770  
FXCA20 KWBC 261138  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
638 AM EST SUN NOV 26 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO FROM NOV 26/06 UTC: GLOBAL  
MODELS FOLLOW SIMILAR EVOLUTION THROUGH 96-108 HRS...THEN DIVERGE  
ON AMPLITUDE OF MID LEVEL TROUGH ENTERING THE FORECAST AREA LATER  
IN THE WEEK.  
 
EARLY IN THE CYCLE THE MID/UPPER LEVEL TROUGH EXTENDED OVER THE  
SOUTHEAST USA-GULF OF MEXICO...WITH AXIS PRESSING AGAINST A BROAD  
CELL OF THE SUBEQUATORIAL RIDGE TO THE EAST OVER THE  
CARIBBEAN/WESTERN ATLANTIC. THE MID LEVEL RIDGE FAVORS A FAIRLY  
STRONG TRADE WINDS CAP ACROSS THE FORECAST AREA THAT IS INHIBITING  
ORGANIZED DEEP CONVECTION. THIS PATTERN IS TO HOLD DURING THE NEXT  
48-60 HRS. EARLY ON WEDNESDAY THE TROUGH STALLS TO PULL OVER THE  
SUBEQUATORIAL RIDGE...FAVORING EROSION OF RIDGE PATTERN TO THE  
NORTH OF 20N. AS THE RIDGE WEAKENS...THE LOW LEVEL TRADE WINDS  
CAPS RELAXES ITS FOOTHOLD ACROSS THE FORECAST AREA...WITH AN  
ELEVATED INVERSION TO THEN ESTABLISH ON WEDNESDAY AND ONWARD. ON  
FRIDAY-SATURDAY THE GFS SHOWS THE MID LEVEL TROUGH DEEPENING INTO  
THE EASTERN CARIBBEAN...WHILE THE ECMWF FORECASTS THE BASE OF THE  
TROUGH TO BOTTOM OUT OVER THE FORECAST AREA. REGARDLESS OF THESE  
DIFFERENCES...BOTH THE GFS AND ECMWF FORECAST THE CAP INVERSION TO  
WEAKEN ENOUGH TO ALLOW FOR PLUME OF TROPICAL MOISTURE TO ADVECT  
ACROSS THE FORECAST AREA...WITH PWAT CONTENT PEAKING BETWEEN  
1.50-1.75 INCHES ON WEDNESDAY-FRIDAY. HOWEVER...THE IMPACT THIS IS  
TO HAVE WILL BE HIGHLY DEPENDENT ON THE INTENSITY OF THE  
TROUGH...WHICH LEADS TO A LOWER CONFIDENCE IN THE FORECAST LATER  
IN THE WEEK.  
 
AT LOW LEVELS...EAST-SOUTHEASTERLY TRADES PREVAIL. THIS PATTERN IS  
TO GENERALLY HOLD DURING THE NEXT FEW DAYS. EMBEDDED IN THIS  
FLOW...A WEEK PERTURBATION IN THE EASTERLY TRADES MOVES ACROSS THE  
FORECAST AREA ON TUESDAY. PLUME OF TROPICAL MOISTURE TRAILS THIS  
WAVE...POOLING ACROSS THE FORECAST AREA ON WEDNESDAY  
MORNING-FRIDAY EVENING. ALSO...AS THE MID/UPPER LEVEL TROUGH LIFTS  
ACROSS THE WESTERN ATLANTIC BY THE MIDDLE OF THE WEEK...IT IS TO  
INDUCE A LOW LEVEL CLOSE CYCLONIC CIRCULATION/SHORT WAVE TROUGH TO  
THE NORTH OF THE ISLANDS. THIS...IN-TURN...WILL THEN FAVOR AN  
INDUCED/INVERTED TROUGH OVER THE EASTERN CARIBBEAN. IN RESPONSE TO  
THIS FEATURE...THE LOW LEVEL WINDS ARE TO VEER FARTHER TO THE  
SOUTHEAST LATER ON THURSDAY/EARLY ON FRIDAY MORNING. THIS IS TO  
SUSTAIN A MOIST ADVECTIVE PATTERN ACROSS THE EASTERN CARIBBEAN TO  
PUERTO RICO/VIRGIN ISLES AND THE DOMINICAN REPUBLIC...TO HELP  
SUSTAIN THE HIGHER PWAT CONTENT ACROSS THE FORECAST AREA.  
 
OVER THE NEXT COUPLE OF DAYS...AS A STRONG TRADE WINDS CAP  
HOLDS...EXPECTING FAIR WEATHER CONDITIONS TO PREVAIL ACROSS THE  
ENTIRE FORECAST AREA. SHOWERS...IF ANY...WILL GENERALLY LIMIT TO  
EL YUNQUE RAINFOREST/EASTERN PORTIONS OF PUERTO RICO IN MORNING  
AND AFTERNOON CONVECTION...AND ACROSS NORTHWEST PUERTO RICO IN  
AFTERNOON CONVECTION. ON WEDNESDAY TO THURSDAY EXPECTING A GRADUAL  
INCREASE ACROSS THE FORECAST AREA. IN LIGHT STEERING WINDS...AIR  
MASS CONVECTION ACROSS PUERTO RICO ON THURSDAY WILL BECOME  
SOMEWHAT MORE WIDESPREAD THAN ON WEDNESDAY. SEE QPF CHARTS FOR  
DETAIL.  
 
DAVISON...WPC (USA)  
 

 
 
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