Area Forecast Discussion
Issued by NWS San Juan, PR

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809
FXCA62 TJSJ 290128
AFDSJU

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
928 PM AST THU APR 28 2016

.UPDATE...SHOWERS AND THUNDERSTORMS WERE OBSERVED IN THE AFTERNOON
ACROSS WESTERN PR. SOME OF THE THUNDERSTORMS PRODUCED INTENSE
LIGHTNING AND THE SHOWERS WERE LOCALLY HEAVY...PRODUCING SOME
URBAN AND SMALL STREAM FLOODING. THIS WAS DUE TO THE AVAILABLE
MOISTURE AND THE UNSTABLE ATMOSPHERE.

REGARDING THE UPCOMING WEATHER...THE NAAPS AEROSOL MODEL INDICATES
THAT WE MAY STILL HAVE SOME SAHARAN DUST FOR THE NEXT DAY OR
TWO...SIMILAR TO WHAT WE HAVE HAD IN THE PAST COUPLE OF DAYS. AS
FAR AS WEATHER GOES...THE OVERALL PATTERN IS EXPECTED TO BE VERY
SIMILAR TOMORROW SINCE THE AVAILABLE MOISTURE AND UPPER AIR
PATTERN CONTINUES TO BE FAVORABLE FOR SHOWER AND THUNDERSTORM
DEVELOPMENT. IN THE SURFACE THOUGH...THERE APPEARS TO BE A SHORT
WAVE TROUGH THAT WILL APPROACH THE LOCAL AREA TOMORROW AFTERNOON
THEN THERE IS THE THETA-E VALUES WHICH ARE INDICATIVE OF ENOUGH
INSTABILITY FOR THE DEVELOPMENT OF THUNDERSTORMS...WHICH ALSO
CONCURS WITH THE FORECAST SOUNDINGS. SO WE DO EXPECT SHOWERS AND
THUNDERSTORMS ON FRIDAY ACROSS THE LOCAL AREA. THE HIGH RESOLUTION
MODELS SEEM TO FAVOR THE INTERIOR...NORTH CENTRAL...AND THE
WESTERN HALF OF PR AS WE WOULD HAVE A LIGHT SOUTHEASTERLY WIND
FLOW. THE IMPACT WILL BE AT LEAST SOME URBAN AND SMALL STREAM
FLOODING.

&&

.AVIATION...VFR CONDS LIKELY CONTINUE AT ALL TAF SITES THRU THE
OVERNIGHT HOURS. HOWEVER...MVFR CONDS WILL REMAIN POSSIBLE ONCE
AGAIN AT TJMZ/TJBQ AND POSSIBLY TJSJ TERMINALS IN SHRA/TSRA BETWEEN
17Z AND 22Z. MOUNTAIN OBSCURATION EXPECTED ACROSS THE INTERIOR OF
PR FRI AFTERNOON. LOW LEVEL WINDS 5-15 KT FROM THE SE...BECOMING
LIGHT AND VARIABLE OVERNIGHT.

&&

.MARINE...SMALL CRAFT SHOULD EXERCISE CAUTION AS SEAS WILL BE UP
TO 6 FEET ACROSS THE ATLANTIC OFFSHORE WATERS TONIGHT AND FRIDAY.
NORTHERLY SWELL IS SUBSIDING AND THE SURF HEIGHTS HAVE ALSO
DIMINISHED SO THERE IS NOW A MODERATE RISK OF RIP CURRENTS ACROSS
THE ATLANTIC COASTS OF THE LOCAL ISLANDS.


&&

.PREV DISCUSSION... /ISSUED /

SYNOPSIS...A BROAD UPPER LEVEL TROUGH ACROSS THE WESTERN
ATLANTIC TO THE NORTHERN CARIBBEAN WILL CONTINUE TO MAINTAIN A
DIVERGENT PATTERN ACROSS THE FORECAST AREA DURING THE NEXT SEVERAL
DAYS...AS UPPER LEVEL PERTURBATIONS MOVE OVER THE ISLANDS FROM
TIME TO TIME. AT LOWER LEVELS...INDUCED SURFACE TROUGH ACROSS
THE WATERS NORTH OF HISPANIOLA AND SURFACE HIGH TO THE NORTHEAST
OF THE REGION WILL PROMOTE A SOUTHEASTERLY WIND FLOW THROUGH MUCH
OF THE FORECAST PERIOD. SAHARAN DUST WILL CONTINUE ACROSS THE AREA
AT LEAST UNTIL SATURDAY.

