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Tropical Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1048Z Jun 21, 2018)
 
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TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
648 AM EDT THU JUN 21 2018

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JUN 21/06
UTC: TUTT LOW NEAR 26N 66W ANCHORS A SHORT WAVE TROUGH JUST NORTH
OF PUERTO RICO/DOMINICAN REPUBLIC...WHILE A CELL OF THE
SUBEQUATORIAL RIDGE TO THE SOUTH EXTENDS FROM THE TROPICAL
ATLANTIC TO THE EASTERN CARIBBEAN. AT LOW LEVELS...AN EASTERLY
WAVE IS MOVING ACROSS THE WINDWARD/FRENCH ISLES...WITH SATELLITE
DERIVED PWAT ANALYSIS SHOWING MOISTURE CONTENT OF AROUND TWO
INCHES. A DRYER AIR MASS LIES TO THE NORTH...ENTRAINING ACROSS THE
LEEWARD/VIRGIN ISLES TO PUERTO RICO. THIS IS ACCOMPANIED BY A
FAIRLY STRONG TRADE WINDS CAP THAT IS EXPECTED TO DOMINATE LOCAL
WEATHER CONDITIONS THROUGH MIDDAY ON FRIDAY.

CONSISTENT WITH PREVIOUS FORECASTS THE TUTT LOW IS TO MEANDER
SOUTH...AND THROUGH FRIDAY EVENING IT IS TO SETTLE JUST
NORTH-NORTHWEST OF PUERTO RICO. AS IT MEANDERS SOUTH...IT IS TO
THEN EXTEND A SHORT WAVE TROUGH ACROSS TO THE NORTHEAST CARIBBEAN.
ON FRIDAY-SATURDAY THIS TROUGH IS TO THEN FAVOR THE EROSION OF THE
TRADE WINDS CAP ENVELOPING THE FORECAST AREA. AT LOW LEVELS...THE
EASTERLY WAVE PULLS ACROSS 70W EARLY ON FRIDAY MORNING...WITH
TRAILING PLUME OF DEEP TROPICAL MOISTURE TO THEN ADVECT NORTH
ACROSS THE FORECAST AREA LATER IN THE DAY. AS THIS COINCIDES WITH
AMPLIFYING TUTT PATTERN ALOFT...THIS WILL ALLOW FOR HIGH MOISTURE
CONTENT ACROSS THE FORECAST AREA LATER ON FRIDAY THROUGH SATURDAY.
NOTE THAT BOTH THE GFS AND ECMWF GDI FORECAST SHOW PEAK IN
CONVECTIVE INSTABILITY DURING THE EARLY MORNING HOURS ON
SATURDAY...WITH ENVIRONMENTAL CONDITIONS APPEARING FAVORABLE FOR
AN MCS FORMING BETWEEN SAINT CROIX AND CARIBBEAN COASTAL WATERS OF
PUERTO RICO. THIS IS TO INITIALLY FAVOR MODERATE TO HEAVY COASTAL
CONVECTION. IN A SOUTHEASTERLY STEERING FLOW...ACTIVITY IS TO
GRADUALLY SPREAD INLAND THROUGH THE EARLY AFTERNOON
HOURS...CLUSTERING OVER NORTHWEST PORTIONS OF PUERTO RICO WHERE WE
EXPECT MODERATE TO HEAVY RAINFALL AMOUNTS.

FOLLOWING WAVE PASSAGE...TIGHTENING PRESSURE GRADIENT WILL SUSTAIN
A TRADE WINDS SURGE ACROSS THE EASTERN CARIBBEAN...WITH 850 HPA
WINDS OF 20-25KT LATER ON SATURDAY THROUGH TUESDAY MORNING. AS A
PLUME OF SHALLOW MOISTURE LINGERS ACROSS THE FORECAST AREA...THIS
WILL FUEL GENERATION OF STREAMERS ACROSS THE VIRGIN
ISLES...VIEQUES AND CULEBRA...WITH CELLS CLUSTERING OVER EASTERN
PORTIONS OF PUERTO RICO. SECONDARY CELL GENERATION IS ALSO
EXPECTED ACROSS EL YUNQUE TO AFFECT NORTHEAST PORTIONS OF PUERTO
RICO DURING THE LATE MORNING AND THROUGH THE AFTERNOON HOURS...TO
SUSTAIN MODERATE TO HEAVY RAINFALL AMOUNTS. BUT AS THE WINDS BACK
TO THE EAST LATER IN THE DAY...CONVECTION IS TO THEN CLUSTER
ACROSS WESTERN PORTIONS OF THE ISLAND.

OVERALL...WE MAINTAIN GENERAL CONTINUITY WITH PREVIOUS FORECAST
AND ONLY MADE SOME MINOR CORRECTIONS TO QPF GRIDS.

DAVISON...WPC (USA)