TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
846 AM EDT THU SEP 21 2023

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI SEP 21/12UTC:
A TROPICAL WAVE IS APPROACHING THE ISLANDS TODAY INTO
FRIDAY...CAUSING A GRADUAL INCREASE IN MOISTURE. AN INCREASE IN
SHOWER ACTIVITY COVERAGE IS ALSO FORECAST..ESPECIALLY LATER TODAY
AND OVERNIGHT. SHOWERS WITH POSSIBLE THUNDERSTORMS ARE FORECAST
ACROSS THE WESTERN SECTIONS OF PR EACH AFTERNOON OVER THE 5-DAY
FORECAST PERIOD. THAT SAID...SOME SHOWER ACTIVITY IS ALSO EXPECTED
ACROSS THE USVI AND EASTERN PR...ESPECIALLY OVERNIGHT TONIGHT AS
THE TROPICAL WAVE APPROACHES...AND THEN AGAIN ON MONDAY AS ANOTHER
INCREASE IN MOISTURE IS EXPECTED. PRECIPITABLE WATER IS EXPECTED
TO BE BETWEEN 1.6 AND 2.1 INCHES THROUGH THE 5-DAY PERIOD...WITH
THE HIGHEST PEAKS IN MOISTURE THIS EVENING AND DURING THE DAY ON
MONDAY...WHILE THE LOWEST MOISTURE IS EXPECTED GENERALLY ON
SATURDAY INTO EARLY SUNDAY.

THERE IS GOOD POTENTIAL FOR SHOWERS AND THUNDERSTORMS TODAY ACROSS
WESTERN PR AND AREAS NEAR THE SAN JUAN METRO INTO NORTH CENTRAL
PR...DUE TO THE INCREASE IN MOISTURE COMBINING WITH THE OROGRAPHIC
LIFTING AND SEA BREEZE CONVERGENCE. OUR FORECAST INDICATES 1-2
INCHES OF RAIN ACROSS SEVERAL SECTORS OF PR...BUT THE HI-RES
GUIDANCE ONCE AGAIN INDICATES THAT ISOLATED HIGHER AMOUNTS ARE
POSSIBLE. THE U.S. VIRGIN ISLANDS ARE EXPECTED TO OBSERVE BETWEEN
A TENTH AND A QUARTER OF AN INCH TODAY AND TONIHGT. THE EGDI
ALGORITHM HAS BEEN CONSISTENT AND IT CONTINUES TO INDICATE
POSSIBLE THUNDERSTORMS THIS AFTERNOON AND FRIDAY. SATURDAY STILL
LOOKS A BIT DRIER...WITH THE PRECIPITABLE WATER AND THE LOW TO MID
LEVEL RELATIVE HUMIDITY DECREASING. THE EGDI ALSO HAS LITTLE TO NO
INDICATION OF THUNDERSTORM POTENTIAL ON SATURDAY. HOWEVER...EVEN
THOUGH AVAILABLE MOISTURE IS EXPECTED TO INCREASE STARING LATE
SUNDAY INTO MONDAY...THE EGDI IS VERY MODEST WITH ITS
THUNDERSTORMS POTENTIAL...POSSIBLY BECAUSE THE MODELS SHOW A
STRONG HIGH PRESSURE IN THE 400-200MB LAYER OVER THE
AREA...PROMOTING SOME STABILITY.

THE 5-DAY RAINFALL TOTAL IS FORECAST TO BE AROUND 5 INCHES ACROSS
THE WESTERN INTERIOR TO NORTHWEST PR...EASTERN PR IS FORECAST
AROUND 2 INCHES...WHILE THE USVI IS FORECAST BETWEEN 0.50 AND 0.75
INCHES.

ALAMO...WPC (USA)