569  
FXCA20 KWBC 181928  
PMDCA  
 
TROPICAL DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
228 PM EST THU JAN 18 2018  
 
PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 18/06  
UTC: A MEANDERING MID LEVEL HIGH OVER PUERTO RICO-MONA PASSAGE  
ANCHORS A SHORT WAVE RIDGE ACROSS THE NORTHEAST CARIBBEAN ISLES.  
THIS FAVORS AN ELEVATED TRADE WINDS CAP THAT IS STRONG ENOUGH TO  
INHIBIT DEEP CONVECTION...BUT NOT STRONG ENOUGH TO SUPPRESS  
SHALLOW DEVELOPMENT. AT LOW LEVELS...IN A DISORGANIZED MEANDERING  
FLOW NORTH OF THE ISLANDS...LIGHT EASTERLY TRADES PREVAIL ACROSS  
THE FORECAST AREA...WITH BOUNDARY LAYER WINDS OF 05-10KT. EMBEDDED  
IN THIS FLOW...A PLUME OF SHALLOW MOISTURE IS PULLING ACROSS THE  
NORTHERN LEEWARD TO THE VIRGIN ISLES EARLY THIS MORNING. AT ITS  
CURRENT PACE IT SHOULD BE REACHING EASTERN PUERTO RICO AROUND  
15UTC TODAY...INTO WESTERN PORTIONS OF THE ISLAND BY 20-21UTC. THE  
PWAT CONTENT WITH THIS CLUSTER IS EXPECTED TO PEAK BETWEEN  
1.50-1.75 INCHES. IT LOOKS LIKE THE GFS HAS A BETTER HANDLE OF  
THIS PLUME OF MOISTURE THAN THE ECMWF. THE MOIST TONGUE IS TO  
REMAIN ACROSS THE FORECAST AREA THROUGH AT LEAST FRIDAY AFTERNOON.  
THE INFLOW OF LOW/MID LEVEL MOISTURE WILL FEED CONVECTION DURING  
THE DAY TODAY AND ON FRIDAY...WITH MOST INTENSE TO CONTINUE  
CLUSTERING OVER INTERIOR PORTIONS OF PUERTO RICO. THE GFS AND NAM  
ARE DOING BETTER THAN THE OTHER MODELS IN DEPICTING DIURNAL  
DEVELOPMENT ACROSS INTERIOR PORTIONS OF PUERTO RICO. RISK OF  
MODERATE TO HEAVY RAINFALL AMOUNTS WILL PERSIST OVER THE NEXT  
COUPLE OF DAYS.  
 
ALTHOUGH THE MID/UPPER LEVEL PATTERN IS TO REMAIN NEARLY UNCHANGED  
OVER THE NEXT SEVERAL DAYS...AT LOW LEVELS A POLAR FRONT OVER THE  
WESTERN ATLANTIC MOVES ACROSS BERMUDA EARLY THIS AFTERNOON.  
BOUNDARY TAKES A BEELINE ACROSS THE CENTRAL ATLANTIC ON  
FRIDAY...BUT AS IT LOSES ITS UPPER LEVEL SUPPORT IT IS EXPECTED TO  
BOTTOM OUT TO THE NORTH OF 22N EARLY ON SATURDAY MORNING. A POLAR  
RIDGE ROLLS BEHIND THIS AXIS...BUILDING OVER THE WESTERN ATLANTIC  
ON SATURDAY-SUNDAY. THIS IS TO FAVOR BRISK EAST-NORTHEASTERLY  
TRADES ACROSS THE FORECAST AREA...WITH 850 HPA WINDS EXPECTED TO  
PEAK AROUND 20KT. THIS WILL BE STRONG ENOUGH TO FAVOR GENERATION  
OF STREAMERS ACROSS THE NORTHERN LEEWARD/VIRGIN ISLES THAT ARE TO  
AFFECT EASTERN PORTIONS OF PUERTO RICO FROM TIME-TO-TIME...WITH  
MOST ACTIVE DURING THE MORNING HOURS. THE SURGING TRADES WILL BE  
ACCOMPANIED BY A SLIGHTLY STRONGER CAP INVERSION...WITH PWAT  
CONTENT DROPPING TO ABOUT 1.0-1.25 INCHES DURING THE WEEKEND-EARLY  
NEXT WEEK. AS A RESULT DIURNAL ACTIVITY ACROSS INTERIOR PORTIONS  
OF PUERTO RICO IS TO BECOME MORE SPARSE.  
 
