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(Caution: Version displayed is not the latest version. - Issued 1220Z Jul 09, 2020)
 
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TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
820 AM EDT THU JUL 09 2020

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JUL 09/12
UTC: ANOTHER MOSTLY FAIR WEATHER DAY IS EXPECTED TODAY AS A DRY
AND DUSTY SAHARAN AIR LAYER LINGERS OVER THE NORTHEAST CARIBBEAN.
THIS...HOWEVER...CHANGES ON FRIDAY AS A RETROGRESSING TUTT INDUCES
A LOW-MID LEVEL PERTURBATION AS IT ENTERS THE NORTHEAST CARIBBEAN.
UNDER INFLUENCE OF THIS PERTURBATION...THE PWAT CONTENT BRIEFLY
INCREASES TO AROUND 1.5 INCHES EARLY ON FRIDAY MORNING...LEADING
TO A ROUND OF MOSTLY LIGHT CONVECTION AS IT STREAMS ACROSS THE
VIRGIN ISLES-EASTERN PUERTO RICO. THROUGH MID AFTERNOON ACTIVITY
WILL GRADUALLY EBB FROM THE EAST...WITH FOCUS OF THE AFTERNOON
CONVECTION SHIFTING TO NORTHWEST PUERTO RICO...WITH MOST ACTIVE
EXPECTED BETWEEN BARCELONETA AND AGUADILLA TO THE NORTHWEST WHERE
IT IS TO LIKELY RESULT IN LOCALLY MODERATE RAINFALL AMOUNTS.

THIS SETS THE STAGE FOR THE GRADUAL EROSION OF THE TRADE WINDS
INVERSION DURING THE WEEKEND...WITH VIGOROUS TROPICAL WAVE
ENTERING THE EASTERN CARIBBEAN ON SATURDAY AFTERNOON. THE GLOBAL
MODELS THEN SHOW THIS PERTURBATION STREAMING ACROSS THE FORECAST
AREA DURING THE MORNING HOURS ON SUNDAY. ALTHOUGH THE MODELS
GENERALLY AGREE ON THE TIMING OF THIS WAVE...THEY DIVERGE ON ITS
IMPACT OVER THE NORTHEAST CARIBBEAN ISLANDS. THE GFS AND ECMWF IN
PARTICULAR...BOTH SHOW PWAT CONTENT SURGING TO AROUND TWO INCHES
AS THE WAVES MAKES LANDFALL OVER THE VIRGIN ISLES-EASTERN PUERTO
RICO. THE EUROPEAN MODEL THEN SHOWS ITS MOISTURE QUICKLY EBBING
DURING THE DAY ON SUNDAY...WHILE THE GFS PREFERS A HIGHER MOISTURE
CONTENT THROUGH THE DAY INTO THE EVENING HOURS. BUT AS THE MJO
TRANSITIONS TO ITS CONVERGENT PHASE EARLY NEXT WEEK...THE GFS
MIGHT BE TOO BULLISH WITH THE MOISTURE CONTENT. THUS A BLEND OF
THESE MODELS MIGHT OFFER THE BETTER SOLUTION...WITH TRAILING
MOISTURE LASTING THROUGH MIDDAY THEN RAPIDLY EBBING DURING THE
AFTERNOON HOURS. OVER THE VIRGIN ISLES-EASTERN PUERTO RICO/SAN
JUAN METRO THE MOST ACTIVE IS EXPECTED BETWEEN 06-15 UTC ON
SUNDAY...WITH SECONDARY CONVECTION EXPECTED OVER NORTHWEST PUERTO
RICO IN THE AFTERNOON. UNCERTAINTY...HOWEVER...REMAINS HIGH AND
CONFIDENCE IN THE FORECAST IS LOW DURING THE MEDIUM RANGE PERIOD.

DAVISON...WPC (USA)