Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 010921
AFDSJU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
521 AM AST SUN MAY 1 2016

.SYNOPSIS...A TROUGH WITH A WEAK CUT-OFF LOW WILL PASS THROUGH
THE AREA TODAY. A SECONDS WEAKER TROUGH WILL PASS THROUGH ON
TUESDAY TO PROLONG THE TIME THE TROUGH STAYS IN PLACE. BY THURSDAY
A RIDGE WILL BEGIN TO BUILD OVER THE AREA...MOVING OVER THE
LEEWARD ISLANDS BY LATE IN THE WEEK.

AT MID LEVELS...CONSIDERABLE DRYING WILL TAKE PLACE TODAY AND ONLY
MODEST MOISTURE WILL PERSIST THROUGH THE END OF THE WEEK. WEAK
TROUGHINESS WILL PERSIST OVER THE AREA THROUGH MID WEEK...THEN
HIGH PRESSURE WILL BUILD INTO THE CENTRAL ATLANTIC AND OVER THE
CARIBBEAN SEA.

AT LOWER LEVELS...HIGH PRESSURE ENTERING THE WESTERN ATLANTIC AND
BUILDING INTO THE CENTRAL ATLANTIC WILL MAINTAIN SOUTHEAST TRADE
WIND FLOW THROUGH THE REST OF THE WEEK. ABUNDANT MOISTURE WILL
INCREASE OVER THE AREA AFTER MONDAY KEEPING SHOWERS AND
THUNDERSTORMS IN THE FORECAST.

&&

.DISCUSSION...SHOWERS AND THUNDERSTORMS HAD CLEARED OFF ALL BUT
THE SOUTHEAST PORTION OF THE FORECAST AREA BY LATE EVENING. BUT
THE LINE OF SHOWERS RETURNED AND NOW STRETCHES ACROSS THE
CARIBBEAN WATERS WITH SCATTERED SHOWERS. SHOWERS WENT OVER SAINT
CROIX AROUND 4 AM AST BUT THE HEAVIEST SHOWERS WENT AROUND THE
RAIN GAGE AT THE AIRPORT IN FREDERIKSTED. SHOWERS ARE WEAKENING AS
THEY APPROACH SAINT JOHN...SAINT THOMAS AND PUERTO RICO...BUT A
FEW ARE EXPECTED TO ARRIVE BY 8 AM AST. WITH SKIES CLEARING
HEATING SHOULD MAKE THINGS UNSTABLE AGAIN AND SHOWERS AND
THUNDERSTORMS WILL DEVELOP OVER PUERTO RICO. MODELS BARELY SHOW
THE AREA WHERE SHOWERS ARE FIRING OFF NOW...BUT RATHER SHOW A
WIDE AREA OF MOISTURE WITH LITTLE FORM...MAKING THE FORECAST
PARTICULARLY DIFFICULT TODAY. HOWEVER THE GFS BRINGS SOME
DIVERGENCE AT UPPER LEVELS ACROSS THE CENTER OF PUERTO RICO TODAY
FROM THE SOUTHWEST WHICH SHOULD HELP CONVECTION. OTHERWISE EXPECT
THE CONVECTION TO REMAIN MAINLY SOUTH AND EAST OF PUERTO RICO AND
POSSIBLY OVER THE U.S. VIRGIN ISLANDS. WINDS CONTINUE FROM THE
SOUTHEAST AND EXPECT WARMER TEMPERATURES TODAY. MOISTURE REACHES A
MINIMUM OVERNIGHT TONIGHT...BUT STILL AROUND 1.5 INCHES OF
PRECIPITABLE WATER. THEN OVER THE WEEK IT GRADUALLY RISES TO TWO
INCHES AGAIN. THE MODEL ALSO HAS 1000-850 MB THICKNESSES RISING
OVER THE AREA WITH THE WARMEST PERIOD FRIDAY THROUGH SUNDAY.
BECAUSE MOISTURE RISES THROUGH THE WEEK...THE FORECAST CAPE AND
LIFTED INDEX ALSO SHOW MORE INSTABILITY AS THE WEEK CONTINUES. THE
ONLY AMELIORATING FACTOR IS THAT MID LEVEL MOISTURE REMAINS MODEST
THROUGHOUT THE PERIOD...SO THE FLOODING POTENTIAL SHOULD REMAIN
LOCALIZED TO URBAN AND SMALL STREAM FLOODING IN THE INTERIOR AND
NORTHWEST PORTIONS OF THE ISLAND FOR THE ENTIRE WEEK. ALTHOUGH THE
SOUTH COAST WILL BE WARM...OF COURSE...THE NORTH COAST WILL ALSO
SEE TEMPERATURES RISE BETWEEN NOW AND THURSDAY... AND SAN JUAN
SHOULD BEGIN TO SEE A FEW LOWER 90S BY THE END OF THE WEEK.

&&

.AVIATION...-SHRA CAN BE EXPECTED FROM TIME TO TIME ACROSS MUCH OF
THE PR/USVI TERMINALS. ALSO...BKN/OVC HIGH CIRRUS WILL CONTINUE
ACROSS THE AREA. -TSRA/SHRA CAN STILL AFFECT THE LEEWARD
TERMINALS FROM TIME TO TIME...AND ALSO CAN MOVE ACROSS OR NEAR
THE USVI/SOUTHERN AND EASTERN PR TERMINALS THIS MORNING. DURING
THE AFTERNOON MTN TOP OBSCURATIONS OVER ERN...INTERIOR AND
NORTHWESTERN PR IN SHRA/LOW CIG ARE EXPECTED BETWEEN 01/15-23Z.
SFC WINDS FROM THE SOUTH AT 5 TO 10 KT WILL INCREASE TO 10 TO 15
KT AND LOCALLY HIGHER IN/NEAR TSRA/SHRA.


&&

.MARINE...SEAS ARE GRADUALLY MELLOWING OVER THE AREA...BUT WILL
INCREASE SOMEWHAT BETWEEN 5 AND 7 MAY. SMALL CRAFT ADVISORIES ARE
NOT EXPECTED DURING THE NEXT 7 DAYS AND SWELL FROM THE ATLANTIC
WILL BE MINIMAL.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  87  77  88  76 /  30  40  30  40
STT  83  78  87  76 /  50  50  50  50

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...NONE.
VI...NONE.
AM...NONE.
&&

$$

27/12


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