Area Forecast Discussion
Issued by NWS San Juan, PR

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000
FXCA62 TJSJ 211508
AFDSJU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
1108 AM AST SUN SEP 21 2014

.UPDATE...THUNDERSTORM ACTIVITY WITH FREQUENT LIGHTNING OVER THE
SOUTHWESTERN CARIBBEAN SEA DISSIPATED JUST BEFORE THE MID MORNING
HOURS. THIS HAS LEFT A DENSE CIRRUS CLOUD COVER OVER THE
SOUTHWESTERN QUADRANT OF THE FORECAST AREA BUT NOT ENOUGH TO
INHIBIT DAYTIME HEATING OVER THE CENTRAL MOUNTAIN RANGE OF PUERTO
RICO. THE 21/12Z TJSJ SOUNDING INDICATED JUST BELOW NORMAL PWAT
VALUES...BUT THIS AVAILABLE MOISTURE COMBINED WITH DAYTIME HEATING
AND LOCAL EFFECTS WILL BE ENOUGH TO CREATE AFTERNOON CONVECTION
OVER THE INTERIOR AND WESTERN SECTIONS OF PR. MIMIC-TPW AND
SAHARAN AIR LAYER PRODUCTS INDICATED A DRIER AIR MASS MOVING OVER
THE NORTHEASTERN LEEWARD ISLANDS...THIS SHOULD MAINTAIN ISOLATED
PASSING SHOWERS THROUGH THE OVERNIGHT OVER THE USVI AND EASTERN
PR. NO CHANGES MADE TO PREVIOUS SHORT TERM GRIDS.

&&

.AVIATION...PERIODS OF MVFR CONDITIONS IN SHRA/TSRA WILL
DEVELOP BETWEEN 16-22Z OVER CENTRAL AND WESTERN PUERTO RICO WITH
MOUNTAIN OBSCURATIONS...AFFECTING MAINLY TJMZ AND TJBQ. VFR
CONDITIONS EXPECTED ELSEWHERE. LOW LEVEL WINDS FM THE EAST AT 10
TO 20 KTS WITH SEA BREEZE VARIATIONS.

&&

.MARINE...BUOYS AROUND THE LOCAL ISLANDS ARE JUST BELOW 4 FEET. NO
CHANGES MADE TO PREVIOUS MARINE FORECAST.

&&

.PREV DISCUSSION... /ISSUED 326 AM AST SUN SEP 21 2014/

DISCUSSION...NUMEROUS SHOWERS/THUNDERSTORMS MOVING THROUGH SOUTH
MONA PASSAGE ARE RIDING JUST AHEAD OF A 850 MB SPEED MAXIMUM
WHICH WILL CLEAR WEST OF THE PASSAGE THIS MORNING NEAR OR SOON
AFTER 12Z. ISOLATED/SCATTERED CONVECTION CONTINUES BEHIND IN
HIGH TOTAL PRECIPITABLE WATER IN EXCESS OF 2 INCHES AS SHOWN BY
SSMI AND MIMIC-TPW IMAGES. 00Z GFS HAS A NICE HANDLE ON THE
DISTRIBUTION OF THIS MOISTURE AND SUGGESTS A HIGH NORTH TO SOUTH
MOISTURE GRADIENT OVER PR/USVI AND EXTENDING TO NORTHERN LEEWARD
ISLANDS TODAY. WRF AND NAM TAKE PRECIPITATION ACROSS EASTERN PR
AND NORTH OF THE ISLAND THIS MORNING WHILE GFS KEEPS IT FARTHER
SOUTH...STILL OVER PR. SIGNIFICANT CONVECTION OVER PR THIS MORNING
SEEMS HIGHLY UNLIKELY GIVEN THE POSITION OF THE 850 MB FLOW...A
CURRENTLY COOL ISLAND AND RELATIVE LACK OF CURRENT UPSTREAM
ACTIVITY.

WIND FLOW SHOULD NOT BE A SIGNIFICANT FACTOR AT ANY LEVEL IN
CONVECTIVE DEVELOPMENT TODAY OR NEXT SEVERAL DAYS...INSTEAD
DEPENDENT MORE ON HOW MUCH SOLAR HEATING OCCURS AND WHERE SEA
BREEZES SET UP. HIGH CLOUDS FROM THUNDERSTORMS OVER CARIBBEAN
COASTAL WATERS ARE SPREADING BUT MOST OF THIS WILL DISSIPATE AND
MOVE WEST BEFORE MUCH SUNSHINE OCCURS. GFS SHOWS 2+ INCHES OF
PRECIPITABLE WATER AT 18Z TODAY OVER WEST AND SOUTH PR...EXPECT
MOST AFTERNOON CONVECTIVE DEVELOPMENT TODAY OVER WEST PR WHERE THE
BEST HEATING WILL OCCUR AND RESULTANT SEA BREEZES FOCUS.

MOISTURE DECREASES SOMEWHAT ON MON BUT REBOUNDS IMMEDIATELY
FOLLOWING ARRIVAL OF A TROPICAL WAVE ON MON NIGHT. GFS SHOWS
PRECIPITABLE WATER PERSISTENTLY NEAR 2 INCHES FROM TUE-THU.
CONVECTION AGAIN HIGHLY DEPENDENT ON SUNSHINE.

MARINE...MINOR SWELL IS MOVING WESTWARD NORTHEAST OF LEEWARD
ISLANDS. BUOY 41044 IS CURRENTLY RECORDING 5 FT SEAS AND ABOUT A
FOOT HIGHER THAN MODELS CURRENTLY SHOW. THESE SEAS WILL PASS
NORTH OF PR ON MON AND MAINLY NORTH OF 20N. MAINTAINING 4 FT IN
COASTAL WATERS GOOD.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  88  79  89  79 /  20  30  30  20
STT  89  79  89  79 /  40  40  40  40

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...NONE.
VI...NONE.
COASTAL WATERS...NONE.
&&

$$

13/23



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