Prognostic Meteorological Discussion
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FXCA20 KWBC 281053
PMDCA

TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
653 AM EDT WED JUN 28 2017

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JUNE
28/06 UTC: UNDER INFLUENCE OF MID LEVEL RIDGE OVER THE NORTHERN
CARIBBEAN ISLES AND A DRY SAHARAN AIR LAYER ENTRAINING ACROSS THE
EASTERN CARIBBEAN...FAIR WEATHER CONDITIONS PREVAIL ACROSS THE
FORECAST AREA. HOWEVER...IN BRISK EASTERLY TRADES...EARLY MORNING
SHOWERS WERE STREAMING ACROSS THE ATLANTIC COASTAL WATERS OF
PUERTO RICO AND THE USVI. THESE ARE TO TRIGGER LIGHT RAINFALL
AMOUNTS IN WIDELY ISOLATED CONVECTION. OTHER CONVECTION IS TO
CLUSTER ACROSS WESTERN MAYAGUEZ-RINCON LATER THIS AFTERNOON...TO
TRIGGER MODERATE RAINFALL AMOUNTS.

THE SYSTEM OF CONCERN IS A NEGATIVELY TILTED TROPICAL WAVE
APPROACHING THE LESSER ANTILLES EARLY THIS MORNING. THIS WAVE...IN
INTERACTION WITH A TUTT TO THE NORTH...SUSTAINS DEEP ORGANIZED
CONVECTION ACROSS THE WINDWARD ISLES. THE WAVE ENTERS THE LESSER
ANTILLES BY MIDDAY TODAY...AND ACROSS 66W BY DAYBREAK ON THURSDAY.
ON ITS WAKE THE LOW LEVEL WINDS ARE TO VEER TO THE
SOUTHEAST...FAVORING A MOIST ADVECTIVE PATTERN. THE PWAT CONTENT
IS TO GRADUALLY INCREASE TO AROUND 50MM LATER DURING THE
DAY...WITH HIGH MOISTURE CONTENT TO PERSIST THROUGH FRIDAY. AS IT
IS TO INITIALLY MIX WITH THE DRY SAL OVER THE FORECAST
AREA...EXPECTING AIR MASS TO GRADUALLY BECOME MORE UNSTABLE LATER
ON THURSDAY-EARLY MORNING HOURS ON FRIDAY. GLOBAL MODEL GUIDANCE
CONTINUE TO AGREE ON MOST ACTIVE CONVECTION WITH THIS FEATURE
CLUSTERING OVER THE VIRGIN ISLES/EASTERN PUERTO RICO. BUT AS THE
LOW LEVEL WINDS SHIFT TO THE SOUTHEAST...A SECONDARY MAXIMA IS
EXPECTED OVER NORTHWEST PUERTO RICO IN AFTERNOON CONVECTION.
ACROSS EASTERN PUERTO RICO/USVI EXPECTING MAXIMA OF 1-2
INCHES...WHILE OVER NORTHWEST PUERTO RICO IT SHOULD PEAK AT 2-4
INCHES.

A DECREASE IN ACTIVITY IS THEN EXPECTED DURING THE WEEKEND AS AN
ELEVATED INVERSION SETTLES ACROSS THE FORECAST AREA. THIS WILL BE
STRONG ENOUGH TO DELAY THE ONSET OF DIURNAL CONVECTION...BUT NOT
STRONG ENOUGH TO SUPPRESS IT. MOST ACTIVE IS LIKELY TO BUILD
ACROSS WESTERN INTERIOR PORTIONS OF PUERTO RICO AND SAINT CROIX IN
AFTERNOON CONVECTION.

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





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