924  
FXCA20 KWBC 071927  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
227 PM EST THU DEC 07 2017  
 
TROPICAL DISCUSSION FROM DEC 07/12 UTC: DEEPENING POLAR TROUGH  
ACROSS THE WESTERN USA IS THE SYSTEM OF CONCERN. THE UPPER TROUGH  
IS FORECAST TO EXTEND ITS AXIS INTO CHIHUAHUA IN NORTHERN MEXICO  
ON THURSDAY...INTO EASTERN TEXAS-TAMAULIPAS ON FRIDAY EVENING...TO  
THEN START PULLING WHILE EXTENDING ITS AXIS ACROSS ALABAMA INTO  
THE SOUTHWESTERN GULF OF MEXICO. THIS TROUGH IS UNUSUALLY STRONG  
AND COLD...AND WILL LEAD TO A HISTORIC SNOWFALL EVENT IN PARTS OF  
NORTHERN MEXICO. THE COMBINATION OF FACTORS FAVORABLE FOR HEAVY  
SNOW IS QUITE UNUSUAL. ON ONE HAND...THE UPPER TROUGH IS VERY  
COLD...WITH CENTRAL TEMPERATURES BELOW -26C AT 500 HPA. ON THE  
OTHER HAND...MOISTURE WILL BE SUFFICIENT...AS A WEAK BUT PRESENT  
SUBTROPICAL MOISTURE PLUME IS TRANSPORTING TROPICAL PACIFIC WATER  
VAPOR INTO THE REGION. ALSO...THE DYNAMICS OF THE TROUGH WILL BE  
FAVORABLE FOR ASCENT...ESPECIALLY OVER COAHUILA/NORTHEASTERN  
MEXICO. AT LOW-LEVELS...COLD AIR MASS IN PLACE EAST OF THE SIERRA  
MADRE ORIENTAL WILL ALLOW THE 0C ISOTHERM TO DEEPEN DOWN TO 925  
HPA. THIS WILL POSE THE THREAT FOR SNOW IN AREAS AS LOW AS  
SALTILLO...AND MONTERREY TO A LESSER EXTENT. EXPECTING SNOWFALL  
AMOUNTS IN THE ORDER OF 10-15CM/DAY WITH MAXIMA OF 20-40CM MOSTLY  
IN COAHUILA AND UPPER ELEVATIONS OF NUEVO LEON. HEAVY SNOW IS ALSO  
POSSIBLE IN AREAS OF THE RIO BRAVO ACROSS WESTERN TAMAULIPAS INTO  
COAHUILA. ALSO...SNOWFALL IS EXPECTED IN MOST OF CHIHUAHUA AND  
DURANGO...WHERE SNOWFALL ACCUMULATIONS WILL REACH 05-10CM/DAY AND  
MAXIMA OF 15-20CM. THE CHANCE FOR SNOW WILL RAPIDLY DECREASE  
DURING THE DAY ON FRIDAY.  
 
THE AFOREMENTIONED UPPER TROUGH WILL SUSTAIN CYCLOGENESIS OVER THE  
GULF OF MEXICO...WHICH WILL IN TERM PRODUCE A STRONG COLD FRONT  
AND NORTES EVENT ACROSS THE GULF. BY THURSDAY EVENING...THE FRONT  
WILL EXTEND ACROSS CENTRAL-NORTHERN FLORIDA INTO THE CENTRAL  
GULF...AND INTO SOUTHERN VERACRUZ. THE NORTES WILL REACH  
TEHUANTEPEC BY FRIDAY MORNING...TO PERSIST INTO SATURDAY. BY  
FRIDAY EVENING...THE FRONT WILL EXTEND ACROSS TAMPA...MERIDA IN  
THE YUCATAN PENINSULA...AND EXTREME NORTHWESTERN GUATEMALA. DURING  
THIS TIME...A PRE-FRONTAL TROUGH IS FORECAST TO ORGANIZE ACROSS  
THE EASTERN YUCATAN PENINSULA INTO NORTH-CENTRAL HONDURAS. BY  
SATURDAY EVENING...THE FRONT WILL CONTINUE ITS RAPID PROGRESSION  
TO EXTEND ACROSS THE CENTRAL BAHAMAS...SOUTH EASTERN CUBA...JUST  
EAST OF THE CAYMAN ISLANDS...EXTREME NORTHEASTERN NICARAGUA. BY  
THIS TIME...A SHEAR LINE WILL ORGANIZE...TO EXTEND ACROSS SAN  
ANDRES/NUEVA PROVIDENCIA IN THE WESTERN CARIBBEAN AND INTO BOCAS  
DEL TORO IN PANAMA. BY SUNDAY EVENING...THE FRONT WILL EXTEND  
ACROSS THE TURKS AND CAICOS...SOUTHEASTERN CUBA...WESTERN  
JAMAICA...CENTRAL NICARAGUA. THE SHEAR LINE WILL EXTEND ACROSS  
HAITI...INTO BOCAS DEL TORO IN PANAMA.  
 
