027  
FXCA20 KWBC 121918  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
218 PM EST TUE DEC 12 2017  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 12/06  
UTC: BROAD POLAR TROUGH EXTENDS FROM THE CENTRAL USA TO THE  
WESTERN ATLANTIC...WITH AXIS BOTTOMING OUT NORTH OF 20N. ALSO AT  
MID LEVELS...A LOW AMPLITUDE RIDGE EXTENDS WEST-TO-EAST ACROSS THE  
CARIBBEAN BASIN...FAVORING A FAIRLY STRONG TRADE WINDS CAP ACROSS  
THE EASTERN CARIBBEAN ISLES. AT LOW LEVELS...THE TROUGH ALOFT  
SUSTAINS A SURFACE FRONT THAT STRETCHES ACROSS THE BAHAMAS-CUBA TO  
THE WESTERN CARIBBEAN. OVER THE EASTERN CARIBBEAN...IN RESPONSE TO  
THIS FEATURE...A LIGHT MEANDERING FLOW DOMINATES.  
 
GLOBAL MODELS FORECAST A CLIPPER TO STREAM ACROSS THE EASTERN USA  
LATER THIS AFTERNOON. AS IT ROUNDS THE BASE OF THE LONG WAVE  
TROUGH NORTH OF THE ISLANDS...IT IS TO PRESS AGAINST THE MID LEVEL  
RIDGE TO THE SOUTH. UNDER PRESSURE...THE RIDGE IS TO RELAX ITS  
FOOTHOLD ACROSS THE EASTERN CARIBBEAN...WITH STRONG TRADE WINDS  
CAP LIFTING ABOVE 700 HPA BY MIDDAY ON WEDNESDAY. DURING THAT  
PERIOD THE PWAT CONTENT IS TO PEAK AROUND 1.25 INCHES. THE  
SLIGHTLY MORE FAVORABLE ENVIRONMENTAL CONDITIONS...HOWEVER...WILL  
BE SHORT LIVED. ON THURSDAY-FRIDAY MID LEVEL RIDGE PATTERN BOUNCES  
BACK...STRENGTHENING THE TRADE WINDS CAP ACROSS THE CARIBBEAN  
WHILE SUSTAINING A DRYING TREND. AT LOW LEVELS...THE POLAR FRONT  
SLOWLY MOVES ACROSS THE TURKS AND CAICOS TO NORTHERN HISPANIOLA ON  
WEDNESDAY MORNING. BUT AS IT LOSES ITS UPPER LEVEL  
SUPPORT...FRONTAL BOUNDARY IS TO THEN GRADUALLY WEAKEN WHILE  
RETROGRESSING ACROSS THE BAHAMAS-CUBA LATER IN THE WEEK. EVOLVING  
LOW/MID LEVEL PATTERN TO THE WEST AND NORTH WILL HAVE AN INFLUENCE  
ON THE LOCAL FLOW PATTERN. EARLY THIS MORNING THE STEERING FLOW  
BECOME SLIGHTLY BETTER DEFINED AS THE WINDS VEER TO THE  
SOUTH-SOUTHEAST...SHIFTING TO THE SOUTH-SOUTHWEST LATER IN THE  
DAY/EARLY WEDNESDAY MORNING. ON THURSDAY-FRIDAY...AS A RIDGE  
GRADUALLY BUILDS TO THE NORTH...THE PREVAILING EASTERLY TRADES  
RETURN TO THE FORECAST AREA. STRONGER WINDS ARE EXPECTED BY THE  
END OF THE WEEK.  
 
AS THE WINDS VEER TO THE SOUTH...EXPECTING SLIGHT INCREASE IN  
DIURNAL CONVECTION ALONG THE CORDILLERA-NORTHERN PUERTO RICO  
DURING THE DAY TODAY. SLIGHTLY HIGHER AMOUNTS ARE EXPECTED ON  
WEDNESDAY AS THE MID LEVEL RIDGE PATTERN WEAKENS AND DEEPER  
MOISTURE POOLS ACROSS THE FORECAST AREA. ON THURSDAY AND ONWARD  
EXPECTING A LULL IN ACTIVITY...WITH EARLY MORNING TRADE WIND  
SHOWERS OVER THE VIRGIN ISLES-EASTERN PUERTO RICO TO ONLY TRIGGER  
LIGHT RAINFALL AMOUNTS. LIKEWISE ACROSS WESTERN INTERIOR PORTIONS  
OF PUERTO RICO IN AFTERNOON CONVECTION.  
 