DISCUSSION...THROUGH THE MORNING HOURS UNTIL NOW...A CLUSTER OF
THUNDERSTORMS HAVE REDEVELOP OVER THE CARIBBEAN WATERS TO THE
SOUTH OF VIEQUES AND ST. CROIX. THIS IS DUE TO THE POSITION OF THE
UPPER TROF AXIS AND A 90+ KT JET ABOVE 200 MB AS INDICATED BY THE
28/12Z TJSJ UPPER AIR SOUNDING. THIS UPPER LEVEL PERTURBATION
WILL CONTINUE TO LIFT TO THE NORTHEAST TODAY...HOWEVER...DIURNALLY
INDUCED AFTERNOON SHOWERS AND THUNDERSTORMS WILL BE ENHANCED BY
FAVORABLE UPPER LEVEL DYNAMICS ACROSS THE MTN RANGES OF PR AND
SLOWLY DRIFT TO THE NORTH-NORTHWEST AS STEERING WINDS PREVAIL
FROM THE SOUTH. ALSO...DRIER AIR BETWEEN 600-500 MB AND LOW
TEMPS(-9.5C) AT 500 MB COULD CREATE SMALL HAIL AND/OR STRONG GUSTY
WINDS AT THE SURFACE WITH THE THUNDERSTORMS THIS AFTERNOON.

TROUGHINESS ALOFT WILL MAINTAIN FAVORABLE CONDITIONS
FOR THE DEVELOPMENT OF SHOWERS AND STRONG THUNDERSTORMS THROUGH
EARLY NEXT WEEK. AS UPPER LEVEL SHORT WAVE VERTICES MOVE ACROSS
THE FORECAST AREA DURING THE WEEKEND AND EARLY NEXT WEEK...THIS
WILL ENHANCE RAINFALL AND THUNDERSTORM ACTIVITY OVER THE INTERIOR
AND THE NORTH/EASTERN SIDE OF THE ISLAND. FRIDAY/SATURDAY SEEMS TO
BE THE NEXT ACTIVE WEATHER DAY WITH DEEPER SHORT WAVE MOVING ALOFT
AND BETTER LOW LEVEL MOISTURE POOLING ACROSS THE AREA. ANOTHER
SHORT WAVE TROF MOVES ON MONDAY ENHANCING CONVECTION ONCE AGAIN.
DUE TO ALREADY SATURATED SOILS...ANY ADDITIONAL HEAVY RAINFALL WILL
LEAD TO URBAN AND SMALL STREAM/RIVER FLOODING. ALSO...LOCALIZED FLASH
FLOODING AND MUDSLIDES CANT BE RULED OUT EACH AFTERNOON.

AVIATION...MVFR CONDS WILL REMAIN POSSIBLE THROUGH AT LEAST 28/22Z
AT TJMZ/TJBQ AND POSSIBLY TJSJ TERMINALS IN SHRA/TSRA. MOUNTAIN
OBSCURATION ACROSS THE INTERIOR OF PR. MOSTLY VFR COND FOR THE REST
OF THE AREA. LOW LEVEL WINDS 5-15 KT FROM THE SE...BECOMING LIGHT
AND VARIABLE OVERNIGHT.

MARINE...LATEST BUOY READINGS INDICATE THAT THE NORTHERLY SWELL
IS SLOWLY FADING AND MODEL GUIDANCE SUGGEST THE SAME WITH
WAVE HEIGHTS AROUND 4 FEET AT 10 SECONDS. SEAS WILL REMAIN
GENERALLY BELOW 6 FEET ACROSS ALL LOCAL WATERS DURING THE NEXT
SEVERAL DAYS. UNSETTLED WEATHER CONDITIONS WILL CONTINUE ACROSS
THE REGIONAL WATERS INTO EARLY NEXT WEEK AS THUNDERSTORMS ARE
FORECAST TO DEVELOP OVER THE WATERS.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  76  88  75  88 /  30  50  40  40
STT  76  83  76  83 /  40  40  40  40

&&

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

$$

11/25



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