166  
FXCA20 KWBC 151913  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
213 PM EST FRI DEC 15 2017  
 
TROPICAL DISCUSSION FROM DEC 15/12 UTC: FAIR CONDITIONS ARE TO  
CONTINUE ACROSS MOST OF THE CARIBBEAN BASIN IN ASSOCIATION WITH  
THE BUILDING OF A ROBUST MID-LEVEL RIDGE. MODERATE RAINFALL WILL  
BE CONSTRAINED TO PARTS OF CENTRAL/NORTHERN MEXICO THROUGH THE  
FORECAST CYCLE.  
 
NOTE: MODELS ARE STARTING TO SHOW A STRONG UPPER TROUGH DIGGING  
INTO THE NORTHEASTERN CARIBBEAN BY DECEMBER 23-25TH. ALTHOUGH IT  
IS TOO EARLY TO HAVE CONFIDENCE IN THIS FORECAST...IT IS  
WORTHWHILE TO START MONITORING THE POTENTIAL FOR THIS EVOLUTION.  
 
IN MEXICO...A LARGE POLAR TROUGH IS DIGGING FROM THE NORTH TO  
EXTEND ACROSS WEST TEXAS-CHIHUAHUA-SOUTHERN SONORA-BAJA CALIFORNIA  
BY FRIDAY EVENING...TO THEN START LIFTING TO THE NORTHEAST AND  
EXTEND ACROSS WEST TEXAS-COAHUILA-EASTERN DURANGO BY SATURDAY  
EVENING. THE UPPER TROUGH IS INTERACTING WITH A SUBTROPICAL  
MOISTURE PLUME/ATMOSPHERIC RIVER THAT ORIGINATES IN THE TROPICAL  
PACIFIC...AND IS LEADING TO PRECIPITABLE WATER AMOUNTS OVER 45MM  
OFF THE COAST OF JALISCO AND NAYARIT. MODERATE PRECIPITATION HAS  
ALREADY STARTED IN PARTS OF WESTERN MEXICO...AND IS FORECAST TO  
PEAK THROUGH EARLY SATURDAY. DURING THIS PERIOD...ADDITIONAL  
ENHANCEMENT WILL BE PROVIDED BY THE DIVERGENT ENTRANCE OF AN UPPER  
JET. THROUGH SATURDAY...EXPECTING ACCUMULATIONS OF 15-20MM/DAY AND  
MAXIMA OF 25-50MM FROM SINALOA/NAYARIT NORTH INTO COAHUILA AND SAN  
LUIS POTOSI. BY SATURDAY-SUNDAY EXPECTING A DECREASE IN  
ACCUMULATIONS...TO 10-15MM/DAY AND MAXIMA OF 20-35MM ACROSS  
NAYARIT/JALISCO...AND 05-10MM/DAY AND MAXIMA OF 15MM IN  
COAHUILA/SAN LUIS POTOSI. BY SUNDAY-MONDAY...EXPECTING REMNANT  
RAIN SHOWERS IN NAYARIT AND JALISCO...TO PRODUCE MAXIMA UNDER  
10MM/DAY.  
 
ALSO IN MEXICO...A SECOND POTENT TROUGH WILL BE SUFFICIENTLY  
STRONG TO STIMULATE PRECIPITATION IN THE SONORAN DESERT.  
ACCUMULATIONS WILL START ON SATURDAY...WHEN EXPECTING 05-10MM/DAY  
AND ISOLATED MAXIMA OF 15MM IN PARTS OF SONORA. THE EVENT WILL  
PEAK ON SUNDAY-MONDAY...WHEN EXPECTING 10-15MM/DAY AND MAXIMA OF  
20-40MM. SNOWFALL IS ALSO EXPECTED WITH THIS EVENT...BUT WILL BE  
CONSTRAINED TO HIGH PEAKS IN THE SIERRA MADRE OCCIDENTAL.  
 
IN THE MEANTIME...THE REMNANTS OF THE SURFACE FRONT IN THE  
NORTHERN CARIBBEAN WILL LEAD TO LIGHT RAIN SHOWERS ACROSS THE  
CENTRAL BAHAMAS...CENTRAL CUBA...WHERE EXPECTING MAXIMA UNDER  
10MM/DAY. ALSO...REMNANT SHOWERS ARE ARRIVING INTO THE YUCATAN  
PENINSULA/BELIZE/NORTHERN GUATEMALA...WHERE EXPECTING 05-10MM/DAY  
AND ISOLATED MAXIMA OF 15MM. THE BOUNDARY WILL DISSIPATE BY EARLY  
SATURDAY.  
 
