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Tropical Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1927Z Dec 07, 2017)
 
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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)