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Tropical Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1127Z Jul 02, 2020)
 
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TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
727 AM EDT THU JUL 02 2020

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JUL 02/11
UTC: TROPICAL WAVE MOVED ACROSS THE EASTERN CARIBBEAN/FORECAST
AREA OVERNIGHT... WITH MEASURABLE RAINFALL AMOUNTS LIMITING TO
SOUTHWEST PORTIONS OF PUERTO RICO. DURING THE DAY...IN A
SOUTHEASTERLY STEERING FLOW...THIS COULD RESULT IN LIGHT
CONVECTION ACROSS NORTHEAST PUERTO RICO/SAN JUAN METRO AREA...WITH
A BETTER CHANCE FOR MEASURABLE RAINFALL AMOUNTS OVER NORTHWEST
PORTIONS OF THE ISLAND BETWEEN CAMUY AND AGUADILLA/RINCON THROUGH
THE EARLY AFTERNOON HOURS. LATER IN THE DAY...A DRYING TREND IS
EXPECTED AS MID LEVEL RIDGE ROLLS FROM THE EAST INTO THE NORTHEAST
CARIBBEAN AND A TRAILING SAHARAN AIR LAYER (SAL) ENTRAINS ACROSS
THE ISLANDS. THE ENVIRONMENTAL CONDITIONS ARE TO THEN REMAIN
RELATIVELY HOSTILE TO DEVELOPMENT THROUGH AT LEAST SATURDAY
MORNING...WITH FAIR WEATHER CONDITIONS TO PREVAIL ON FRIDAY.

SIMULTANEOUSLY...A DEEP UPPER LEVEL LOW OVER THE CENTRAL ATLANTIC
IS TO GRADUALLY EXTEND A TROUGH TO THE NORTHEAST CARIBBEAN. THIS
IS TO SHEAR A SHORT WAVE TROUGH THAT EVOLVES INTO A CLOSED LOW
OVER THE NORTHEAST CARIBBEAN. THE LOW IS FORECAST TO RETROGRESS
ACROSS THE FORECAST AREA TO HISPANIOLA DURING THE WEEKEND. AS IT
MEANDERS WEST...THE LOW/TROUGH ALOFT IS TO SUSTAIN AN INVERTED MID
LEVEL TROUGH OVER THE NORTHEAST CARIBBEAN. THIS APPEARS TO BE
STRONG ENOUGH TO SUSTAIN A WEAKNESS IN THE MID LEVEL RIDGE
PATTERN...WEAKENING THE TRADE WINDS INVERSION WHILE ALLOWING FOR
SHALLOW MOISTURE TO START POOLING FROM THE NORTHEAST. THIS WILL
LEAD TO A SLIGHT INCREASE IN DIURNAL CONVECTION DURING THE
WEEKEND. IN A NORTHEASTERLY STEERING FLOW ON SATURDAY...THE MOST
ACTIVE IS TO CLUSTER ALONG THE CORDILLERA IN SOUTHWEST PUERTO
RICO...WHILE ON SUNDAY IT WILL TEND TO FAVOR NORTHWEST PORTIONS OF
THE ISLAND. OVER THE VIRGIN ISLES TO EASTERN PUERTO RICO...FAIR
WEATHER CONDITIONS ARE LIKELY TO PREVAIL...WITH CHANCE FOR EARLY
MORNING SHOWERS TO STREAM FROM TIME-TO-TIME.

EARLY NEXT WEEK THE GLOBAL MODELS AGREE ON A VIGOROUS TROPICAL
WAVE TO ENTER THE ISLAND CHAIN LATER ON SUNDAY...AND BY DAYBREAK
ON MONDAY THE WAVE REACHES THE VIRGIN ISLES/EASTERN PUERTO RICO.
THIS IS TO SUSTAIN EARLY MORNING CONVECTION AS IT STREAMS ACROSS
THE VIRGIN ISLES TO EASTERN PUERTO RICO...BUT AS IT MOVES ACROSS
THE ISLAND DURING THE DAYTIME HOURS...THIS ONE MIGHT HAVE A BETTER
CHANCE OF TRIGGERING MORE WIDESPREAD CONVECTION AS ENHANCED BY
DIURNAL HEATING THAN THE ONE EARLY THIS MORNING. THIS IS A
SOLUTION THAT BOTH THE UKMET AND GFS SUPPORTS...BUT NOT THE
ECMWF...WHICH ADDS UNCERTAINTY AND A LOWER CONFIDENCE TO THE
MEDIUM RANGE FORECAST.

DAVISON...WPC (USA)