TROPICAL DISCUSSION FROM DEC 12/12 UTC: A ROBUST UPPER RIDGE IS  
ESTABLISHED ACROSS MOST OF THE CARIBBEAN BASIN WHILE CENTERING  
OVER NORTHERN COLOMBIA/SOUTHERN CARIBBEAN. THIS RIDGE WILL LIMIT  
THE PROGRESSION OF MID-LATITUDE TROUGHS AND ASSOCIATED SURFACE  
FRONTS INTO THE BASIN. REGIONS PRONE TO UPPER TROUGH  
INTENSIFICATION WILL LIMIT TO EXTREME WESTERN MEXICO AND THE  
CENTRAL PORTIONS OF THE TROPICAL NORTH ATLANTIC.  
 
A SURFACE FRONT IS LINGERING ACROSS THE NORTHWESTERN CARIBBEAN. AS  
THE ASSOCIATED TROUGH HAS LIFTED INTO THE NORTH ATLANTIC...THE  
FRONT IS REMAINING MOSTLY STATIONARY AND WEAKENING. YET...AS  
ANOTHER STRONG UPPER TROUGH EXITS THE EAST COAST OF THE UNITED  
STATES ON TUESDAY...NORTHERLY WINDS WILL INTENSIFY INTO WEDNESDAY  
TO PRODUCE A TRANSIENT INTENSIFICATION OF THE AFOREMENTIONED  
FRONT. YET...AS THIS SECOND UPPER TROUGH LIFTS ON THURSDAY AND  
MID-UPPER RIDGE PATTERN ESTABLISHES OVER THE CENTRAL AND NORTHERN  
CARIBBEAN BASIN...EXPECTING THE SURFACE FRONT TO RAPIDLY WEAKEN  
DURING THURSDAY AND INTO FRIDAY. BY TUESDAY EVENING...THE  
STATIONARY FRONT WILL EXTEND ACROSS THE SOUTHEASTERN  
BAHAMAS...NORTHERN HAITI...JAMAICA INTO NORTHERN NICARAGUA. GIVEN  
THE INTENSIFICATION OF THE NORTHERLIES LATE ON TUESDAY/EARLY  
WEDNESDAY...THE FRONT WILL ADVANCE SLIGHTLY ACROSS NORTHERN  
HISPANIOLA AS A COLD FRONT. BY WEDNESDAY EVENING...IT IS FORECAST  
TO EXTEND JUST TO THE SOUTHEAST OF THE TURKS AND CAICOS  
ISLANDS...THE NORTHERN DOMINICAN REPUBLIC...CENTRAL  
HAITI...JAMAICA...WITH AN ILL-DEFINED TAIL INTO NORTHERN  
NICARAGUA. AS THE NORTHERLY WINDS INCREASE TEMPORARILY...SHEAR  
LINE CONVERGENCE WILL ALSO INCREASE ON WEDNESDAY. THIS WILL REFORM  
THE SHEAR LINE ACROSS THE CENTRAL-WESTERN CARIBBEAN...BUT THE  
FEATURE WILL LOSE DEFINITION DURING THURSDAY MORNING. BY THURSDAY  
EVENING...THE FRONT WILL BECOME STATIONARY ONCE AGAIN ACROSS/JUST  
TO THE SOUTHEAST OF THE TURKS AND CAICOS AND INTO THE WINDWARD  
PASSAGE. THE WESTERN TIER OF THE FRONT WILL RETURN AS A WEAKENING  
WARM FRONT TO EXTEND ACROSS THE SOUTHEAST OF THE CAYMAN ISLANDS  
AND INTO THE HONDURAS/NICARAGUA BORDER. A SECOND FRONT WILL ALSO  
ARRIVE IN THE BASIN...BUT IT WILL EXERT LIMITED EFFECTS IN  
PRECIPITATION AS DEEP-LAYER MOISTURE REMAINS IN THE CENTRAL  
CARIBBEAN. THE NEW FRONT WILL EXTEND ACROSS THE CENTRAL FLORIDA  
PENINSULA INTO NORTHERN TAMAULIPAS IN MEXICO BY TUESDAY EVENING.  
BY WEDNESDAY EVENING IT WILL EXTEND ACROSS THE CENTRAL BAHAMAS  
INTO CENTRAL CUBA. DURING THURSDAY...IT WILL MERGE WITH THE  
AFOREMENTIONED FRONT TO SUSTAIN THE BOUNDARY ALONG THE TURKS AND  
CAICOS-SOUTHEASTERN CUBA.  
 