TROPICAL DISCUSSION FROM JAN 18/12 UTC: THE UPPER LEVEL ANALYSIS  
SHOWS THE SUBEQUATORIAL RIDGE EXTENDING FROM NORTHERN SOUTH  
AMERICA ACROSS THE CENTRAL AND EASTERN CARIBBEAN BASIN. AT  
MID-LEVELS...A RIDGE CENTERS ON A HIGH OVER/JUST TO THE NORTH OF  
PUERTO RICO WHILE COVERING THE ENTIRE EASTERN AND CENTRAL  
CARIBBEAN...INCLUDING HISPANIOLA AND JAMAICA. THE PREVALENCE OF A  
MID-UPPER RIDGE PATTERN ACROSS THE CENTRAL AND EASTERN CARIBBEAN  
IS BLOCKING THE INCURSION OF POLAR TROUGHS INTO THE BASIN. THIS IS  
LIMITING THE SOUTHEASTWARD PROGRESSION OF ASSOCIATED SURFACE  
FRONTS TO AREAS TO THE NORTH OF THE GULF OF HONDURAS...JAMAICA AND  
NORTHERN HISPANIOLA...WHILE A SHEAR LINE IS REORGANIZING FROM THE  
HAITIAN PENINSULA SOUTHWEST INTO EASTERN COSTA RICA. THE LARGEST  
VALUES OF PRECIPITABLE WATER IN THE BASIN ARE CONSTRAINED TO THE  
SOUTHERN CARIBBEAN AND GULF OF HONDURAS...WHERE READINGS OVER 50MM  
ARE PRESENT. UNSEASONABLY HIGH VALUES ARE ALSO PRESENT ALONG THE  
ATLANTIC COASTS OF EASTERN CUBA AND HISPANIOLA...AND IN THE  
SOUTHERN BAHAMAS/TURKS AND CAICOS...WHERE ESTIMATIONS RANGE  
BETWEEN 40-45MM.  
 
STRONGEST CONVECTION IN THE BASIN IS OCCURRING IN THE GULF OF  
HONDURAS AND IN THE SOUTHERN CARIBBEAN. IN THE GULF OF  
HONDURAS...CONVECTION IS RESPONDING TO ENHANCED NORTHERLY  
TRADES...DEEP-LAYER MOISTURE...OROGRAPHIC FORCING AND ENHANCED  
UPPER DIVERGENCE IN THE RIGHT ENTRANCE OF THE SUBTROPICAL UPPER  
JET. CONVECTION IN THE SOUTHERN CARIBBEAN...FROM NORTHWESTERN  
COLOMBIA TO EASTERN COSTA RICA...IS RESPONDING TO AN INCREASED  
NORTHERLY COMPONENT IN THE TRADES ACROSS THE SOUTHERN CARIBBEAN  
AND TO THE ORGANIZATION OF THE SHEAR LINE.  
 
