549  
FXCA20 KWBC 191909  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
209 PM EST TUE DEC 19 2017  
 
TROPICAL DISCUSSION FROM DEC 19/12 UTC: UNDER THE INFLUENCE OF A  
ROBUST MID-LEVEL RIDGE ACROSS MOST OF THE CARIBBEAN...FAIR WEATHER  
CONDITIONS ARE EXPECTED TO PERSIST THROUGH THE FORECAST CYCLE. A  
FEATURE OF INTEREST IS AN UPPER TROUGH FORECAST TO DIG FROM THE  
NORTH ATLANTIC INTO THE NORTHEASTERN CARIBBEAN DURING CHRISTMAS  
WEEKEND. UNDER THE INFLUENCE OF A WEAK DIVERGENT PULSE OF THE  
MADDEN-JULIAN OSCILLATION...THE EVOLUTION OF THIS SYSTEM IS WORTH  
MONITORING. ALTHOUGH MODELS ARE UNCLEAR ABOUT THE AMOUNTS OF  
RAINFALL...THEY ARE ALL SHOWING A PEAK IN PRECIPITATION BETWEEN  
DECEMBER 24TH AND DECEMBER 26TH IN AREAS OF THE NORTHEASTERN  
CARIBBEAN.  
 
IN MEXICO...MODELS ARE SUGGESTING ANOTHER ROBUST UPPER TROUGH  
APPROACHING FROM THE NORTHWEST ON THURSDAY-FRIDAY. THIS TROUGH  
WILL REVIVE THE SUBTROPICAL MOIST PLUME OBSERVED DURING LAST  
WEEKEND. THE PLUME IS FORECAST TO REFORM ON THURSDAY WHEN IT WILL  
EXTEND FROM THE TROPICAL PACIFIC INTO SINALOA/NAYARIT. THIS WILL  
RESULT IN A GRADUAL INCREASE IN SHOWERS IN PORTIONS OF THE SIERRA  
MADRE OCCIDENTAL OF SINALOA ON THURSDAY...DECREASING LATE ON  
FRIDAY. THIS WILL PRODUCE ACCUMULATIONS OF 05-10MM/DAY AND MAXIMA  
OF 15MM ON BOTH DAYS.  
 
IN THE CARIBBEAN BASIN...QUIET PATTERN IS TO PERSIST THROUGH THE  
REST OF THE WORK WEEK. AT UPPER LEVELS...A LARGE AND PERSISTENT  
RIDGE IS FORECAST TO CONTINUE CENTERING OVER THE  
SOUTHERN/SOUTHWESTERN CARIBBEAN WHILE EXTENDING ACROSS MOST OF THE  
BASIN. AT MID-LEVELS...A ROBUST RIDGE HAS ESTABLISHED OVER  
CUBA/FLORIDA STRAIGHT/CAYMAN ISLANDS/YUCATAN PENINSULA...AND COVER  
ALL OF THE CARIBBEAN...CENTRAL AMERICA AND NORTHERN SOUTH AMERICA.  
THE MID-LEVEL RIDGE CONTINUES STRENGTHENING THE SUBSIDENCE  
INVERSION...ESPECIALLY OVER THE NORTHERN CARIBBEAN ISLES.  
PRECIPITABLE WATER HAS DECREASED SIGNIFICANTLY IN THE  
BAHAMAS/NORTHERN CARIBBEAN. THIS FAVORS MOSTLY FAIR  
CONDITIONS...AND ANY ACTIVITY WILL BE EXPECTED IN THE FORM OF VERY  
LIGHT TRADE WIND SHOWERS. SHOWERS ARE TO CLUSTER IN EASTERN  
CUBA/NORTHEASTERN HISPANIOLA AND NORTHEASTERN JAMAICA EARLY IN THE  
CYCLE...DECREASING IN COVERAGE AND INTENSITY AS THE CYCLE  
PROGRESSES.  
 
