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Tropical Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1146Z Dec 05, 2017)
 
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TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
646 AM EST TUE DEC 05 2017

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM DEC 05/06
UTC: MID/UPPER LEVEL SHORT WAVE TROUGH MOVED ACROSS THE BAHAMAS
EARLY THIS MORNING...AND AS IT STREAMS ACROSS THE WESTERN ATLANTIC
IT IS RAPIDLY MERGING WITH BROAD TROUGH TO THE EAST. AS THEY
MERGE...SUBTROPICAL JET MAXIMA NORTH OF THE FORECAST AREA IS
FORECAST TO STRENGTHEN. DIVERGENCE ON ITS RIGHT ENTRANCE REGION IS
TO THEN ENVELOP THE NORTHEAST CARIBBEAN ISLES THROUGH THURSDAY.
BUILDING MID/UPPER LEVEL RIDGE WILL THEN BECOME THE DOMINANT UPPER
FEATURE LATER IN THE WEEK.

AT LOW LEVELS...A MEANDERING FRONT LIES NORTH OF THE ISLANDS WITH
AXIS EXTENDING EAST-TO-WEST ALONG 21N/22N TO THE BAHAMAS-CUBA.
HIGH PRESSURE RIDGE ROLLING ACROSS THE EASTERN USA TO THE WESTERN
ATLANTIC EARLY THIS MORNING CONTINUES TO TIGHTEN THE PRESSURE
GRADIENT. THIS...IN-TURN...FAVORS THE GRADUAL INTENSIFICATION OF
THE EASTERLY TRADES WHILE TRIGGERING A PREFRONTAL SHEAR LINE
ACROSS PUERTO RICO-VIRGIN ISLES. THE SHEAR LINE WILL CONTINUE TO
THE LEEWARD ISLES EARLY THIS AFTERNOON...WITH BOUNDARY LAYER WINDS
INCREASING TO 18-21KT. THE STRONGER WINDS WILL FAVOR GENERATION OF
STREAMERS ACROSS THE LEEWARD/VIRGIN ISLES THAT ARE TO AFFECT
EASTERN PUERTO RICO FROM TIME-TO-TIME. THIS IS TO ALSO FAVOR A
COLD ADVECTIVE PATTERN...DRIVING THE MEANDERING FRONT SOUTH ACROSS
THE VIRGIN ISLES TO EASTERN PUERTO RICO LATER THIS EVENING/EARLY
WEDNESDAY MORNING. THE FRONT IS TO THEN REMAIN NEARLY STATIONARY
WHILE GRADUALLY WEAKENING LATER IN THE WEEK. AS MOISTURE CONVERGES
ALONG THIS BOUNDARY...PWAT CONTENT OF 1.50-2.0 INCHES IS EXPECTED
TO LAST THROUGH MIDAFTERNOON HOURS ON THURSDAY. MODELS AGREE ON A
DRYER AIR MASS TO RAPIDLY ENTRAIN FROM THE EAST STARTING ON
THURSDAY EVENING...TO INTENSIFY THROUGH THE DAY ON FRIDAY.

AS MOISTURE CONVERGES ALONG THE SURFACE FRONT...AND UNDER
FAVORABLE UPPER LEVEL JET DYNAMICS...ENVIRONMENTAL CONDITIONS ARE
TO REMAIN HIGHLY FAVORABLE FOR MODERATE TO HEAVY CONVECTION TO
AFFECT THE FORECAST AREA DURING THE NEXT 48-72 HRS. GLOBAL MODEL
GUIDANCE SHOWS THE MOST INTENSE CLUSTERING BETWEEN EASTERN PUERTO
RICO AND THE VIRGIN ISLES...WITH TOTAL RAINFALL AMOUNTS OVER THE
NEXT FOUR DAYS PEAKING BETWEEN 2-3 INCHES. HOWEVER...LOCALLY
HIGHER AMOUNTS ARE HIGHLY PROBABLE DUE TO STRONGER TRADE WINDS
FLOW AND TOPO FORCING ACROSS EASTERN PORTIONS OF PUERTO RICO.
MODERATE RAINFALL AMOUNTS ARE TO ALSO AFFECT WESTERN INTERIOR
PORTIONS OF PUERTO RICO IN AFTERNOON CONVECTION. A DRYING TREND IS
EXPECTED LATER IN THE WEEK.

DAVISON...WPC (USA)