IN TERMS OF PRECIPITATION IN ASSOCIATION WITH THE FRONTS AND SHEAR  
LINE...LARGEST ACCUMULATIONS ARE EXPECTED IN NORTHERN SLOPES OF  
JAMAICA ON TUESDAY-EARLY WEDNESDAY...WHERE ENHANCED MOISTURE  
CONVERGENCE WILL PRODUCE 10-15MM/DAY AND MAXIMA OF 20-45MM. IN  
SOUTH EASTERN CUBA...NORTHWESTERN HAITI AND THE SOUTHEASTERN  
BAHAMAS/TURKS AND CAICOS...EXPECTING 10-15MM/DAY AND MAXIMA OF  
20-35MM. POST-FRONTAL COLD AIR CUMULUS WILL CONTINUE STIMULATING  
PRECIPITATION IN NORTHERN HONDURAS...WHERE EXPECTING 10-15MM/DAY  
AND MAXIMA OF 20-35MM...WHILE IN SOUTHERN VERACRUZ/TABASCO  
EXPECTING 05-10MM/DAY AND MAXIMA OF 15-20MM. SHOWERS PRODUCED BY  
THE COLD AIR CUMULUS ARE TO RAPIDLY DECREASE IN INTENSITY DURING  
THE DAY ON WEDNESDAY. ALSO ON TUESDAY-WEDNESDAY...CONVERGENCE  
AHEAD OF A NORTHERLY LOW-LEVEL JET WILL CONTINUE STIMULATING  
MOISTURE CONVERGENCE IN THE CARIBBEAN COAST OF COSTA RICA AND  
WESTERN PANAMA...WHERE EXPECTING 15-20MM/DAY AND MAXIMA OF  
25-50MM. BY WEDNESDAY-THURSDAY...LARGEST ACCUMULATIONS WITH THE  
FRONT ARE EXPECTED IN THE NORTHERN DOMINICAN REPUBLIC...WHERE  
ENHANCED MOISTURE CONVERGENCE WILL LEAD TO 15-20MM/DAY AND MAXIMA  
OF 30-60MM. ACROSS WESTERN HAITI AND JAMAICA...WANING FRONTAL  
CONVECTION WILL LEAD TO 10-15MM/DAY AND MAXIMA OF 20-30MM.  
MOISTURE CONVERGENCE IN COSTA RICA AND WESTERN PANAMA WILL PRODUCE  
10-15MM/DAY AND MAXIMA OF 20-40MM. BY THURSDAY-FRIDAY...EXPECTING  
A SIGNIFICANT DECREASE IN ACCUMULATIONS AS THE SURFACE FRONT  
WEAKENS...AND THE MID-LEVEL RIDGE PATTERN STRENGTHENS OVER THE  
NORTHERN ANTILLES. DURING THIS PERIOD...FRONTAL CONVECTION WILL  
LEAD TO AMOUNTS OF 05-10MM/DAY AND MAXIMA OF 15MM IN THE TURKS AND  
CAICOS/SOUTHEASTERN BAHAMAS INTO NORTHERN HAITI. ACROSS NORTHERN  
JAMAICA AND SOUTHEASTERN CUBA EXPECTING 00-05MM/DAY AND MAXIMA OF  
10MM. AS THE EASTERLY TRADES REESTABLISH...MOISTURE WILL RETURN  
INTO NORTHEASTERN NICARAGUA...WHERE IT WILL LEAD TO 05-10MM/DAY  
AND MAXIMA OF 15-20MM. ALSO ON THURSDAY-FRIDAY...WANING MOISTURE  
CONVERGENCE IN COSTA RICA WILL LEAD TO 10-15MM/DAY AND MAXIMA OF  
20-35MM.  
 