IN CENTRAL AMERICA...HOWEVER...RAINFALL ACCUMULATIONS WILL BE  
LARGER DUE TO INTERACTIONS BETWEEN THE TRADES AND OROGRAPHY. THE  
CARIBBEAN LOW-LEVEL JET HAS REESTABLISHED...AND IS ENHANCING  
MOISTURE CONVERGENCE BETWEEN WESTERN PANAMA AND EASTER HONDURAS.  
LARGEST ACCUMULATIONS ARE EXPECTED TO OCCUR IN EASTERN  
NICARAGUA/EASTERN HONDURAS ON TUESDAY...WHERE NUMEROUS SHOWERS AND  
ISOLATED EMBEDDED THUNDERSTORMS WILL LEAD TO 05-10MM/DAY AND  
MAXIMA 15-20MM. IN COSTA RICA/WESTERN PANAMA EXPECTING ISOLATED  
MAXIMA OF 15MM. BY WEDNESDAY-THURSDAY...ACCUMULATIONS INCREASE TO  
05-10MM/DAY AND MAXIMA OF 15-25MM IN COSTA RICA/PANAMA/SOUTHERN  
NICARAGUA...WHILE DECREASING TO MAXIMA OF 15MM IN  
NICARAGUA/EASTERN HONDURAS. BY THURSDAY-FRIDAY...EXPECTING A  
FURTHER DECREASE IN WESTERN PANAMA TO AMOUNTS OF 05-10MM/DAY AND  
ISOLATED MAXIMA OF 15-20MM. IN NICARAGUA EXPECTING ONCE AGAIN  
MAXIMA OF 15MM. ISOLATED SHOWERS ARE EXPECTED ALSO FURTHER NORTH  
INTO NORTHERN HONDURAS/BELIZE/GUATEMALA...YET ACCUMULATIONS ARE TO  
REMAIN UNDER 10MM/DAY WHERE SHOWERS OCCUR.  
 
IN THE NORTHERN VENEZUELA/LESSER ANTILLES...LIGHT TRADE WIND  
SHOWERS WILL CONTINUE THROUGH THE CYCLE TO PRODUCE MAXIMA UNDER  
10MM/DAY. LARGER ACCUMULATIONS ARE EXPECTED IN THE GUIANAS...WHERE  
A DEEPER MOIST LAYER EXISTS. THIS WILL LEAD TO DAILY ACCUMULATIONS  
OF 05-10MM/DAY AND MAXIMA OF 15-25MM. IN WESTERN COLOMBIA...UNDER  
THE INFLUENCE OF AN ACTIVATING EASTERN PACIFIC ITCZ...EXPECTING  
SHOWERS AND ISOLATED THUNDERSTORMS TO CONTINUE AND INCREASE IN  
INTENSITY BY THE LATE WORK WEEK. INITIALLY...ACCUMULATIONS WILL BE  
IN THE 05-10MM/DAY AND MAXIMA OF 15-20MM RANGE ACROSS NORTHERN  
ECUADOR INTO CENTRAL COLOMBIA. THIS WILL INCREASE TO 10-15MM/DAY  
AND MAXIMA OF 20-30MM FROM WEDNESDAY THROUGH FRIDAY.  
 
TROPICAL/EASTERLY WAVES INITIALIZED AT 12UTC:  
INITIAL 24 36 48 60 72 84 96 TYPE  
SOF  
49W 52W 55W 58W 61W 64W DISSIPATES TUTT INDCD  
22N  
85W 88W 91W 94W 97W 100W 103W 106W EW  
17N  
 
THE UPPER TROUGH IN THE CENTRAL TROPICAL ATLANTIC IS INDUCING A  
WAVE IN THE TRADES. THIS WAVE IS INITIALIZED AT 49W AND TO THE  
SOUTH OF 22N. THE WAVE IS FORECAST TO PROPAGATE WEST INTO THE  
BASIN AS THE FORECAST CYCLE PROGRESSES. BY TUESDAY EVENING...THE  
WAVE WILL APPROACH FRENCH GUIANA WHERE IT WILL SUSTAIN 05-10MM/DAY  
AND MAXIMA OF 15-25MM. BY WEDNESDAY EVENING...IT WILL LIE JUST TO  
THE EAST OF THE ARC OF THE ANTILLES/NORTHEASTERN VENEZUELA. UNDER  
THE ABSENCE OF A DEEP MOIST LAYER...RAINFALL MAXIMA WILL REMAIN  
UNDER 10MM/DAY. SIMILAR RAINFALL ACCUMULATIONS ARE EXPECTED ON  
THURSDAY EVENING...WHEN THE WAVE IS FORECAST TO EXTEND ACROSS  
CENTRAL VENEZUELA. NOTE THAT MODELS ARE ALL SHOWING THE WAVE  
LOOSING DEFINITION DURING THURSDAY.  
 
AN ILL-DEFINED EASTERLY WAVE IS INITIALIZED AT 85W AND TO THE  
SOUTH OF 17N. THIS WAVE IS RAPIDLY CROSSING CENTRAL AMERICA  
PRODUCING LIMITED EFFECTS ON PRECIPITATION.  
 
GORDON...NMS (BELIZE)  
PECK...MS (JAMAICA)  
RIVAS...SENAMHI (PERU)  
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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