Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 211513
AFDSJU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
1113 AM AST THU AUG 21 2014

.UPDATE...DOPPLER WEATHER RADAR DETECTED ISOLATED TO SCATTERED
PASSING SHOWERS MOVING SOUTHWESTWARD FROM THE ATLANTIC WATERS ACROSS
THE USVI...VIEQUES...CULEBRA AND EASTERN AND NORTHERN SECTIONS OF
PUERTO RICO THIS MORNING. THIS ACTIVITY WAS ASSOCIATED WITH THE
MOISTURE OF THE LEADING EDGE OF A STRONG TROPICAL WAVE TO THE EAST
OF THE EAST OF THE LESSER ANTILLES. THE MOISTURE ASSOCIATED WITH THE
WAVE WILL CONTINUE TO MOVE WESTWARD ACROSS THE LOCAL ISLANDS TONIGHT
AND FRIDAY.

THIS SYSTEM WILL BRING A GENERAL INCREASE IN CLOUDINESS...SHOWERS
AND THUNDERSTORMS ACROSS THE REGION UNTIL AT LEAST FRIDAY NIGHT.
RESIDUAL MOISTURE WILL CONTINUE TO PROVIDE A WET WEATHER PATTERN
OVER THE LOCAL ISLANDS AND SURROUNDING WATERS SATURDAY. FOR THOSE
REASONS A FLASH FLOOD WATCH IS IN EFFECT FOR ALL PUERTO RICO...
INCLUDING VIEQUES AND CULEBRA...AND FOR THE USVI UNTIL SATURDAY
MORNING. GUIDANCE SUGGESTS THAT A GRADUAL IMPROVEMENT IN THE WEATHER
WILL BEGIN ON SUNDAY.

&&

.AVIATION...AFTER 21/16Z TSRA/SHRA WILL DEVELOP OVER THE INTERIOR AND
WESTERN PR WITH BRIEF PERIODS OF MVFR CONDITIONS AND MTN TOP
OBSCURATIONS AT TJMZ AND TJPS. MVFR OR EVEN IFR CONDS POSSIBLE
TONIGHT AS LOW PRESSURE MOVES INTO THE EASTERN CARIBBEAN.
DETERIORATING CONDS ACROSS USVI AND PR TERMINALS THIS EVENING AS THE
SFC LOW PRESSURE CONTINUES TO MOVE W-NW. E-NE AT 10-25 KTS WITH
HIGHER GUSTS ARE EXPECTED.

&&

.MARINE...SMALL CRAFT SHOULD EXERCISE CAUTION THIS AFTERNOON. SMALL
CRAFT ADVISORIES WILL GO INTO EFFECT STARTING TONIGHT. THE STRONG
TROPICAL WAVE/LOW PRESSURE SYSTEM WILL RAPIDLY DETERIORATE THE
MARINE CONDITIONS TONIGHT AND SATURDAY. SEAS WILL IMPROVE BY SUNDAY.

&&

.PREV DISCUSSION... /ISSUED 625 AM AST THU AUG 21 2014/

SYNOPSIS...LOW PRESSURE SYSTEM A FEW HUNDRED MILES EAST OF THE
WINDWARD ISLANDS IS EXPECTED TO MOVE INTO THE LOCAL AREA LATE
TONIGHT AND LINGER UNTIL SATURDAY...CAUSING THUNDERSTORMS WITH
HEAVY RAIN...GUSTY WINDS AND SQUALLS.

DISCUSSION...FOR TODAY...BEFORE THE LOW PRESSURE ARRIVES TO THE LOCAL
AREA...PASSING SHOWERS ARE EXPECTED IN THE MORNING WITH CONVECTION
DEVELOPING ACROSS THE SOUTHWESTERN QUADRANT OF PUERTO RICO.
MEANWHILE...SCATTERED TO NUMEROUS SHOWERS AND ISOLATED
THUNDERSTORMS ARE EXPECTED TO BE MAKING THEIR WAY INTO THE LOCAL
AREA FROM THE EAST AS THEY START AFFECTING THE USVI IN THE LATE
AFTERNOON HOURS.

LATEST IR SATELLITE IMAGERY SHOWS THE LOW PRESSURE
SYSTEM JUST EAST OF THE WINDWARD ISLANDS SHOWED SIGNS OR WAKENING
DURING THE NIGHT BUT NOW ONCE AGAIN SEEMS TO BE GAINING SOME
CONVECTION AND COOLING CLOUD TOPS. THE CENTER OF CIRCULATION IS
MOVING TO THE WEST NORTHWEST ACCORDING AT 10-15 MPH ACCORDING TO
THE NHC. THE LATEST PUBLIC ADVISORY FROM THE NHC GIVES THIS SYSTEM
A 50 PERCENT CHANCE OF DEVELOPING INTO A TROPICAL CYCLONE WITHIN
THE NEXT 48 HOURS. HOWEVER REGARDLESS OF DEVELOPMENT...SIGNIFICANT
SHOWERS AND THUNDERSTORMS WITH STRONG GUSTY WINDS ARE EXPECTED
LATE TONIGHT THROUGH SUNDAY...WITH FRIDAY LOOKING LIKE THE MOST
ACTIVE DAY.

THIS SYSTEM WILL BRING A SHARP INCREASE IN AVAILABLE MOISTURE WITH
PRECIPITABLE WATER VALUES GETTING CLOSE TO 2.5 INCHES AND FORECAST
SOUNDINGS SHOWING A DEEP SATURATED PROFILE ON FRIDAY INTO EARLY
SATURDAY ACCORDING TO THE 21/00Z GFS RUN.