THROUGH EARLY FRIDAY...LARGEST ACCUMULATIONS ARE EXPECTED IN  
VERACRUZ/TABASCO/NORTHERN CHIAPAS...WHERE OROGRAPHIC UPLIFT AS THE  
FRONT APPROACHES WILL LEAD TO 10-15MM/DAY AND MAXIMA OF 20-35MM.  
BY FRIDAY-SATURDAY...EXPECTING AN INCREASE IN ACTIVITY IN SOUTHERN  
BELIZE/NORTHWESTERN HONDURAS AND EASTERN GUATEMALA...WHERE  
ENHANCED OROGRAPHIC UPLIFT AS THE NORTHERLIES ARRIVE...WILL LEAD  
TO 15-20MM/DAY AND MAXIMA OF 25-50MM. PRE-FRONTAL CONFLUENCE WILL  
ALSO ENHANCE ACCUMULATIONS IN WESTERN CUBA...WHERE EXPECTING  
15-30MM/DAY AND MAXIMA OF 40-80MM. IN SOUTHERN VERACRUZ/CHIAPAS  
STILL EXPECTING 10-15MM/DAY AND MAXIMA OF 20-35MM. ELSEWHERE  
ACROSS THE YUCATAN PENINSULA/BELIZE EXPECTING MAXIMA OF 15MM. BY  
SATURDAY-SUNDAY...ENHANCED OROGRAPHIC UPLIFT ALONG THE NORTHERN  
COAST OF HONDURAS WILL LEAD TO 75-100MM/DAY AND ISOLATED MAXIMA OF  
150-250MM. ALSO...FRONTAL AND SHEAR LINE CONVERGENCE WILL INTERACT  
WITH MOISTURE POOL IN THE WESTERN CARIBBEAN...TO SUSTAIN HEAVY  
RAIN FROM SAN ANDRES/NUEVA PROVIDENCIA NORTH INTO THE CAYMAN  
ISLANDS/WESTERN JAMAICA AND CENTRAL CUBA. IN THESE  
REGIONS...EXPECTING 15-20MM/DAY AND MAXIMA OF 35-70MM. ENHANCED  
OROGRAPHIC UPLIFT IN EASTERN COSTA RICA/WESTERN PANAMA...AS SHEAR  
LINE DEVELOPS AND LOW-LEVEL JET FROM THE NORTH ESTABLISHES...WILL  
LEAD TO 75-100MM/DAY AND MAXIMA OF 150-250MM. NOTE THAT HEAVY RAIN  
IN COSTA RICA AND PANAMA WILL EXTEND INTO LATE MONDAY...WITH  
SIMILAR AMOUNTS EXPECTED ON SUNDAY-MONDAY. IN THE BAHAMAS AND  
WESTERN CUBA...EXPECTING 10-15MM/DAY AND MAXIMA OF 20-40MM ON  
SATURDAY-SUNDAY.  
 
