TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
651 AM EDT THU JUL 20 2017

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JUL 20/06
UTC: NOTE THAT A NEWER VERSION OF THE GFS MODEL IS NOW IN EFFECT.

THE FORECAST AREA REMAINS UNDER THE INFLUENCE OF A BROAD CELL OF
THE SUBTROPICAL RIDGE THAT DOMINATES THE WESTERN ATLANTIC-NORTHERN
CARIBBEAN ISLES. THIS FAVORS A FAIRLY STRONG SUBSIDENCE
CAP...WHICH IN COMBINATION WITH A DENSE SAHARAN AIR LAYER TENDS TO
FAVOR FAIR WEATHER CONDITIONS. AT LOW LEVELS...AN EASTERLY WAVE IS
MOVING TOWARDS THE LEEWARD/FRENCH ISLES EARLY THIS MORNING. A
TROPICAL WAVE ALONG 50W/51W FOLLOWS THIS AXIS.

THE EASTERLY WAVE REACHES THE VIRGIN ISLES/EASTERN PUERTO RICO
EARLY ON FRIDAY MORNING. ALTHOUGH THE MODELS FAVOR A WELL DEFINED
PERTURBATION...IT IS GOING TO HAVE A HARD TIME OVERCOMING NEGATIVE
INFLUENCE OF PERSISTENT MID LEVEL RIDGE PATTERN AND ENSUING
EROSION OF AVAILABLE MOISTURE AS IT MIXES WITH THE DRYER SAHARAN
AIR LAYER. THUS WE ONLY EXPECT WIDELY ISOLATED CONVECTION WITH
THIS FEATURE AND LIGHT RAINFALL AMOUNTS...WITH MOST ACTIVE
CLUSTERING ACROSS THE VI-EASTERN PUERTO RICO AND POSSIBLY THE SAN
JUAN METRO AREA IN MORNING CONVECTION. RAINFALL AMOUNTS ARE TO
GENERALLY PEAK AROUND A QUARTER OF AN INCH.

THE TROPICAL WAVE THAT FOLLOWS MOVES ACROSS THE ISLAND CHAIN LATER
ON SATURDAY...AND ACROSS THE VIRGIN ISLES TO EASTERN PUERTO RICO
BY DAYBREAK ON SUNDAY. BY THAT TIME THE SAL SHOULD BE DILUTED
ENOUGH TO ALLOW FOR DEEPER MOISTURE TO ENVELOP THE FORECAST AREA.
THE GLOBAL MODELS ARE IN GOOD AGREEMENT WITH THE TIMING OF THIS
FEATURE...WITH ALL SHOWING AN INCREASE IN CONVECTION DURING THE
DAY ON SUNDAY TO MONDAY. THIS IS TO INITIALLY FAVOR LIGHT TO
MODERATE RAINFALL AMOUNTS OVER THE VIRGIN ISLES-EASTERN PUERTO
RICO. HOWEVER...EXPECTING PERSISTENT MID LEVEL RIDGE PATTERN TO
DELAY ONSET OF DEEP CONVECTION TO MID AFTERNOON HOURS ON SUNDAY
AND AGAIN ON MONDAY...WITH MOST ACTIVE TO CLUSTER OVER WESTERN
INTERIOR PORTIONS OF PUERTO RICO.

GAJADHAR...SLMS (SAINT LUCIA)
DAVISON...WPC (USA)