ALSO IN THE NORTHERN PART OF THE FORECAST DOMAIN...MODELS ARE  
SHOWING UPPER TROUGH ORGANIZING OFF THE COAST OF WESTERN MEXICO.  
THIS WILL INTERACT WITH A SUBTROPICAL MOISTURE PLUME THAT  
ORIGINATES IN THE TROPICAL PACIFIC...AND SUSTAIN SIGNIFICANT  
FORCING TO STIMULATE RAINFALL ON THURSDAY THROUGH SATURDAY.  
ALTHOUGH DISCREPANCIES ON MODEL SOLUTIONS START APPEARING  
THURSDAY...THE PATTERN WILL LIKELY PRODUCE MODERATE RAINFALL IN  
THE SIERRA MADRE OCCIDENTAL OF SINALOA AND NAYARIT...WHERE  
OROGRAPHIC UPLIFT WILL ENHANCE ACTIVITY LOCALLY. BY  
THURSDAY-FRIDAY EXPECTING 10-15MM/DAY AND MAXIMA OF 20-40MM...WITH  
THE POTENTIAL FOR A SLIGHT INCREASE ON FRIDAY-SATURDAY.  
 
IN NORTHERN SOUTH AMERICA AND THE WINDWARD ISLANDS...A  
TUTT-INDUCED WAVE IS APPROACHING THE REGION AND IS FORECAST TO  
ORGANIZE AS IT CROSSES NORTHERN VENEZUELA ON WEDNESDAY AND  
THURSDAY. THIS WILL LEAD TO A TRANSIENT INCREASE IN TRADE WIND  
SHOWERS AND RAINFALL ACCUMULATIONS (SEE BELOW). IN COLOMBIA...AS A  
DRY AIR MASS ARRIVES FROM THE EAST...EXPECTING A GRADUAL DECREASE  
IN ACCUMULATIONS. INITIALLY...EXPECTING 10-15MM/DAY AND MAXIMA OF  
20-30MM ACROSS THE CHOCO/EJE CAFETERO...DECREASING TO 00-05MM/DAY  
AND ISOLATED MAXIMA OF 10MM DURING WEDNESDAY. A SLIGHT INCREASE IS  
EXPECTED ON THURSDAY AS AN EASTERLY WAVE APPROACHES.  
 
TROPICAL/EASTERLY WAVES INITIALIZED AT 12UTC:  
INITIAL 24 36 48 60 72 84 96 TYPE SOF  
59W 62W 66W 69W 73W 76W 80W 83W TUTT INDCD/EW 10N  
 
A TUTT-INDUCED WAVE IN THE WESTERN ATLANTIC IS STARTING TO  
PROPAGATE AS AN EASTERLY WAVE. THIS WAVE WILL SLOWLY REGAIN  
DEFINITION AS IT PROPAGATES ACROSS THE SOUTHERN CARIBBEAN. IT WILL  
STIMULATE PRECIPITATION IN TRINIDAD AND TOBAGO/NORTHERN GUYANA ON  
TUESDAY...WHERE IT WILL PRODUCE 05-10MM/DAY AND MAXIMA OF 15MM.  
SIMILAR AMOUNTS WILL EXPAND INTO CENTRAL VENEZUELA ON WEDNESDAY.  
DURING THURSDAY...SHOWERS WILL EXPAND INTO NORTHWESTERN  
VENEZUELA/ABC...BUT EXPECTING A DECREASING TREND IN ACCUMULATIONS  
TO 00-05MM/DAY AND MAXIMA OF 10MM.  
 
GORDON...NMS (BELIZE)  
PECK...MS (JAMAICA)  
RIVAS...SENAMHI (PERU)  
GALVEZ/DAVISON...WPC (USA)  
 

 
 
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