OTHER SYSTEM OF INTEREST...IS A COLD FRONT MEANDERING SOUTHWARD  
ACROSS THE EASTERN CARIBBEAN. BY THURSDAY EVENING...THE COLD FRONT  
IS FORECAST TO EXTEND ACROSS GUADELOUPE/DOMINICA INTO AREAS TO THE  
SOUTH OF PUERTO RICO...AND THE EASTERN DOMINICAN REPUBLIC. BY  
FRIDAY EVENING THE FRONT WILL START LOSING DEFINITION...BUT A WEAK  
BOUNDARY WILL EXTEND ACROSS MARTINIQUE. ALTHOUGH THE THERMAL  
PROPERTIES OF THE FRONT WILL DILUTE AFTER...LINGERING SHOWERS WILL  
CONTINUE MEANDERING SOUTH INTO THE NORTH COAST OF  
VENEZUELA...WHERE THEY WILL LEAD TO ENHANCEMENT OF ACCUMULATIONS  
ON SATURDAY THROUGH MONDAY. THROUGH EARLY FRIDAY...EXPECTING  
10-15MM/DAY AND MAXIMA OF 20-40MM IN THE FRENCH ANTILLES. IN  
PUERTO RICO AND HISPANIOLA...EXPECTING 05-10MM/DAY AND MAXIMA OF  
15-25MM. BY FRIDAY-SATURDAY...EXPECTING 05-10MM/DAY AND MAXIMA OF  
15MM IN THE WINDWARD ISLANDS. BY SATURDAY-SUNDAY...EXPECTING  
SIMILAR AMOUNTS ACROSS MOST OF THE COAST OF VENEZUELA INTO  
TRINIDAD AND TOBAGO...TO PERSIST INTO MONDAY.  
 
IN NORTHERN SOUTH AMERICA...MOST ACTIVE WILL CLUSTER IN THE AMAZON  
BASIN...WHERE EXPECTING GENERALLY 10-15MM/DAY AND MAXIMA OF  
20-35MM. SIMILAR AMOUNTS ARE EXPECTED IN THE GUIANAS EARLY IN THE  
CYCLE...IN ASSOCIATION WITH AN ACTIVE ATLANTIC ITCZ AND A WEAK  
SHEAR LINE FROM THE NORTH ATLANTIC. IN COLOMBIA...MOST ACTIVE WILL  
CLUSTER TO THE WEST IN THE CHOCO AND CAUCA REGION/EJE  
CAFETERO...WHERE AMOUNTS WILL BE GENERALLY IN THE 10-15MM/DAY  
RANGE AND MAXIMA OF 20-35MM. EXPECTING A SLIGHT INCREASE TOWARD  
THE LATE WEEKEND TO ISOLATED MAXIMA OF 25-50MM.  
 
TROPICAL/EASTERLY WAVES INITIALIZED AT 12UTC:  
INITIAL 24 36 48 60 72 84 96 TYPE SOF  
68W 71W 74W 77W 79W DISSIPATES EW 15N  
85W 87W 90W DISSIPATES EW 17N  
 
AN EASTERLY WAVE ALONG 68W AND SOUTH OF 15N IS ENHANCING ACTIVITY  
ALONG A MOISTURE PLUME IN THE CARIBBEAN. IT WILL HAVE LIMITED  
EFFECTS IN CONTINENTAL REGIONS...AS IT IS FORECAST TO BECOME  
EMBEDDED IN A DEVELOPING SHEAR LINE ON SUNDAY-MONDAY IN THE  
WESTERN CARIBBEAN.  
 
AN EASTERLY WAVE ALONG 85W AND SOUTH OF 17N IS ENHANCING  
CONVECTION IN NICARAGUA/HONDURAS...YET IT IS FORECAST TO RAPIDLY  
LOSE DEFINITION AS IT ENTERS THE DRY PACIFIC BASIN OF CENTRAL  
AMERICA LATE ON THURSDAY.  
 
GORDON...NMS (BELIZE)  
PECK...MS (JAMAICA)  
RIVAS...SENAMHI (PERU)  
DAVISON/GALVEZ...WPC (USA)  
 

 
 
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