GUIDANCE SUGGESTS THAT A GRADUAL IMPROVEMENT IN THE WEATHER WILL
BEGIN ON SUNDAY BUT IT IS NOT UNTIL MONDAY THAT WE SEE A MORE
SIGNIFICANT IMPROVEMENT. FOR THE UPCOMING WEEK...MOISTURE WILL
DECREASE AND DRIER WEATHER IS EXPECTED...HOWEVER MOISTURE RETURNS
ON WEDNESDAY...POSSIBLY CAUSING ANOTHER DAY WITH SHOWERS AND
THUNDERSTORMS ACROSS PORTIONS OF THE LOCAL AREA.

AVIATION...VFR CONDITIONS EXPECTED AT PR AND USVI TERMINALS IN THE
MORNING. AFTER 21/16Z TSRA/SHRA DEVELOPMENT OVER THE
INTERIOR/WESTERN PR MAY RESULT IN BRIEF MVFR CONDS WITH MTN TOP
OBSCURATIONS AT TJMZ AND TJPS. MVFR OR EVEN IFR CONDS
POSSIBLE LATER THIS MORNING AT LEEWARD SITES AS LOW PRESSURE
MOVES INTO THE EASTERN CARIBBEAN. DETERIORATING CONDS ACROSS USVI AND
PR TERMINALS LATE THIS EVENING AS THE SFC LOW PRESSURE CONTINUES TO
MOVE W-NW. E-NE AT 10-25 KTS WITH HIGHER GUSTS.

MARINE...SMALL CRAFT ADVISORIES WILL GO INTO EFFECT STARTING
TONIGHT. THE ARRIVAL OF THE LOW PRESSURE SYSTEM WILL DETERIORATE
THE MARINE CONDITIONS AND CAUSE HAZARDOUS SEAS. SMALL CRAFT SHOULD
EXERCISE CAUTION TODAY. SEAS IMPROVING STARTING ON SUNDAY.

HYDROLOGY...A MUCH WETTER PATTERN IS EXPECTED TONIGHT THROUGH AT LEAST SATURDAY
AS AN ELONGATED AREA OF LOW PRESSURE AND ASSOCIATED DEEP TROPICAL
MOISTURE MOVES ACROSS THE EASTERN CARIBBEAN. BASED ON THE LATEST
HURRICANE CENTER GUIDANCE...ENVIRONMENTAL CONDITIONS ARE EXPECTED TO
BE CONDUCTIVE FOR DEVELOPMENT DURING THE NEXT COUPLE OF DAYS WHILE
THE SYSTEM MOVES WEST-NORTHWESTWARD AT 10 TO 15 MPH.

REGARDLESS DEVELOPMENT...PERIODS OF MODERATE TO HEAVY RAINFALL ARE
POSSIBLE LATE TONIGHT THROUGH AT LEAST SATURDAY ACROSS PUERTO RICO
AND THE U.S. VIRGIN ISLANDS WITH AN INCREASING POTENTIAL FOR URBAN
AND FLASH FLOODING. AREAS WITH RAINFALL TOTALS BETWEEN 4 AND 6 INCHES
WITH LOCALLY HIGHER AMOUNTS ARE POSSIBLE. FFA MAY BE REQUIRED FOR
THIS EVENING THROUGH SATURDAY.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  89  78  86  78 /  40  60  90  80
STT  89  79  88  79 /  40  60  90  80

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH SATURDAY MORNING FOR
     CENTRAL INTERIOR-CULEBRA-EASTERN INTERIOR-MAYAGUEZ AND
     VICINITY-NORTH CENTRAL-NORTHEAST-NORTHWEST-PONCE AND
     VICINITY-SAN JUAN AND VICINITY-SOUTHEAST-SOUTHWEST-VIEQUES-
     WESTERN INTERIOR.

VI...FLASH FLOOD WATCH FROM LATE TONIGHT THROUGH SATURDAY MORNING FOR
     ST CROIX-ST. THOMAS/ST. JOHN/ADJACENT ISLANDS.

COASTAL WATERS...
     SMALL CRAFT ADVISORY FROM 9 AM FRIDAY TO 6 PM AST SATURDAY FOR
     COASTAL WATERS OF SOUTHERN PUERTO RICO OUT 10 NM.

     SMALL CRAFT ADVISORY FROM 3 AM FRIDAY TO MIDNIGHT AST SATURDAY
     NIGHT FOR CARIBBEAN WATERS OF PUERTO RICO FROM 10 NM TO 17N.

     SMALL CRAFT ADVISORY FROM 9 AM FRIDAY TO NOON AST SATURDAY FOR
     COASTAL WATERS OF NORTHERN PUERTO RICO OUT 10 NM.

     SMALL CRAFT ADVISORY FROM 9 PM THIS EVENING TO MIDNIGHT AST
     SATURDAY NIGHT FOR ATLC WATERS OF PUERTO RICO AND USVI FROM
     10NM TO 19.5N.

     SMALL CRAFT ADVISORY FROM 3 AM FRIDAY TO NOON AST SATURDAY FOR
     COASTAL WATERS OF NORTHERN USVI AND CULEBRA OUT 10 NM-
     COASTAL WATERS OF SOUTHERN USVI VIEQUES AND EASTERN PUERTO
     RICO OUT 10 NM.

     SMALL CRAFT ADVISORY FROM 9 PM THIS EVENING TO NOON AST SATURDAY
     FOR ANEGADA PASSAGE SOUTHWARD TO 17N.

     SMALL CRAFT ADVISORY FROM 3 PM FRIDAY TO MIDNIGHT AST SATURDAY
     NIGHT FOR MONA PASSAGE SOUTHWARD TO 17N.

&&

$$

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