584  
FXCA20 KWBC 181931  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
231 PM EST MON DEC 18 2017  
 
TROPICAL DISCUSSION FROM DEC 18/12 UTC: FAIR WEATHER CONDITIONS  
AND VERY LIGHT RAINFALL ACCUMULATIONS ARE EXPECTED ACROSS THE  
BASIN THROUGH THE FORECAST CYCLE. THIS IS BECAUSE OF THE INFLUENCE  
OF A LARGE AND ROBUST MID-LEVEL RIDGE THAT IS FORECAST TO EXTEND  
ACROSS MOST OF THE NORTHERN-CENTRAL CARIBBEAN THROUGH THE CYCLE.  
 
AT UPPER LEVELS...A LARGE AND PERSISTENT RIDGE IS FORECAST TO  
CONTINUE EXTENDING OVER MOST OF THE CARIBBEAN WHILE CENTERING ON A  
HIGH OVER NORTHERN COLOMBIA/SOUTHERN CARIBBEAN. UPPER TROUGHS WILL  
MEANDER ALONG THE PERIPHERY OF THIS RIDGE. IN MEXICO...THESE WILL  
PROPAGATE FROM SOUTHWEST TO NORTHEAST ALONG NORTHWESTERN  
MEXICO/SOUTHWESTERN UNITED STATES. IN THE ATLANTIC OCEAN...THESE  
WILL PROPAGATE FROM NORTHWEST TO SOUTHEAST TOWARDS THE CENTRAL  
TROPICAL ATLANTIC. AT MID-LEVELS MODELS ALL AGREE ON A STRONG  
MID-LEVEL RIDGE STRENGTHENING ACROSS THE NORTHERN CARIBBEAN. THE  
CENTER OF THE RIDGE WILL MEANDER BETWEEN THE EASTERN GULF OF  
MEXICO/FLORIDA/THE BAHAMAS...YET IT WILL EXTEND INTO EQUATORIAL  
LATITUDES OF SOUTH AMERICA. THE RIDGE WILL CONTINUE STRENGTHENING  
THE CURRENT SUBSIDENCE CAP...AND WILL CONTINUE LIMITING ANY  
VERTICAL DEVELOPMENT TO SHALLOW CUMULUS CAPABLE OF PRODUCING ONLY  
LIGHT RAIN SHOWERS. AREAS WITH MODERATE PRECIPITATION IN THE  
FORECAST DOMAIN WILL LIMIT TO THE GUIANAS AND THE CENTRAL AMAZON  
BASIN.  
 
IN MEXICO...A SUBTROPICAL MOISTURE PLUME IS WANING WHILE STILL  
ENHANCING LIGHT RAIN SHOWERS ACROSS SINALOA/NAYARIT INTO  
NORTHEASTERN MEXICO. THIS IN INTERACTION WITH STRONG UPPER TROUGH  
MEANDERING ACROSS NORTHWESTERN MEXICO TOWARDS NEW MEXICO/TEXAS.  
THROUGH EARLY TUESDAY...EXPECTING 05-10MM/DAY AND MAXIMA OF 15MM  
ACROSS SINALOA/NAYARIT...WHILE MAXIMA UNDER 10MM IS EXPECTED  
ELSEWHERE ACROSS ZACATECAS/DURANGO/COAHUILA. A SIGNIFICANT DRYING  
TREND FOLLOWS.  
 
ACROSS THE NORTHERN AND EASTERN CARIBBEAN...UNDER THE INFLUENCE OF  
THE STRONG MID-LEVEL RIDGE...ACTIVITY WILL BE CONSTRAINED TO LIGHT  
RAIN SHOWERS. TRADE WINDS ARE FORECAST TO REMAIN FROM THE  
EAST-NORTHEAST AND ACCELERATED THROUGH THE CYCLE. WIND SPEEDS WILL  
RANGE BETWEEN 15-25KT RANGE TO THE SOUTH OF 22N...AND 10-20KT FROM  
22N TO 25N. THIS WILL CONTINUE FAVORING SHALLOW INSTABILITY AND  
FAST-MOVING LIGHT TRADE WIND SHOWERS ACROSS THE EASTERN AND  
NORTHEASTERN COASTS OF CUBA...HISPANIOLA...PUERTO RICO AND MOST OF  
THE LESSER ANTILLES. RAINFALL ACCUMULATIONS WITH THESE PERSISTENT  
SHOWERS WILL BE UNDER 10MM/DAY. EXCEPTIONS WILL BE ISOLATED HIGHER  
AMOUNTS ALONG NORTHEASTERN COASTS OF HISPANIOLA...WHERE ENHANCED  
OROGRAPHIC UPLIFT WILL LEAD TO LOCALIZED MAXIMA.  
 
