Prognostic Meteorological Discussion
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FXCA20 KWBC 201126
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TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
726 AM EDT FRI OCT 20 2017

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE VI FROM OCT
20/00UTC: FAIR AND WINDY CONDITIONS ARE TO PERSIST INTO EARLY NEXT
WEEK. ANY PRECIPITATION WILL BE LIGHT AND ISOLATED...GENERALLY IN
THE FORM OF FAST MOVING TRADE WIND SHOWERS.

AT UPPER LEVELS...A LARGE TROUGH SITS OVER THE TROPICAL ATLANTIC.
THIS TROUGH CENTERS ON A LOW AT 21N 42W...AND EXTENDS AN AXIS INTO
THE CENTRAL LESSER ANTILLES. THE UPPER TROUGH IS INTERACTING WITH
A TROPICAL WAVE THAT EXTENDS NEAR 40W. TO THE WEST OF THE
LOW...HOWEVER...AN UPPER CONVERGENT PATTERN IS DOMINATING.
ALSO...AN UPPER TROUGH EXTENDS OFF THE EAST COAST OF THE UNITED
STATES AND IS PROPAGATING TO THE SOUTHEAST. AT LOW-LEVELS...A
SIGNIFICANT FEATURE IS A READE WIND SURGE THAT CROSSED THE ISLANDS
LATE ON WEDNESDAY...AND NOW EXTENDS ACROSS CENTRAL HISPANIOLA. THE
AIR MASS TO THE EAST OF THE LEADING EDGE OF THE SURGE IS
CHARACTERIZED BY A SUBSIDENCE CAP SITTING AT 900 HPA...AND BY A
VERY DRY AIR MASS THAT SITS OVER THE CAP AND EXTENDS INTO THE
MID/UPPER TROPOSPHERE. TRADE WINDS ARE FAST...WITH SPEEDS BETWEEN
20 AND 30KT. THIS IS LIMITING ACTIVITY TO VERY ISOLATED AND
RAPIDLY MOVING TRADE WIND SHOWERS.

MODELS ARE SHOWING THIS PATTERN PERSISTING INTO EARLY NEXT WEEK.
THE UPPER TROUGH EXITING THE EAST COAST OF THE UNITED STATES IS
FORECAST TO CONTINUE PROPAGATING TO THE SOUTHEAST TO SIT JUST TO
THE NORTH OF THE ISLANDS BY SUNDAY. YET...THE LOW AND MID-LEVELS
WILL BE TOO DRY TO SUSTAIN ANY SIGNIFICANT PRECIPITATION. FROM
SUNDAY TO TUESDAY...THE UPPER TROUGH WILL SIT JUST TO THE NORTH OF
THE LEEWARD ISLANDS NEAR 24N 63W. IT APPEARS THAT THE POSITIVE
INTERACTION BETWEEN THE UPPER TROUGH AND THE MOIST PLUME
ASSOCIATED WITH THE TROPICAL WAVE AND THE ITCZ WILL HAPPEN TO THE
SOUTH OF THE FRENCH ANTILLES WHILE AN UPPER CONVERGENT PATTERN
STRENGTHENS OVER THE ISLANDS. THIS WILL CONTINUE SUSTAINING
CURRENT FAIR WEATHER PATTERN INTO EARLY NEXT WEEK.

IN TERMS OF THE DISTRIBUTION OF PRECIPITATION...EXPECTING LARGEST
AMOUNTS IN EASTERN PUERTO RICO IN THE FORM OF LIGHT AND
SHORT-LIVED TRADE WIND SHOWERS. THESE ARE TO PRODUCE ACCUMULATIONS
OF 0.2-0.3 INCHES/DAY. SIMILAR AMOUNTS ARE EXPECTED IN AFTERNOON
SHOWERS IN EXTREME WESTERN PUERTO RICO. EASTERLY TRADES OF 15-20KT
ARE TO PERSIST THROUGH LATE FRIDAY...TO THEN DECREASE TO 10-15KT.

GUY...NMS (BELIZE)
PECK...MS (JAMAICA)
GALVEZ...WPC (USA)
$$





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