THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


CARIBBEAN FORECAST DISCUSSION




TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
627 AM EST TUE NOV 21 2017

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM NOV 21/06
UTC: AT 500/250 HPA...CELL OF THE SUBEQUATORIAL RIDGE IS QUICKLY
BECOMING THE DOMINANT FEATURE ACROSS THE EASTERN CARIBBEAN...WITH
AXIS TO CENTER AT 500 HPA ON A CLOSED HIGH OVER PUERTO RICO LATER
ON WEDNESDAY. THIS PATTERN IS FORECAST TO HOLD NEARLY UNCHANGED
THROUGH SUNDAY EVENING. AS IT BUILDS ACROSS THE EASTERN
CARIBBEAN...THE MID LEVEL RIDGE FAVORS A STRONG CAP INVERSION THAT
IS TO SUSTAIN A SHARP DROP IN AVAILABLE WATER DURING THE DAY
TODAY...WITH A PWAT MINIMA AND DRY/STABLE CONDITIONS TO PERSIST
THROUGH AT LEAST THURSDAY AFTERNOON.

AT LOW LEVELS...BROAD POLAR RIDGE OVER THE WESTERN NORTH ATLANTIC
FAVORS A TIGHT GRADIENT AND BRISK EASTERLY TRADES ACROSS THE
EASTERN CARIBBEAN ISLES. LOW LEVEL RIDGE PATTERN IS EXPECTED TO
WEAKEN ON WEDNESDAY...WITH LIGHTER WINDS TO THEN ENVELOP THE
FORECAST AREA LATER IN THE DAY. EMBEDDED IN THIS FLOW...A TUTT
INDUCED PERTURBATION IN THE EASTERLY TRADES MOVES ACROSS THE
ISLAND CHAIN TO THE VIRGIN ISLES LATER ON THURSDAY...REACHING
EASTERN PUERTO RICO BY FIRST LIGHT ON FRIDAY. THIS WILL BE
ACCOMPANIED BY A SHALLOW MOIST TONGUE THAT IS FORECAST TO ADVECT
FROM THE EAST. THIS...HOWEVER...WILL BE SHORT LASTING...WITH A
DRYER AIR MASS TO ONCE AGAIN SETTLE ACROSS THE NORTHEAST CARIBBEAN
ISLES ON SATURDAY-SUNDAY.

ALTHOUGH IN AGREEMENT ON HOW PATTERN EVOLVES THROUGH THE MEDIUM
RANGE PERIOD...THE GLOBAL MODELS CONTINUE TO DISAGREE ON ITS
IMPACT ACROSS THE FORECAST AREA...WITH THE ECMWF CONSISTENTLY
FORECASTING TRACE RAINFALL AMOUNTS WITH THIS WAVE. THE UKMET
REVERSED TRENDS AND NOW JOINS THE GFS IN FORECASTING LIGHT TO
MODERATE CONVECTION IN ASSOCIATION WITH THIS FEATURE LATER ON
THURSDAY-FRIDAY. ALTHOUGH THERE IS POTENTIAL FOR SHOWERS TO
DEVELOP...UNDER NEGATIVE INFLUENCE OF MID LEVEL HIGH/RIDGE...THE
ENVIRONMENTAL CONDITIONS WILL REMAIN HOSTILE FOR ORGANIZED DEEP
CONVECTION TO DEVELOP. AS A RESULT...EXPECTING SLIGHTLY WETTER
CONDITIONS THAN WHAT THE ECMWF SUGGESTS...BUT NOT AS WET AS
FORECAST BY THE GFS.

DAVISON...WPC (USA)                                               
                     




Last Updated: 627 AM EST TUE NOV 21 2017