120
FXCA20 KWBC 181923
PMDCA
TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
323 PM EDT THU SEP 18 2025
FORECAST BULLETIN 18 SEPTEMBER 2025 AT 1930 UTC:
THE BAHAMAS AND CARIBBEAN:
A VERTICALLY STACKED MID-LATITUDE UPPER-LEVEL TROUGH OVER THE
SOUTHEAST U.S. WILL BE MOVING SLOWLY TO THE EAST DURING THE
FORECAST PERIOD. WITH THE BASE OF THE TROUGH IN THE YUCATAN,
ENHANCED DIVERGENCE IS LOCATED OVER CUBA AND THE BAHAMAS,
PARTICULARLY FROM THURSDAY INTO FRIDAY. THE UPPER-LEVEL TROUGH, IN
COMBINATION WITH A LOW-LEVEL TROUGH AND MOISTURE, IS EXPECTED TO
INCREASE INSTABILITY, RESULTING IN CONVECTIVE ACTIVITY AND
MODERATE PRECIPITATION. THE ENHANCEMENT OF DIVERGENCE DUE TO THE
PRESENCE OF THE UPPER-LEVEL TROUGH WILL CONTINUE ON FRIDAY, MOSTLY
FOR THE BAHAMAS, AND AS THE LOW-LEVEL TROUGH AND MOISTURE ARE
EXPECTED TO CONTINUE, PRECIPITATION IS LIKELY FOR THE FORECAST
PERIOD. A TROPICAL WAVE PROPAGATING WESTWARD OVER HISPANIOLA ON
THURSDAY IS EXPECTED TO BRING PRECIPITATION TO THE CARIBBEAN IN
THE NEXT THREE DAYS. INITIALLY, THIS TROPICAL WAVE, IN COMBINATION
WITH STRONG UPPER-LEVEL WINDS AND TOPOGRAPHY, IS EXPECTED TO
PRODUCE CONVECTIVE ACTIVITY OVER HISPANIOLA FROM THURSDAY INTO
FRIDAY. THEN, IT WILL ENHANCE PRECIPITATION OVER CUBA FROM FRIDAY
TO SATURDAY. A TUTT, CURRENTLY LOCATED NORTH OF PUERTO RICO, WILL
CONTINUE PROPAGATING NORTHWESTWARD. THIS TUTT WILL ENHANCE
INSTABILITY OVER PUERTO RICO AND THE U.S. VIRGIN ISLANDS, AND WITH
AN INDUCED TROUGH AT LOWER LEVELS, PRECIPITATION IS EXPECTED FROM
THURSDAY TO FRIDAY. THIS INSTABILITY WILL CONTINUE FROM FRIDAY TO
SATURDAY. MORE STABLE CONDITIONS ARE EXPECTED FROM SATURDAY INTO
SUNDAY.
MEXICO AND CENTRAL AMERICA:
IN NORTHWEST MEXICO, AN UPPER-LEVEL RIDGE WILL DOMINATE THE REGION
THROUGH THE NEXT THREE DAYS. SUPPORT FROM PROPAGATING MID-LEVEL
SHORTWAVE TROUGHS, AND MOISTURE CONVERGENCE ALONG THE WESTERN
COAST OF MEXICO, WILL INCREASE INSTABILITY AND DEEP CONVECTION IN
THE AREA.
MOST OF THE PRECIPITATION THE NEXT THREE DAYS IS EXPECTED OVER
SOUTHERN MEXICO AND PORTIONS OF CENTRAL AMERICA, INCLUDING
GUATEMALA, HONDURAS AND EL SALVADOR. THIS PRECIPITATION WILL BE
DUE TO THE ENHANCEMENT OF DIVERGENCE FROM THE UPPER-LEVEL TROUGH
IN THE U.S THAT EXTENDS INTO THE YUCATAN PENINSULA, ENHANCED
VERTICAL MOTION FROM MID-LEVEL TROUGHS AND ENHANCEMENT OF
LOW-LEVEL MOISTURE CONVERGENCE WITH A LOW-PRESSURE SYSTEM EMBEDDED
IN THE MONSOON TROUGH AND A TROPICAL WAVE CURRENTLY LOCATED AROUND
90W AS IT MOVES ALONG THE COAST IN THE EASTERN PACIFIC.
THE MONSOON TROUGH OVER COSTA RICA AND PANAMA WILL FAVOR LOW-LEVEL
CIRCULATION AND ENHANCE MOIST ONSHORE FLOW FROM THE PACIFIC OCEAN
AND FROM THE CARIBBEAN SEA. THE HIGHEST PRECIPITATION VALUES ARE
EXPECTED ON THURSDAY AND ARE EXPECTED TO DECREASE ON FRIDAY AND
SATURDAY.
SOUTH AMERICA:
ACROSS TROPICAL SOUTH AMERICA, SHOWERS AND THUNDERSTORMS WILL BE
OBSERVED EACH DAY. THE PRIMARY DRIVERS WILL BE LOW-LEVEL MOISTURE
CONVERGENCE AND DIURNAL HEATING. MOST OF THE PRECIPITATION WILL BE
OVER THE NORTHWESTERN REGION AS LONG-FETCH PACIFIC MOISTURE FEEDS
INTO WESTERN COLOMBIA, AND SOME DIVERGENCE IS EXPECTED FROM AN
UPPER LEVEL TROUGH IN THE CARIBBEAN ENTERING COLOMBIA AND
VENEZUELA, PARTICULARLY FROM THURSDAY INTO FRIDAY. THE REST OF
TROPICAL SOUTH AMERICA, HOWEVER, WILL SEE THE INFLUENCE OF DRY AIR
ADVECTION, ESPECIALLY ACROSS THE EAST AND CENTRAL REGION, WHERE
MINIMAL PRECIPITATION IS EXPECTED.
POSITIONS OF TROPICAL/EASTERLY WAVES INITIALIZED AT 12 UTC
TYPE SOF INIT 19/00 19/12 20/00 20/12 21/00 21/12 22/00 22/12
TW 22N 72W 74W 75W 76W 77W 78W 80W 82W 84W
TW 18N 88W 90W 92W 94W 96W 97W 98W 99W 100W
TW 21N 109W 111W 113W 114W 116W EXITS
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