TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
812 AM EDT THU SEP 19 2024

WEEKLY DISCUSSION FOR PUERTO RICO AND THE USVI SEP 19/12UTC:

CURRENT CONDITIONS SHOW A LARGELY AMPLIFIED MID-UPPER FLOW PATTERN
FOR MID-SEPTEMBER. THIS CONSISTS OF A POTENT QUASI-STATIONARY
UPPER TROUGH ACROSS THE EAST COAST OF THE USA INTO WEST CUBA AND
THE GULF OF HONDURAS...AN ELONGATED RIDGE TO THE EAST...AND
ANOTHER UPPER TROUGH THAT EXTENDS IN THE NORTH ATLANTIC ACROSS 30N
51W INTO THE VI AND INTO NORTHERN COLOMBIA. IN ASSOCIATION WITH
THE TUTT...A LOW CENTERS NEAR 25N 54W. THIS ASSOCIATES WITH A
BROAD CYCLONIC CIRCULATION LOCATED JUST TO THE NORTH AND EAST OF
THE NORTHEAST CARIBBEAN. EMBEDDED IN THIS CIRCULATION...SEVERAL
WEAK FEEDER BAND-LIKE STRUCTURES ARE PROPAGATING SOUTH INTO THE
ISLANDS. ONE OF THEM CURRENTLY EXTENDS ACROSS THE NORTHERN LEEWARD
ISLANDS INTO THE VI. ALTHOUGH THE TUTT AND THE LOW-LEVEL BROAD
TROUGH ARE PRESENT...PRECIPITABLE WATER VALUES CONTINUE LIMITED
FOR THIS TIME OF THE YEAR. THEY LIE IN THE 45-48MM RANGE WHICH IS
BELOW NORMAL FOR MID-SEPTEMBER. THE LARGE SCALE PATTERN ASSOCIATED
WITH EQUATORIAL WAVES CONTINUES TO BE UPPER CONVERGENT. BOTH
CONSERVATIVE AMOUNTS OF AVAILABLE MOISTURE AND ENHANCED UPPER
CONVERGENCE ARE FAVORING GENERALLY FAIR WEATHER CONDITIONS ACROSS
THE NORTHEAST CARIBBEAN...WITH THE EXCEPTION OF TRADE WIND SHOWERS
OCCURRING IN THE CONVERGENCE BANDS.

THE TUTT IS FORECAST TO MOVE LITTLE THROUGH LATE SATURDAY...TO
THEN MEANDER EAST DUE TO A BUILDING UPPER RIDGE IN THE WESTERN
CARIBBEAN. THE LATTER IS ASSOCIATED WITH A DEVELOPING CENTRAL
AMERICAN GYRE (CAG)...WHICH IS OF RATHER MODERATE TO HIGH
CONFIDENCE AT THIS TIME. THIS IMPLIES THAT FROM SUNDAY INTO MID
NEXT WEEK...PUERTO RICO AND THE VI WILL BE UNDER THE INFLUENCE OF
NORTHERLY UPPER FLOW IN THE PERIPHERY OF THIS RIDGE. A TROUGH IS
FORECAST TO DEEPEN ACROSS THE NORTH ATLANTIC INTO THE LEEWARD
ISLANDS DURING TUESDAY/WEDNESDAY...WHICH COULD LIKELY ENHANCE
VENTILATION. YET...AVAILABLE MOISTURE WILL LIKELY REMAIN NEAR TO
BELOW NORMAL. THIS AND ENHANCE SUBSIDENCE IN THE PERIPHERY OF THE
CAG WILL LIKELY CONTINUE TO FAVOR LIMITED COVERAGE OF
PRECIPITATING CONVECTION THROUGH THE FORECAST CYCLE.

ON THURSDAY...EXPECT A CONVERGENCE BAND TO ENTER THE ISLANDS
DURING THE MORNING FAVORING MOISTURE ADVECTION DURING THE TIME OF
DIURNAL HEATING. THIS...IN COMBINATION WITH VERY WEAK TRADE WINDS
AND THE TUTT WILL FAVOR STRONG AFTERNOON CONVECTION ACROSS CENTRAL
PORTIONS OF PUERTO RICO. DRIER CONDITIONS ARE EXPECTED ON FRIDAY
AS THE FLOW CHANGES AND BECOMES SOUTHERLY...WHILE ADVECTING A
DRIER AIR MASS PRESENT JUST TO THE SOUTH OF THE ISLANDS.
STILL...WEAK WINDS AND THE INFLUENCE OF THE TUTT WILL FAVOR
AFTERNOON DEVELOPMENT ALONG MOST OF THE CORDILLERA TO FAVOR
LOCALLY MODERATE RAINFALL AMOUNTS. A DECREASING TREND FOLLOWS...AS
TRADE WINDS ACQUIRE A SOUTHEASTERLY COMPONENT TO ADVECT A DRIER
AIR MASS PRESENT IN THE EASTERN CARIBBEAN. THIS...IN COMBINATION
WITH THE STRENGTHENING OF A MID-LEVEL RIDGE WILL LIKELY YIELD TO
MORE CONSERVATIVE RAINFALL AMOUNTS ON SATURDAY AND SUNDAY. ON
MONDAY...EXPECT A SLIGHT INCREASE IN AVAILABLE MOISTURE AS A MOIST
PLUME IS ADVECTED INTO THE ISLANDS FROM THE SOUTH. THIS WILL FAVOR
AN INCREASE IN RAINFALL AMOUNTS IN DIURNAL CONVECTION...WHICH WILL
LIKELY CLUSTER IN CENTRAL AND NORTHERN PORTIONS OF PUERTO RICO
RESPONDING TO SOUTH-SOUTHEASTERLY TRADE WIND DIRECTIONS. THIS WILL
LIKELY HOLD THROUGH TUESDAY AND WEDNESDAY...WHEN EXPECT
POTENTIALLY A SLIGHT INCREASE IN AMOUNTS GIVEN THE INFLUENCE OF A
NORTH ATLANTIC TROUGH DEEPENING AND REACHING THE FAR NORTHEAST
CARIBBEAN.

NOTE: MADDEN-JULIAN CONDITIONS ARE LIKELY TO BECOME UPPER
DIVERGENT STARTING EARLY NEXT WEEK...LASTING INTO MID-OCTOBER.
ALTHOUGH THE MOST INTENSE ACTIVITY IN THE TROPICAL AMERICAS IS
FORECAST TO CLUSTER IN CENTRAL AMERICA AND THE WESTERN CARIBBEAN
EARLY AND MID NEXT WEEK...THIS CHANGE IN THE LARGE SCALE PATTERN
WILL POTENTIALLY YIELD TO A GENERAL INCREASE IN THE CHANCE FOR
HEAVY RAINFALL EVENTS DURING THE FIRST TWO WEEKS OF OCTOBER.

GALVEZ...WPC (USA)
ACOSTA...WPC (USA)
TINOCO...WPC (USA)