498  
FXCA20 KWBC 061943  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
243 PM EST WED DEC 06 2017  
 
TROPICAL DISCUSSION FROM DEC 06/12 UTC: DEEPENING POLAR TROUGH  
OVER THE USA REMAINS THE SYSTEM OF CONCERN. CONSISTENT WITH  
PREVIOUS FORECAST...ON THURSDAY THE TROUGH IS TO DEEPEN ACROSS THE  
CENTRAL USA WHILE TRAILING ACROSS TEXAS TO THE NORTHERN STATES OF  
MEXICO. ON FRIDAY-SATURDAY IT PUSHES FARTHER EAST AND SOUTH INTO  
THE GULF AND THE NORTHERN STATES OF MEXICO. AT LOW LEVELS THE  
TROUGH ALOFT SUSTAINS A SURFACE FRONT ACROSS THE NORTHERN GULF TO  
THE NORTHERN STATES OF MEXICO. THE FRONT IS TO REMAIN NEARLY  
STATIONARY THROUGH THURSDAY MORNING. LATER IN THE DAY...UNDER  
INFLUENCE OF 25-30KT WIND SURGE OVER THE WESTERN GULF...THE FRONT  
MOVES SOUTH ACROSS TAMAULIPAS TO VERACRUZ. ON FRIDAY MORNING THE  
WINDS INCREASE TO 35-40KT...WITH THE FRONT MOVING SOUTH TO THE  
YUCATAN-SOUTHERN MEXICO LATER IN THE DAY. THE ENSUING  
TEHUANTEPECER JET IS TO PEAK AT 45-50KT. ON SATURDAY MORNING THE  
FRONT MOVES TO GRAND BAHAMA...WHILE TRAILING ACROSS WESTERN CUBA  
TO THE GULF OF HONDURAS. STRONG LOW LEVEL NORTHERLIES OVER THE  
WESTERN CARIBBEAN ARE FORECAST TO PEAK AT 35-40KT. THIS WILL  
COMBINE WITH A COLD ADVECTIVE PATTERN TO SUSTAIN HEAVY CONVECTION  
ALONG THE NORTH COAST OF HONDURAS/ISLAS DE LA BAHIA. LATER ON  
SATURDAY THE FRONT WILL MOVE TO THE CENTRAL BAHAMAS-CENTRAL  
CUBA-CAYMAN ISLES TO NORTHEAST HONDURAS/ NICARAGUA.  
 
AS IT MEANDERS OVER THE NORTHERN STATES OF MEXICO THE FRONT IS TO  
FAVOR MODERATE RAINFALL AMOUNTS ACROSS COAHUILA-NORTHERN  
CHIHUAHUA...WITH RAINFALL AMOUNTS OF 10-15MM/DAY AND MAXIMA OF  
20-35MM. ON THURSDAY SOLID PRECIPITATION WILL PREVAIL...WITH  
SNOWFALL AMOUNTS OF 15-20CM AND MAXIMA OF 25-50CM ACROSS  
COAHUILA/PARTS OF NUEVO LEON. LIGHTER SNOWFALL ACCUMULATIONS ARE  
EXPECTED IN CHIHUAHUA. ACROSS THE STATE OF VERACRUZ MOST ACTIVE IS  
EXPECTED TO THE SOUTH...TO ALSO AFFECT NORTHERN PORTIONS OF  
CHIAPAS ON WEDNESDAY TO FRIDAY WHERE IT IS TO FAVOR RAINFALL  
AMOUNTS OF 10-15MM/DAY AND MAXIMA OF 20-35MM/DAY. OVER THE  
NORTHWEST BAHAMAS EXPECTING RAINFALL AMOUNTS 10-15MM/DAY AND  
MAXIMA OF 20-35MM ON FRIDAY-SATURDAY...WHILE OVER WESTERN CUBA  
THIS IS TO TRIGGER ACCUMULATION OF 15-30MM/DAY AND MAXIMA OF  
40-80MM ALSO ON FRIDAY-SATURDAY. THIS AFFECTS THE CAYMAN ISLES  
EARLY ON SATURDAY MORNING...WITH ACCUMULATION OF 10-15MM/DAY AND  
MAXIMA OF 20-35MM. OVER NORTHERN HONDURAS EXPECTING RAINFALL  
AMOUNTS OF 15-20MM/DAY AND MAXIMA OF 25-50MM ON  
FRIDAY-SATURDAY...INCREASING TO 75-100MM/DAY AND MAXIMA OF  
150MM-250MM AS OROGRAPHICALLY-FORCED MOISTURE CONVERGENCE PEAKS  
ALONG THE NORTHERN COAST  
 
