALLOW FOR  
DIURNAL HEAT CYCLE TO TAKE OVER ACROSS THE FORECAST AREA...WITH  
AIR MASS CONVECTION TO LIKELY BUILD ACROSS SAINT CROIX. LIKEWISE  
ACROSS PUERTO RICO WITH A BETTER CHANCE FOR HIGHER RAINFALL  
AMOUNTS AS ENHANCED BY SEA BREEZE CONVERGENCE. ON SATURDAY-MONDAY  
(CHRISTMAS DAY)...A SECONDARY RIDGE BUILDS FARTHER TO THE NORTH  
BEHIND THE MEANDERING SURFACE FRONT. AS THE GRADIENT TIGHTENS 850  
HPA WINDS ARE TO THEN CLOCK AT 15-20KT. THIS WILL BE STRONG ENOUGH  
TO FAVOR GENERATION OF STREAMERS ACROSS THE VIRGIN ISLES THAT ARE  
TO AFFECT EASTERN PUERTO RICO DURING THE AFTERNOON HOURS.  
FURTHERMORE...AS THE TUTT LOW ALOFT MEANDERS EAST OF THE LEEWARD  
ISLES...IT IS TO GENERATE CLOUD CLUSTERS THAT ARE TO THEN ADVECT  
IN THE PREVAILING EASTERLY TRADES ACROSS THE NORTHEAST  
CARIBBEAN...TO CONTINUE FAVORING PERIODS OF SCATTERED SHOWERS AND  
LIGHT/LOCALLY MODERATE RAINFALL AMOUNTS.  
 
MODELS THEN DIVERGE ON HOW THE REMNANTS OF THE MID LEVEL VORTEX IS  
TO EVOLVE ON TUESDAY-WEDNESDAY...WITH ECMWF BRINGING A STRONGER  
MID LEVEL PERTURBATION ACROSS THE LESSER ANTILLES TO THE NORTHEAST  
CARIBBEAN BY MIDMORNING ON TUESDAY. THE GFS FAVORS A SIMILAR  
SOLUTION BUT A DAY LATER. AS IT ENTERS THE EASTERN CARIBBEAN  
EXPECTING DEEPER INSTABILITY AS TRADE WINDS CAP RELAXES ITS  
FOOTHOLD ACROSS THE FORECAST AREA. HOWEVER...UNTIL THE MODELS  
RESOLVE THEIR TIMING ISSUES...THE CONFIDENCE IN THE FORECAST  
BEYOND 120-132 HRS WILL REMAIN LOW.  
 
DAVISON...WPC (USA)  
 

 
 
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