THE FORECAST SUGGESTS THAT THE MID-UPPER RIDGE PATTERN WILL HOLD  
THROUGH THE FORECAST CYCLE. WHILE THE UPPER RIDGE IS FORECAST TO  
LINGER CLOSE TO ITS ACTUAL POSITION...THE MID-LEVEL RIDGE IS  
FORECAST TO STRENGTHEN OVER THE GULF OF HONDURAS...NORTHERN  
CENTRAL AMERICA...AND SOUTHERN MEXICO BY SATURDAY EVENING/SUNDAY.  
THIS WILL LEAD TO A RELOCATION OF THE REGIONS OF HEAVIEST  
PRECIPITATION TO PANAMA/COSTA RICA AND TO CUBA/BAHAMAS DURING THE  
WEEKEND. A PROGRESSIVE SURFACE FRONT IS FORECAST TO EXTEND ACROSS  
THE SOUTHEASTERN BAHAMAS/TURKS AND CAICOS...SOUTHEASTERN  
CUBA...CAYMAN ISLANDS AND NORTHERN HONDURAS/SOUTHERN BELIZE BY  
THURSDAY EVENING. AT THIS TIME...THE SHEAR LINE IS FORECAST TO  
ORGANIZE ACROSS HISPANIOLA...INTO SOUTHERN NICARAGUA. BY FRIDAY  
EVENING THE FRONT WILL EXTEND ACROSS THE SOUTHEASTERN  
BAHAMAS...CENTRAL CUBA...CAYMAN ISLANDS...INTO THE GULF OF  
HONDURAS. THE SHEAR LINE WILL START LOSING DEFINITION WHILE  
EXTENDING ACROSS EASTERN HISPANIOLA...JUST TO THE SOUTH OF  
JAMAICA. BY SATURDAY EVENING...THE FRONT WILL RETROGRESS NORTHWARD  
TO EXTEND ACROSS THE CENTRAL BAHAMAS AND CUBA STARTING TO LOSE  
DEFINITION.  
 
REGARDING RAINFALL ACCUMULATIONS...ON THURSDAY-FRIDAY...ENHANCED  
MOISTURE CONVERGENCE WILL SUSTAIN ADDITIONAL RAINFALL  
ACCUMULATIONS OF 15-30MM/DAY AND MAXIMA OF 40-80MM IN NORTHERN  
HONDURAS/EASTERN GUATEMALA AND EXTREME SOUTHERN BELIZE. SHEAR-LINE  
CONVECTION IN BOCAS DEL TORO-PANAMA THROUGH EASTERN COSTA RICA  
WILL LEAD TO 15-30MM/DAY AND MAXIMA OF 40-80MM...WHILE IN NORTHERN  
COSTA RICA/NICARAGUA EXPECTING MAXIMA OF 20-40MM. NUMEROUS SHOWERS  
ARE ALSO EXPECTED IN SOUTHEASTERN CUBA AND NORTHERN JAMAICA WHERE  
ACCUMULATIONS WILL REACH 05-10MM/DAY AND ISOLATED MAXIMA OF  
15-25MM IN REGIONS OF OROGRAPHIC INTERACTIONS. IN THE SOUTHEASTERN  
BAHAMAS AND NORTHEASTERN HISPANIOLA EXPECTING 05-10MM/DAY AND  
MAXIMA OF 15MM. BY FRIDAY-SATURDAY...HEAVY RAINFALL WILL CONTINUE  
AFFECTING THE CARIBBEAN COASTS OF SOUTHEASTERN COSTA RICA/WESTERN  
PANAMA...WHERE MOISTURE CONVERGENCE WILL PEAK TO SUSTAIN  
ACCUMULATIONS OF 30-60MM/DAY AND MAXIMA OF 100-150MM. IN NORTHERN  
COSTA RICA/SOUTHEASTERN NICARAGUA EXPECTING 15-20MM/DAY AND MAXIMA  
OF 30-60MM. SHEAR LINE CONVECTION WILL ALSO INCREASE IN EASTERN  
HISPANIOLA...TO PRODUCE 10-15MM/DAY AND MAXIMA OF 20-35MM. FRONTAL  
CONVECTION IN SOUTHEASTERN/CENTRAL CUBA AND THE SOUTHEASTERN  
BAHAMAS WILL ALSO PRODUCE MODERATE ACCUMULATIONS...AS IT INTERACTS  
WITH APPROACHING UPPER TROUGH. THIS WILL PRODUCE AMOUNTS OF  
05-10MM/DAY AND MAXIMA OF 15-25MM. BY SATURDAY-SUNDAY...FRONTAL  
CONVECTION IN CENTRAL-EASTERN CUBA AND THE CENTRAL BAHAMAS WILL  
LEAD TO RAINFALL AMOUNTS OF 05-10MM/DAY AND MAXIMA OF 15-20MM.  
CONTINUED CONVECTION IN THE CARIBBEAN COASTS OF COSTA RICA AND  
WESTERN PANAMA WILL LEAD TO 15-20MM/DAY AND MAXIMA OF 30-60MM. AS  
THE EASTERLY TRADES RETURN IN EASTERN CENTRAL AMERICA...EXPECTING  
A NEW INCREASE IN SHOWERS TO PRODUCE AMOUNTS OF 10-15MM/DAY AND  
MAXIMA OF 20-40MM FROM QUINTANA ROO AND NORTHERN BELIZE INTO  
EASTERN NICARAGUA...ALTHOUGH LARGEST AMOUNTS ARE EXPECTED  
OFFSHORE. BY SUNDAY-MONDAY...CONVECTION ALONG THE REMNANTS OF THE  
FRONT WILL LEAD TO 05-10MM/DAY AND MAXIMA OF 15-20MM IN THE  
CENTRAL BAHAMAS AND CENTRAL CUBA. TRADE WIND CONVERGENCE IN THE  
CARIBBEAN COASTS OF NICARAGUA...COSTA RICA AND WESTERN PANAMA WILL  
LEAD TO 10-15MM/DAY AND ISOLATED MAXIMA OF 20-40MM. LIGHTER  
ACCUMULATIONS ARE EXPECTED OTHERWISE.  
 