SIMULTANEOUSLY...AS THE MID/UPPER LEVEL TROUGH AMPLIFIES ACROSS  
THE USA...IT IS TO INDUCE THE NORTHWARD AMPLIFICATION OF A RIDGE  
FROM THE EASTERN CARIBBEAN NORTHWARD TO THE WESTERN ATLANTIC EARLY  
IN THE CYCLE. AS THE RIDGE BUILDS...A TROUGH TO THE EAST WILL  
RELOCATE TO THE CENTRAL ATLANTIC EARLY ON FRIDAY MORNING. A  
SUBTROPICAL JET MAXIMA BOUNDS THE SOUTHERN FRINGES OF THE UPPER  
LEVEL TROUGH...WITH ENTRANCE AT 250 HPA JUST TO THE NORTH OF  
PUERTO RICO/VIRGIN ISLES. THIS FAVORS AN UPPER DIVERGENT PATTERN  
THAT IS TO PERSIST DURING THE NEXT TWO DAYS. LATER IN THE WEEK THE  
JET MAXIMA IS TO ALSO PULL AWAY AS THE RIDGE BUILDS TO THE WEST.  
BY MIDMORNING ON FRIDAY...AS THE RIDGE PATTERN STRENGTHENS OVER  
THE WESTERN ATLANTIC...A STRONG SUBSIDENCE INVERSION IS TO  
ESTABLISH ACROSS THE NORTHEAST CARIBBEAN ISLES. THIS IS TO THEN  
FAVOR THE RAPID EROSION OF AVAILABLE MOISTURE FROM EAST TO WEST.  
AT LOW LEVELS...THE TROUGH ALOFT SUSTAINS A SURFACE FRONT THAT  
MEANDERS OVER THE VIRGIN ISLES-EASTERN PUERTO RICO TO THE BAHAMAS.  
THE FRONT REMAINS NEARLY STATIONARY DURING THE NEXT COUPLE OF  
DAYS...MEANWHILE WEAKENING AS IT LOSES ITS UPPER LEVEL SUPPORT.  
BROAD POLAR RIDGE TO THE NORTH OF THE ISLANDS IS TO FAVOR BRISK  
EASTERLY TRADES AND A COLD ADVECTIVE PATTERN ACROSS THE NORTHEAST  
CARIBBEAN ISLES. THIS...IN-TURN...WILL RESULT IN SHALLOW  
CONVECTION BUILDING TO THE EAST AND NORTH...STREAMING ACROSS THE  
VIRGIN ISLES-PUERTO RICO-HISPANIOLA FROM TIME-TO-TIME. BRISK  
EASTERLY TRADES ARE TO PERSIST ACROSS THE NORTHEAST CARIBBEAN  
ISLES THROUGH THE END OF THE WEEK. THIS ALSO FAVORS A PREFRONTAL  
SHEAR LINE OVER THE FRENCH ISLES TO THE EASTERN CARIBBEAN. THE  
SHEAR LINE RAPIDLY MOVES ACROSS THE WINDWARD ISLES TO NORTHEAST  
VENEZUELA-NORTHERN GUYANA LATER THIS EVENING. AS THE FRONT  
MEANDERS ACROSS THE VIRGIN ISLES-PUERTO RICO IT IS TO RESULT IN  
RAINFALL AMOUNTS OF 10-14MM/DAY AND MAXIMA OF 20-30MM THROUGH  
EARLY THURSDAY. ON THURSDAY-FRIDAY THIS DECREASES TO 05-10MM/DAY  
AND MAXIMA OF 15-25MM. OVER THE GUIANAS...THE SHEAR LINE WILL  
TRIGGER RAINFALL AMOUNTS OF 10-15MM/DAY AND MAXIMA OF 20-35MM  
THROUGH EARLY SATURDAY. MUCH LESSER AMOUNTS ARE EXPECTED TO THE  
WEST OF THE SHEAR LINE IN THE WINDWARD ISLANDS AND NORTHEASTERN  
VENEZUELA.  
 
