Prognostic Meteorological Discussion
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FXCA20 KWBC 271106
PMDCA

TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
705 AM EDT THU JUL 27 2017

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JUL 27/06
UTC: ALTHOUGH WEAKENING...A MID LEVEL CELL OF THE SUBTROPICAL
RIDGE NORTH OF THE GREATER ANTILLES REMAINS STRONG ENOUGH TO BLOCK
A TUTT LOW TO THE NORTH-NORTHEAST OF THE ISLAND CHAIN. AS IT
MEANDERS NORTH OF THE FORECAST AREA...THE TUTT IS ENHANCING
CONVECTIVE INSTABILITY TO THE NORTH OF PUERTO RICO/SAINT THOMAS.
IN RESPONSE...PWAT ANALYSIS SHOWS MOISTURE CONTENT OF AROUND 2
INCHES ON THE ATLANTIC WATERS...WHILE OVER THE CARIBBEAN A DRY
SLOT PERSISTS. AT LOW LEVELS...THE MEANDERING TUTT SUSTAINS AN
INDUCED TROUGH IN THE EASTERLY TRADES...NEARING THE LESSER
ANTILLES EARLY THIS MORNING.

THE TUTT LOW IS FORECAST TO SLOWLY WEAKEN WHILE DRIFTING TO THE
NORTH-NORTHEAST DURING THE NEXT FEW DAYS. AS IT PULLS AWAY... AND
ITS INFLUENCE ACROSS FORECAST AREA WANES...CONVECTIVE ENHANCEMENT
WILL GENERALLY LIMIT TO NORTHWEST PUERTO RICO IN AFTERNOON
CONVECTION DURING THE DAY TODAY. THE DEEP MOISTURE POOL IS TO
REMAIN NORTH OF THE FORECAST AREA...WITH DRY SLOT TO THE SOUTH
ENTRAINING ACROSS THE FORECAST AREA. THIS IS TO GENERALLY FAVOR
FAIR WEATHER CONDITIONS FOR THE DAY TODAY...WITH CONVECTIVE
ENHANCEMENT LIMITING TO NORTHWEST PUERTO RICO IN AFTERNOON
CONVECTION. IN THIS AREA THE HIGHER RESOLUTION MODEL GUIDANCE IS
SHOWING RAINFALL MAXIMA OF 1-3 INCHES...AND THIS SEEMS REASONABLE
UNDER CURRENT CONDITIONS. ON FRIDAY MORNING...TUTT INDUCED
PERTURBATION MOVES ACROSS THE VIRGIN ISLES-EASTERN PUERTO
RICO...REACHING THE MONA PASSAGE IN THE AFTERNOON. BUT LACKING
UPPER LEVEL SUPPORT...THE GLOBAL MODEL GUIDANCE AND HIGHER
RESOLUTION REGIONAL MODELS ONLY SHOW LIGHT CONVECTION AS IT MAKES
LANDFALL EARLY IN THE MORNING. MEASURABLE AMOUNTS WILL GENERALLY
LIMIT TO NORTHWEST PUERTO RICO LATER IN THE MORNING/EARLY
AFTERNOON...WITH MAXIMA GENERALLY PEAKING AROUND AN INCH.

DURING MEDIUM RANGE PERIOD...A SECONDARY TUTT IS TO THEN MEANDER
ACROSS THE ISLAND CHAIN TO THE EASTERN CARIBBEAN EARLY ON
SATURDAY...TO MEANDER OVER THE EASTERN CARIBBEAN/SOUTH OF THE
FORECAST AREA DURING THE WEEKEND. MODELS AGREE ON SOME CONVECTIVE
ENHANCEMENT WITH THIS FEATURE AS IT MEANDERS TO THE SOUTH...WITH
THE GFS AND EUROPEAN MODELS SHOWING POTENTIAL FOR SCATTERED
SHOWERS ACROSS THE ENTIRE FORECAST AREA. IN THIS PATTERN GENERALLY
EXPECTING LIGHT TO MODERATE RAINFALL AMOUNTS TO AFFECT THE VIRGIN
ISLES-EASTERN PUERTO RICO...WITH HIGHER AMOUNTS CLUSTERING TO THE
WEST OF THE ISLAND. AT MID LEVELS...A TUTT INDUCED PERTURBATION
NEARS THE ISLAND CHAIN LATER ON SUNDAY...REACHING THE VIRGIN
ISLES/EASTERN PUERTO RICO BY DAYBREAK ON MONDAY. THIS WILL THEN
QUICKLY MOVE ACROSS PUERTO RICO...TRIGGERING SCATTERED SHOWERS.
DURING THIS PERIOD...THE CLIMATE FORECAST SYSTEM (CFS) IS SHOWING
A BROAD AREA OF NEGATIVE VELOCITY POTENTIAL ANOMALIES IN
ASSOCIATION WITH AN ATMOSPHERIC KELVIN WAVE CROSSING THE
CARIBBEAN/NORTHERN SOUTH AMERICA. AS ENVIRONMENTAL CONDITIONS
BECOME MORE FAVORABLE THE MODELS HAVE STARTED TO TREND IN FAVOR OF
MORE WIDESPREAD CONVECTION BUILDING OVER THE CARIBBEAN WATERS OF
PUERTO RICO AND THE VIRGIN ISLES.

GAJADHAR...SLMS (SAINT LUCIA)
DAVISON...WPC (USA)
$$




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