TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
749 AM EDT MON SEP 27 2021

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM SEP
27/12UTC: NHC IS ISSUING ADVISORIES FOR HURRICANE SAM...A MAJOR
HURRICANE. PLEASE REFER TO THEIR DISCUSSION/ADVISORIES FOR
OFFICIAL FORECAST GUIDANCE.

A MEANDERING MID/UPPER LEVEL TROUGH DOMINATES THE FLOW OVER THE
WESTERN ATLANTIC...WITH AXIS SOUTH ACROSS THE BAHAMAS-CUBA TO THE
NORTHERN CARIBBEAN. UPPER LEVEL AXIS IS TO REMAIN NEARLY
STATIONARY THROUGH TUESDAY. ON WEDNESDAY...AS A SHORT WAVE TROUGH
ROUNDS THE LONG WAVE AXIS...IT IS FORECAST TO SHIFT EASTWARD OVER
THE FORECAST AREA. THIS WILL LEAD TO A GRADUAL INCREASE IN
CONVECTIVE INSTABILITY ACROSS THE FORECAST AREA THROUGH MIDWEEK AS
SUGGESTED BY THE GFS AND ECMWF GDI FORECAST. AT LOW LEVELS...EAST
TO SOUTHEASTERLY WINDS PREVAIL...A PATTERN THAT HOLDS THROUGH
WEDNESDAY MORNING. LATER IN THE DAY...AS HURRICANE SAM LIFTS EAST
OF THE ISLANDS AND AN INDUCED TROUGH ENTERS THE EASTERN
CARIBBEAN...THE LOW/MID LEVEL WINDS ARE TO BACK TO THE NORTHEAST.
THIS COULD LAST THROUGH THURSDAY AFTERNOON...WITH STEERING FLOW
BECOMING ILL DEFINED LATER IN THE DAY. BUT LATER IN THE
WEEK...EAST-SOUTHEASTERLY WINDS RETURN AS THE HURRICANE MOVES
FARTHER TO THE NORTH AND THE WANING INDUCED TROUGH SETTLES OVER
THE EASTERN CARIBBEAN. THE LIGHTER STEERING WINDS WILL COMBINE
WITH THE DIURNAL SEA BREEZE TO SUSTAIN MOISTURE CONVERGENCE ACROSS
INTERIOR PORTIONS OF PUERTO RICO.

AS EVIDENT ON THE PWAT AND GDI FORECASTS...OUTER BAND CONVECTION
WITH HURRICANE SAM WILL LIMIT TO THE NORTHERN WINDWARD ISLANDS
STARTING ON WEDNESDAY AND LASTING THROUGH THURSDAY MORNING. OVER
PUERTO RICO AND THE VIRGIN ISLES AIR MASS TYPE/SEA BREEZE DRIVEN
CONVECTION WILL THEN PREVAIL. THIS IS TO GROW MORE PROLIFIC AND
BETTER ORGANIZED THROUGH WEDNESDAY AS ENHANCED BY THE UPPER LEVEL
TROUGH. BY MIDWEEK THERE IS A STRONG CONSENSUS AMOUNT THE
DETERMINISTIC AND ENSEMBLE MODEL GUIDANCE ON POTENTIAL FOR HEAVY
RAINFALL AMOUNTS TO CLUSTER OVER INTERIOR PORTIONS OF PUERTO RICO.
ON THURSDAY...HOWEVER...THE CONSENSUS AMONG THE MODELS IS NOT AS
STRONG...WITH SOME OF THE MODELS SUGGESTING A SECOND ROUND OF
MODERATE RAINFALL AMOUNTS AS SHALLOW CONVECTIVE INSTABILITY
LINGERS OVER THE ISLAND. THIS SEEMS QUITE PLAUSIBLE AS SUGGESTED
BY MID/UPPER LEVEL PATTERN EVOLUTION.

DAVISON...(USA)