IN THE NORTHEASTERN CARIBBEAN...THE PERSISTING MID-LEVEL RIDGE  
PATTERN WILL LEADING TO MOSTLY FAIR CONDITIONS AND ISOLATED TRADE  
WIND SHOWERS. ON THURSDAY AFTERNOON AND EVENING...LARGEST  
ACCUMULATIONS ARE EXPECTED IN WESTERN PUERTO RICO AS A MOISTURE  
PLUME CROSSES THE REGION FROM EAST TO WEST. THIS WILL PRODUCE  
SCATTERED SHOWERS WITH AMOUNTS OF 10-15MM/DAY AND ISOLATED MAXIMA  
OF 20-40MM. A DECREASING TREND FOLLOWS...AS TRADE WINDS INCREASE  
IN SPEED AND ACQUIRE A NORTHEASTERLY COMPONENT ON SATURDAY AND  
SUNDAY. YET...THIS SHALLOW COLD-ADVECTIVE PATTERN WILL ENHANCE THE  
FREQUENCY AND VERTICAL DEVELOPMENT OF LATE NIGHT/EARLY MORNING  
TRADE WIND SHOWERS ACROSS THE CARIBBEAN ISLES...WHICH WILL SUSTAIN  
AN INCREASE IN DAILY ACCUMULATIONS TO 05-10MM/DAY AND MAXIMA OF  
15MM. THIS IS PARTICULARLY TRUE IN EASTERN PORTIONS OF CARIBBEAN  
ISLANDS FROM SATURDAY THROUGH MONDAY...INCLUDING THE LESSER  
ANTILLES.  
 