AT 250 HPA...A TUTT EXTENDS NORTH-TO-SOUTH BETWEEN 110W-85W ACROSS  
THE EQUATOR TO 10S. THIS IS TO GENERALLY PERSIST OVER THE NEXT FEW  
DAYS. EARLY IN THE CYCLE THIS IS TO ENHANCE CONVECTION ACROSS  
EASTERN NICARAGUA TO FAVOR RAINFALL AMOUNTS OF 10-15MM/DAY AND  
MAXIMA OF 20-45MM THROUGH FRIDAY. OVER COSTA-RICA TO PANAMA THIS  
IS TO FAVOR RAINFALL AMOUNTS OF 05-10MM/DAY AND MAXIMA OF 15-20MM.  
OVER WESTERN COLOMBIA...UNDER INFLUENCE OF THE PANAMANIAN  
LOW/TROUGH...DEEP MOISTURE IS TO CONVERGE ALONG THE COAST. OVER  
THE EJE CAFETERO TO THE NORTHWEST EXPECTING RAINFALL AMOUNTS OF  
05-10MM/DAY AND MAXIMA OF 15-20MM THROUGH EARLY  
THURSDAY...INCREASING TO 10-15MM/DAY AND MAXIMA OF 20-35MM AFTER.  
A SIMILAR TREND IS EXPECTED IN THE CAUCA VALLEY. ISOLATED HIGHER  
AMOUNTS ARE EXPECTED IN THE CHOCO ON FRIDAY-SATURDAY. ON THE  
ANDEAN REGION AND ACROSS EASTERN COLOMBIA/AMAZONIA EXPECTING  
RAINFALL AMOUNTS OF 05-10MM/DAY AND MAXIMA OF 15-20MM EARLY IN THE  
CYCLE. IN THE AMAZON...ACCUMULATIONS ARE TO GRADUALLY INCREASE TO  
10-15MM/DAY AND MAXIMA OF 20-35MM BY FRIDAY-SATURDAY.  
 
TROPICAL/EASTERLY WAVES INITIALIZED AT 12UTC:  
INITIAL 24 36 48 60 72 84 96 TYPE SOF  
62W 65W 68W 70W 72W 74W 76W 78W EW 14N  
81W 83W 85W 87W 90W DISSIPATES EW 17N  
 
AN EASTERLY WAVE ALONG 62W AND SOUTH OF 14N IS TO TRIGGER  
SCATTERED CONVECTION ACROSS AMAZONIA IN SOUTHERN VENEZUELA TO  
EASTERN COLOMBIA...WITH ACCUMULATION OF 05-10MM/DAY AND MAXIMA OF  
15-20MM. ON THE ANDEAN REGION OF COLOMBIA THIS IS TO ALSO SUSTAIN  
RAINFALL AMOUNTS OF 05-10MM/DAY AND MAXIMA OF 15-20MM ON  
THURSDAY-FRIDAY...WITH HIGHER AMOUNTS ACROSS THE WESTERN COASTAL  
PLAINS...WITH ACCUMULATION OF 10-15MM/DAY AND MAXIMA OF 20-35MM.  
 
AN EASTERLY WAVE ALONG 81W AND SOUTH OF 17N WILL SUSTAIN ORGANIZED  
CONVECTION ACROSS EASTERN NICARAGUA...WITH ACCUMULATION OF  
10-15MM/DAY AND MAXIMA OF 20-45MM ON WEDNESDAY AND THURSDAY. OVER  
COSTA RICA EXPECTING LIGHT CONVECTION. WAVE IS TO DISSIPATE ON  
FRIDAY.  
 
PECK...MS (JAMAICA)  
RIVAS...SENAMHI (PERU)  
DAVISON/GALVEZ...WPC (USA)  
 

 
 
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