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Tropical Discussion
 
(Latest Discussion - Issued 1205Z Oct 13, 2020)
 
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TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
805 AM EDT TUE OCT 13 2020

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM OCT
13/12UTC: OVER THE WESTERN ATLANTIC...A MID/UPPER LEVEL RIDGE
DOMINATES THE FLOW NORTH OF THE ISLANDS. EAST OF THIS AXIS...A
TUTT LOW OVER THE CENTRAL ATLANTIC EXTENDS A TROUGH SOUTH ACROSS
THE LEEWARD ISLANDS INTO THE NORTHEAST CARIBBEAN. AT LOW
LEVELS...A TROPICAL WAVE IS STREAMING ACROSS THE EASTERN
CARIBBEAN...WHILE A MORE ROBUST PERTURBATION LIES TO THE EAST OF
THE ISLANDS. THE FORMER IS TO ENHANCE MOISTURE TRANSPORT ACROSS
THE FORECAST AREA DURING THE DAY TODAY...TO FEED DIURNAL
DEVELOPMENT. MOST INTENSE IS TO CLUSTER BETWEEN AGUADA/AGUADILLA
AND BARCELONETA LATER THIS MORNING INTO THE AFTERNOON HOURS...WITH
A SECONDARY MAXIMA TO AFFECT SOUTHERN PORTIONS OF THE SAN JUAN
METRO AREA. UNDER INFLUENCE OF TUTT AXIS ALOFT...LIGHT NOCTURNAL
CONVECTION IS TO THEN AFFECT THE VIRGIN ISLES-EASTERN PUERTO RICO
AS THE WAVE PULLS AWAY AND TRAILING PLUME OF MOISTURE ENVELOPS THE
ISLANDS. THE MOIST PLUME WILL THEN FEED ANOTHER ROUND OF DEEP
DIURNAL CONVECTION...WITH MOST INTENSE CLUSTERING  TO CLUSTER OVER
WESTERN INTERIOR PORTIONS OF PUERTO RICO ON WEDNESDAY.

ALONG THE MEANDERING TUTT...MODELS THEN FORECAST A LOW TO CLOSE
OVER THE NORTHEAST CARIBBEAN ON WEDNESDAY EVENING...BUT THEN
DIVERGE ON HOW THIS IS GOING TO EVOLVE. UNDER THE INFLUENCE OF THE
RIDGE TO THE NORTH...THE GFS FORECASTS THE LOW TO RETROGRESS
ACROSS THE FORECAST AREA TO EASTERN HISPANIOLA ON THURSDAY...WHILE
THE ECMWF FORECASTS THE UPPER LOW TO REMAIN NEARLY STATIONARY OVER
THE NORTHEAST CARIBBEAN. THIS LEADS TO MAJOR DIFFERENCES AMONG THE
MODELS ON HOW THIS IS TO AFFECT THE NORTHEAST CARIBBEAN...WITH THE
GFS FORESEEING DEEPER CONVECTIVE INSTABILITY AND A BETTER
POTENTIAL FOR ORGANIZED DEEP CONVECTION THAN THE ECMWF.
LIKEWISE...AT LOW LEVELS...THE GFS IS FORECASTING THE NEXT
TROPICAL WAVE TO STREAM ACROSS THE ISLAND CHAIN EARLY ON
THURSDAY...AND THROUGH THE DAY TO CONTINUE ACROSS THE FORECAST
AREA WHILE PHASING WITH THE RETROGRESSING UPPER LEVEL LOW. THE
ECMWF DIGRESSES FROM THIS SOLUTION...FAVORING A MUCH SLOWER WAVE
THAT TENDS TO SPLIT AS IT INTERACTS WITH THE MEANDERING UPPER
LEVEL LOW. THUS DIVERGING ON ITS IMPACT OVER THE FORECAST
AREA...WITH THE GFS FAVORING A WETTER SOLUTION THAN THE EUROPEAN
MODEL. THIS LEADS TO A HIGH UNCERTAINTY AND LOW CONFIDENCE IN THE
FORECAST BEYOND 48 HRS. THE LATEST RUNS OF THE EC
ENSEMBLES...HOWEVER...LEND THEIR SUPPORT TO THE MORE AGGRESSIVE
GFS SOLUTION. THUS TILTING THE SCALE IN FAVOR OF THE GFS. THIS IS
REFLECTED IN OUR SHORT RANGE FORECAST FOR THE NEXT THREE DAYS.

DAVISON...WPC (USA)