IN NORTHERN SOUTH AMERICA...UNSEASONABLY ACTIVE PATTERN OF  
CONVECTION IS PRODUCING NUMEROUS SHOWERS AND A FEW THUNDERSTORMS  
IN COLOMBIA AND VENEZUELA. STRONGEST CONVECTION IS OCCURRING IN  
THE AMAZON BASIN. VENTILATION IS BEING PROVIDED BY THE NORTHERN  
DIFFLUENT AND DIVERGENT TIER OF THE BOLIVIAN HIGH/SUBTROPICAL  
RIDGE OF SOUTH AMERICA. THIS IS TO DECREASE AS THE CYCLE  
PROGRESSES...WHICH WILL RESULT IN DECREASING ACCUMULATIONS. AT  
LOW-LEVELS...AN INCREASE IN THE SPEED OF THE TRADES IN THE  
SOUTHERN CARIBBEAN/VENEZUELA THROUGH SUNDAY WILL LEAD TO AN  
INCREASE IN LOW-LEVEL SHEAR AND TO THE ADVECTION OF A DRIER AIR  
MASS FROM THE TROPICAL ATLANTIC. THIS WILL LEAD TO A DECREASE IN  
ACCUMULATIONS IN VENEZUELA AND NORTHERN COLOMBIA...AND A RETURN TO  
MORE SEASONAL CONDITIONS. AMAZON CONVECTION WILL CONTINUE LEADING  
TO RAINFALL AMOUNTS OF 15-20MM/DAY AND MAXIMA OF 25-50MM THROUGH  
SATURDAY...WHILE ANDEAN CONVECTION WILL LEAD TO 05-10MM/DAY AND  
VERY ISOLATED MAXIMA OF 15-20MM ALSO THROUGH SATURDAY. OROGRAPHIC  
CONVECTION ALONG THE WESTERN SLOPES OF ECUADOR AND SOUTHERN  
COLOMBIA WILL LEAD TO ISOLATED AMOUNTS OF 10-15MM/DAY AND MAXIMA  
OF 20-35MM FROM FRIDAY THROUGH SUNDAY.  
 
THE INCREASE IN THE SPEED OF THE TRADES ACROSS THE SOUTHERN  
CARIBBEAN AND ATLANTIC WILL CONTINUE SUSTAINING TRADE WIND SHOWERS  
IN EASTERN VENEZUELA AND THE GUIANAS DUE TO TRADE WIND  
CONVERGENCE. IN VENEZUELA/GUYANA...THESE WILL SUSTAIN AMOUNTS OF  
05-10MM/DAY AND MAXIMA OF 15MM FROM FRIDAY THROUGH MONDAY. IN  
FRENCH GUIANA AND SURINAME...A DECREASING TREND WILL CONTINUE AS  
THE ATLANTIC ITCZ MEANDERS TO THE SOUTH TO POSITION ACROSS  
AMAPA-BRASIL BY SATURDAY-SUNDAY. INITIALLY...EXPECTING  
ACCUMULATIONS OF 10-15MM/DAY AND MAXIMA OF 20-30MM THROUGH EARLY  
FRIDAY...DECREASING SIGNIFICANTLY AFTER.  
 
TROPICAL/EASTERLY WAVES INITIALIZED AT 12UTC:  
INITIAL 24 36 48 60 72 84 96 TYPE  
SOF  
58W 60W 62W 64W 66W 69W 73W 77W TUTT INDCD 18N  
 
A TUTT-INDUCED WAVE IS INITIALIZED AT 58W AND BETWEEN 18N AND  
BARBADOS. THIS WAVE HAS A WEAK SIGNATURE IN 700 HPA...AND WILL  
PRODUCE LIMITED EFFECTS ON PRECIPITATION AS IT PROPAGATES TO THE  
WEST ACROSS THE GREATER ANTILLES. THE WAVE IS FORECAST TO AMPLIFY  
AND SPEED UP AS IT CROSSES HISPANIOLA ON SUNDAY...AIDED BY A  
DEVELOPING INCREASE IN THE SPEED OF THE TRADES BEHIND.  
 
GORDON...NMS (BELIZE)  
PECK...MS (JAMAICA)  
DAVISON/GALVEZ...WPC (USA)  
 

 
 
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