IN CENTRAL AMERICA...THE RETURN OF MOIST EASTERLY TRADES WILL ONCE  
AGAIN ESTABLISH A PATTERN OF TRADE WIND SHOWERS IN EASTERN COASTS  
AND SLOPES. YET...UNDER THE INFLUENCE OF BUILDING MID-LEVEL  
RIDGE...CONVECTION WILL STRUGGLE TO BREAK THE SUBSIDENCE CAP...AND  
THUNDERSTORMS WILL GENERALLY BE VERY ISOLATED. THE STRENGTHENING  
TRADE WIND SUBSIDENCE CAP WILL ALSO LIMIT THE POTENTIAL RAINFALL  
ACCUMULATIONS. THROUGH EARLY SATURDAY...THE ARRIVAL OF A  
FAST-PROPAGATING EASTERLY WAVE WILL LEAD TO AMOUNTS OF 05-10MM/DAY  
AND ISOLATED MAXIMA OF 15-20MM IN EXTREME NORTHEASTERN  
HONDURAS/NICARAGUA...WHILE MAXIMA UNDER 15MM IS EXPECTED IN  
EASTERN COSTA RICA/NORTHERN PANAMA...AND INLAND PORTIONS OF  
HONDURAS AND NICARAGUA. LATER IN THE CYCLE...AS A CORRIDOR OF  
ENHANCED TRADE WIND CONVERGENCE ESTABLISHES FROM THE LEEWARD  
ISLANDS INTO HONDURAS/NICARAGUA...EXPECTING PERSISTENT TRADE WIND  
SHOWERS TO PRODUCE DAILY ACCUMULATIONS OF 05-10MM/DAY AND MAXIMA  
OF 15MM BETWEEN NORTHERN COSTA RICA AND NORTHEASTERN HONDURAS.  
FURTHER NORTH ACROSS BELIZE/GUATEMALA...A PEAK IN ACCUMULATIONS IS  
EXPECTED ON SATURDAY-EARLY SUNDAY...WHEN LOW-LEVEL JET CONVERGENCE  
AND OROGRAPHIC INTERACTIONS WILL LEAD TO 05-10MM/DAY AND MAXIMA OF  
15-25MM IN SOUTHERN BELIZE/CENTRAL GUATEMALA. OTHERWISE...MAXIMA  
WILL BE UNDER 15MM.  
 
ACROSS THE EASTERN CARIBBEAN...ALTHOUGH SUBSIDENCE CAP CONTINUES  
STRENGTHENING...THE INCREASE IN THE SPEED OF THE TRADES IS  
STIMULATING EVAPORATION AND MOISTENING OF THE BOUNDARY LAYER. THIS  
IS AIDING WITH THE DEVELOPMENT OF TRADE WIND SHOWERS ACROSS THE  
REGION. MODELS ARE CONFIDENT ABOUT A CORRIDOR OF ENHANCED TRADE  
WIND SHOWERS ESTABLISHING FROM THE CENTRAL ANTILLES INTO THE  
LEEWARD ISLANDS EARLY IN THE CYCLE. THIS WILL ALLOW FOR DAILY  
ACCUMULATIONS UNDER 10MM/DAY. QUIET PATTERN WILL ALSO CONTINUE IN  
NORTHERN SOUTH AMERICA. LARGEST ACCUMULATIONS ARE EXPECTED IN  
FRENCH GUIANA AND SOUTHERN PARTS OF SURINAME UNDER NORTHEASTERLY  
TRADES/ATLANTIC ITCZ CONVERGENCE. EXPECTING 10-15MM/DAY AND MAXIMA  
OF 20-30MM THROUGH EARLY SUNDAY...DECREASING TO 05-10MM/DAY AND  
MAXIMA OF 15-20MM AFTER.  
 
TROPICAL/EASTERLY WAVES INITIALIZED AT 12UTC:  
INITIAL 24 36 48 60 72 84 96 TYPE  
SOF  
81W 85W 89W 94W 97W 101W 104W DISSIPATES EW  
17N  
 
AN EASTERLY WAVE IS INITIALIZED AT 81W. THIS WAVE HAS GAINED  
DEFINITIONS...AND IS PROPAGATING WESTWARD VERY RAPIDLY. IT IS  
FORECAST TO ENHANCE SHOWERS IN COSTA RICA/NICARAGUA/HONDURAS ON  
FRIDAY-SATURDAY...IN BELIZE/GUATEMALA DURING THE DAY ON SATURDAY  
AND INTO EARLY SUNDAY...TO THEN PRODUCE LIMITED EFFECTS ON  
CONTINENTAL CONVECTION.  
 
GORDON...NMS (BELIZE)  
PECK...MS (JAMAICA)  
RIVAS...SENAMHI (PERU)  
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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