Prognostic Meteorological Discussion
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FXCA20 KWBC 201121
PMDCA

TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
721 AM EDT WED AUG 20 2014

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI. TUTT LOW NEAR
23N 63W EXTENDS A SHORT WAVE TROUGH ACROSS THE NORTHEAST CARIBBEAN
ISLANDS. THIS IS ENHANCING DEEP LAYER CONVECTIVE
INSTABILITY...FAVORING GENERATION OF EARLY MORNING CONVECTION
ACROSS THE FORECAST AREA. THE TUTT IS TO PULL TO THE
WEST-NORTHWEST AS A RIDGE BUILDS FROM THE EAST ACROSS THE TROPICAL
ATLANTIC-NORTHEAST CARIBBEAN...WITH RIDGE EXPECTED TO ENVELOP THE
FORECAST AREA LATER ON FRIDAY-EARLY ON SATURDAY. MEANWHILE...UNDER
INFLUENCE OF THE TUTT AND ASSOCIATED MID LEVEL PERTURBATION...DEEP
LAYER CONVECTIVE INSTABILITY IS TO PERSIST ACROSS THE FORECAST
AREA. GDI SHOWS BEST CHANCE FOR THUNDERSTORMS THROUGH THE MORNING
HOURS INTO MID AFTERNOON. DURING THAT PERIOD...THE GLOBAL MODEL
GUIDANCE SHOWS MOST ACTIVE CONVECTION CLUSTERING ALONG THE
NORTHERN HALF OF PUERTO RICO...WITH RAINFALL MAXIMA OF 1-2 INCHES.
HIGHER AMOUNTS ARE EXPECTED ACROSS WESTERN PUERTO RICO...WITH HIGH
RESOLUTION MODELS SHOWING MAXIMA PEAKING AT 3-4 INCHES.

UPSTREAM OVER THE TROPICAL ATLANTIC...TWO WAVES DOMINATE THE FLOW
AS THEY INTERACT WITH THE ITCZ...WITH ONE ALONG 52W/53W AND THE
OTHER NEAR 44W/45W. THE NHC IS MONITORING THESE PERTURBATIONS AND
ASSOCIATED CYCLONIC CIRCULATION FOR POSSIBILITY OF TROPICAL
CYCLONE FORMATION OVER THE NEXT FEW DAYS. PER GLOBAL
MODELS...LOOKS LIKE THESE PERTURBATIONS ARE TO GRADUALLY COMBINE
AS THEY MEANDER WEST ACROSS THE GUIANAS TO THE LESSER ANTILLES.
THEY NOW FAVOR A MUCH FASTER PROGRESSION...WITH STRONG WAVE
ENTERING THE FORECAST AREA DURING THE DAY ON FRIDAY-EARLY SATURDAY
MORNING. THE UKMET FAVORS A STRONGER PERTURBATION/ASSOCIATED
CIRCULATION THAN WHAT THE ECMWF AND GFS SUGGEST...WITH THE GFS
FAVORING A SLIGHTLY STRONGER WAVE THAN THE ECMWF. THE HIGH
VARIABILITY IS ALSO DISPLAYED BY THE NCEP ENSEMBLE
MEMBERS...CREATING A LOT OF UNCERTAINTY FOR SHORT-MEDIUM RANGE
FORECAST PERIOD. THE VELOCITY POTENTIAL ANOMALIES ANALYSIS SHOWS
NEUTRAL TO SLIGHTLY UNFAVORABLE MJO CONDITIONS ACROSS THIS PORTION
OF THE BASIN. THIS MIGHT BE ENOUGH TO SLOW DOWN DEVELOPMENT...BUT
NOT STRONG ENOUGH TO SUPPRESS IT. SO AT THIS POINT...IT LOOKS LIKE
THE SAFEST BET IS A BLEND BETWEEN THE GFS AND ECMWF...AS BOTH TEND
TO BE CONSIDERABLY WEAKER THAN THE UKMET. NEVERTHELESS...THIS HAS
THE POTENTIAL OF BEING A MODERATE TO HEAVY RAINFALL
PRODUCER...WITH RAINFALL MAXIMA OF 4-6 INCHES ACROSS SAINT CROIX
AND SOUTHERN HALF OF PUERTO RICO.

VALVERDE...IMN (COSTA RICA)
MEDINA...ONAMET (REPUBLICA DOMINICANA)
DAVISON...WPC(USA)
GALVEZ...WPC(USA)

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