Severe Thunderstorm Warning 102, NWS Charleston, SC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
153 PM EDT FRI MAY 29 2009

.SYNOPSIS...
A COLD FRONT WILL APPROACH FROM THE NW LATE TODAY THEN MOVE THROUGH
THE AREA TONIGHT. ATLC HIGH PRES WILL THEN REBUILD OVER THE SOUTHEAST
UNITED STATES OVER THE WEEKEND AND REMAIN THE DOMINANT WEATHER FEATURE
THROUGH THE UPCOMING WEEK.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
A FAIRLY MOIST AIRMASS CONTINUES ACROSS THE AREA EAST OF A COLD
FRONT DRAPED FROM THE SC UPSTATE ACROSS CENTRAL GA INTO THE FL
PANHANDLE. THIS FEATURE WILL DRIFT EAST THROUGH EARLY THIS EVENING
BEING DRIVEN BY A STRONG SHORTWAVE NOW OVER THE SOUTHERN
APPALACHIANS. A PATCH OF STRATUS IN SOUTHEAST GA HAS TEMPORARILY
STALLED WARMING...HOWEVER IT SHOULD DISSIPATE FAIRLY QUICKLY
DURING THE NEXT TWO HOURS AND WILL HAVE CREATED SOME DIFFERENTIAL
HEATING BOUNDARIES. MUCH OF THE GUIDANCE CONTINUES TO HOLD OFF ON
SIGNIFICANT CONVECTIVE DEVELOPMENT ALONG THE SEABREEZE EARLY THIS
AFTERNOON. BY MID TO LATE AFTERNOON...WITH THE COLD FRONT SHIFTING
INTO WESTERN ZONES AROUND PEAK HEATING...SCATTERED THUNDERSTORMS
SHOULD DEVELOP.

THE NSSL WRF IS AN OUTLIER WITH AN ORGANIZED CLUSTER OF STORMS
MOVING INTO THE AREA FROM THE WEST LATE THIS AFTERNOON THROUGH THE
EARLY EVENING. DESPITE IT BEING AN OUTLIER...IT OFTEN PERFORMS
VERY WELL IN THESE SETUPS INVOLVING MESOSCALE CONVECTIVE
PROCESSES. THE CONVECTIVE CHECKLIST FROM THE 12Z RAOB FROM CHS
INDICATES AN AIRMASS FAIRLY WELL PRIMED FOR STRONG TO SEVERE
CONVECTION. INSTABILITY PARAMETERS SHOW THE POTENTIAL FOR STRONG
UPDRAFTS WHILE SHEAR VALUES ARE MARGINAL. A GOOD AMOUNT OF LOW-
LEVEL DRY AIR SHOWS THE POTENTIAL FOR DAMAGING WIND TO BE THE MAIN
THREAT. OVERALL...CURRENT THINKING IS SIMILAR TO THAT OF THE
PREVIOUS SHIFT IN THAT THE SEABREEZE WILL STRUGGLE TO LIGHT UP DUE
TO A CONSIDERABLE CAP...BUT AN ACTIVE LATE AFTERNOON AND EVENING
WILL OCCUR ONCE THE CAP BREAKS AND BOUNDARIES INTERACT. NO CHANGES
TO MOST FORECAST GRIDS BUT DID ADD MENTION OF SEVERE THUNDERSTORMS
TO THE WEATHER FORECAST.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/...
RECENT MODEL TRENDS SHOW SIGNALS OF STRONG TO SEVERE NEAR-COASTAL
CONVECTION LINGERING INTO THE EARLY EVENING HOURS AS THE 60-70 KT
UPPER JET SEGMENTS MAKE A RUN AT PHASING OVER THE EASTERN HALF OF
OUR FORECAST AREA. THE WRF AND GEM MODELS INDICATE SOME VERY
INTERESTING MODEL UNDULATIONS OF CONVECTIVE FIELDS ALONG THE
GEORGIA COAST WHICH COULD INDICATE A TIP THAT ORGANIZED STORMS MAY
TRY TO FIRE AND SURGE OFF THE COAST PROMOTING MARINE HAZARDS. AS A
TREND...WE INCREASED EVENING POPS TO CHANCE RANGE FROM DARIEN TO
SAVANNAH TO CHARLESTON BUT STILL MANY TIMING FACTORS HAVE TO COME
TOGETHER FOR AN EARLY EVENING EVENT.

THE FRONT WILL BE SAGGING THROUGH THE REGION LATE WITH CLOUDS
THINNING OUT AFTER MIDNIGHT. SURFACE WINDS WILL GRADUALLY BE
VEERING TO A LIGHT NNW COMPONENT LATE OVER THE NORTHERN HALF OF THE
FORECAST AREA. IF MANY AREAS SEE RNFL AT SOME POINT TODAY...SOME
FOG IS POSSIBLE IN THE FRONTAL ZONE OVERNIGHT. CONFIDENCE IS NOT
THERE YET TO INCLUDE IN OUR FORECAST.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
GUIDANCE IS IN GOOD AGREEMENT WITH A PROGRESSIVE COLD FRONTAL
PASSAGE FRIDAY NIGHT...AND ATLANTIC HIGH PRESSURE THEN REBUILDING
OVER THE FORECAST AREA OVER THE WEEKEND. HIGH PRESSURE IS THEN
EXPECTED TO REMAIN THE DOMINANT WEATHER FEATURE THROUGH MUCH OF THE
UPCOMING WORK WEEK.