UNDER THE STRENGTHENING OF THE TRADES...THE CARIBBEAN LOW-LEVEL  
JET IS INTENSIFYING. MODELS ARE SUGGESTING WIND SPEEDS EXCEEDING  
35-45KT TO THE NORTH OF NORTHERN COLOMBIA/NORTH OF THE  
ABC...STRETCHING WEST INTO THE NICARAGUA COAST. THIS IS TO LAST  
THROUGH WEDNESDAY...WHEN TRADES ARE FORECAST TO WEAKEN  
SLIGHTLY...LEADING TO A WEAKENING OF THE LOW-LEVEL JET TO 25-35KT.  
THE STRONG LOW-LEVEL JET IS FAVORING MOISTURE CONVERGENCE ALONG  
THE CARIBBEAN COAST OF CENTRAL AMERICA...ESPECIALLY FROM WESTERN  
PANAMA ACROSS COSTA RICA INTO NICARAGUA/EASTERN HONDURAS. ALTHOUGH  
A MOIST PLUME IS IN PLACE...THE RAPID MOVEMENT OF THE SHOWERS WILL  
LIMIT POTENTIAL RAINFALL ACCUMULATIONS. THROUGH EARLY  
WEDNESDAY...STREAMING SHOWERS WILL LEAD TO 05-10MM/DAY AND MAXIMA  
OF 15MM ACROSS COSTA RICA AND EASTERN NICARAGUA. BY WEDNESDAY INTO  
THURSDAY...FOCUS OF CONVERGENCE WILL CONCENTRATE IN SOUTHERN  
NICARAGUA/WESTERN PANAMA...WHERE EXPECTING 10-15MM/DAY AND MAXIMA  
OF 20-30MM.  
 
IN NORTHERN SOUTH AMERICA...LARGEST ACCUMULATIONS ARE EXPECTED IN  
THE GUIANAS IN ASSOCIATION WITH ITCZ CONVERGENCE. DEEP-MOIST LAYER  
IS CONSTRAINING TO LATITUDES TO THE SOUTH OF 02N/03N...WHILE A DRY  
AIR MASS CONTINUES IN PLACE IN THE CENTRAL AND NORTHERN GUIANAS  
OVER 500 HPA. THIS WILL CONTINUE LIMITING ACCUMULATIONS TO SUSTAIN  
AMOUNTS OF 10-15MM/DAY AND MAXIMA OF 20-35MM ON  
MONDAY-TUESDAY...DECREASING GRADUALLY ON 05-10MM/DAY AND MAXIMA OF  
15-20MM ACROSS THE SOUTHERN GUIANAS ON WEDNESDAY-THURSDAY. ALSO IN  
NORTHERN SOUTH AMERICA...ENHANCED MOISTURE CONVERGENCE WHERE A  
TRADE WIND MOIST PLUME INTERACTS WITH THE COAST...WILL LEAD TO  
SCATTERED SHOWERS IN TRINIDAD AND TOBAGO/NORTHEASTERN VENEZUELA  
THROUGH THE CYCLE. THIS WILL LEAD TO ACCUMULATIONS OF 05-10MM/DAY  
AND ISOLATED MAXIMA OF 15MM. IN COLOMBIA...ACTIVITY WILL BE  
CONSTRAINED TO THE PACIFIC BASIN...WHERE ITCZ CONVERGENCE WILL  
FAVOR 05-10MM/DAY AND MAXIMA OF 15MM THROUGH EARLY WEDNESDAY. THIS  
IS INCREASING TO 10-15MM/DAY AND MAXIMA OF 20-30MM IN SOUTHWESTERN  
COLOMBIA BY WEDNESDAY-THURSDAY.  
 
TROPICAL/EASTERLY WAVES INITIALIZED AT 12UTC:  
INITIAL 24 36 48 60 72 84 96 TYPE SOF  
80W 82W 85W 87W 90W 93W 96W 100W EW 17N  
 
AN ILL-DEFINED EASTERLY WAVE IS INITIALIZED AT 80W AND TO THE  
SOUTH OF 17N. THIS WAVE WILL RAPIDLY CROSS CENTRAL AMERICA ON  
MONDAY/TUESDAY...PRODUCING LIMITED EFFECTS ON PRECIPITATION.  
 
GORDON...NMS (BELIZE)  
PECK...MS (JAMAICA)  
RIVAS...SENAMHI (PERU)  
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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