DEEP MOISTURE IS EXPECTED TO SHIFT OFFSHORE LATE FRIDAY NIGHT...AND
THIS SHOULD LIMIT THE POTENTIAL FOR ANY SHOWER/THUNDERSTORM ACTIVITY
SATURDAY...SO WILL KEEP THE FORECAST RAIN FREE. WITH LACK OF DEEP
MOISTURE RETURN...WEST-SOUTHWESTERLY DOWNSLOPE FLOW...AND LACK OF
ANY SIGNIFICANT FORCING MECHANISM ASIDE FROM THE SEA BREEZE...EXPECT
RAIN FREE CONDITIONS TO CONTINUE SATURDAY NIGHT THROUGH SUNDAY NIGHT.

MODEL SOLUTIONS SUGGEST THAT A WEAKENING COLD FRONT WILL SLIDE
SOUTHWARD INTO THE FORECAST AREA LATE SUNDAY NIGHT AND
MONDAY...BRINGING WITH IT SLIGHTLY DEEPER MOISTURE AND ANOTHER
FOCUSING MECHANISM IN ADDITION TO THE AFTERNOON SEA BREEZE. AT THIS
POINT AM NOT ANTICIPATING MUCH IN THE WAY OF SHOWER/THUNDERSTORM
COVERAGE...BUT SHOULD SEE ENOUGH TO JUSTIFY SLIGHT CHANCE POPS
ACROSS PRIMARILY NORTHWEST ZONES MONDAY AFTERNOON.

THE WEAKENING FRONTAL BOUNDARY WILL DISSIPATE MONDAY NIGHT...WITH
ATLANTIC HIGH PRESSURE THEN REBUILDING OVER THE REGION TUESDAY
THROUGH WEDNESDAY. NOT TOO EXCITED ABOUT SHOWER/THUNDERSTORM CHANCES
EACH AFTERNOON...BUT CAN/T RULE OUT AN ISOLATED SHOWER/THUNDERSTORM
ACROSS FAR WESTERN ZONES IN CLOSER PROXIMITY TO A DEVELOPING LEE
SIDE TROUGH. THEREFORE...WILL CARRY SLIGHT CHANCE POPS HERE WITH
RAIN FREE CONDITIONS FOR COASTAL ZONES.

GUIDANCE IS IN FAIRLY GOOD AGREEMENT BREAKING DOWN THE ATLANTIC
RIDGE THURSDAY AS A COLD FRONT BEGINS TO APPROACH THE REGION FROM
THE WEST. THE SOUTHERLY FLOW IN ADVANCE OF THE COLD FRONT WILL ALLOW
DEEP MOISTURE TO BEGIN INCREASING OVER THE AREA FROM BOTH THE
ATLANTIC AND GULF OF MEXICO...SO WILL CARRY CHANCE POPS MOST AREAS.

TEMPERATURES ARE EXPECTED TO RUN SLIGHTLY ABOVE CLIMATOLOGY
THROUGHOUT THE EXTENDED PERIODS.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
HAVE MAINTAINED VFR CONDITIONS FOR KCHS AND KSAV THROUGH THE 18Z TAF
PERIOD. INCLUDED A CB GROUP FOR KCHS PRIOR TO 21Z AS RADAR SHOWS
DEVELOPMENT ALONG THE SEABREEZE. YET CONFIDENCE REMAINS LOW THAT KCHS
WILL BE AFFECTED...THUS WILL KEEP SHRA OUT OF TAFS FOR NOW. ADDITIONAL
ACTIVITY IS EXPECTED AS A COLD FRONT APPROACHES THE AREA FROM THE
WEST LATER THIS AFTERNOON INTO THE EVENING FOR BOTH TERMINALS. KEPT
CB GROUPS WHILE INCLUDING VCSH AS INSTABILITY LOOKS FAIRLY
IMPRESSIVE FOR THUNDERSTORM DEVELOPMENT AFTER 21Z. MODELS REMAIN
INCONSISTENT IN TIMING OF CONVECTION THUS WILL CONTINUE TO MONITOR
RADAR TRENDS AND AMEND TAFS IF NEEDED...ESPECIALLY FOR KSAV AS
INTERACTIONS BETWEEN THE GULF COAST SEA BREEZE AND A PINNED ATLC SEA
BREEZE AID THUNDERSTORM DEVELOPMENT. GUIDANCE ALSO REMAINS
INCONSISTENT REGARDING FOG FORMATION EARLY SATURDAY MORNING. YET
WITH SKIES CLEARING...RELATIVELY LIGHT WINDS AND LINGERING LOW LEVEL
MOISTURE...CANNOT RULE OUT SOME MVFR VISIBILITIES/CEILINGS AFTER 08Z.
WILL REEVALUATE FOG POTENTIAL FOR THE NEXT TAF PERIOD.

EXTENDED AVIATION OUTLOOK...VFR CONDITIONS EXPECTED SAT THROUGH
TUE.

&&

.MARINE...
LOW LEVEL TROF WILL RESIDE INLAND TODAY AND A SYNOPTIC SW FLOW
WILL RESIDE OVER THE WATERS THROUGH THIS EVENING. NEAR SHORE...
THERE MAY BE A BIT A W COMPONENT THIS MORNING AND A SOUTHERLY
DIRECTION POST SEA BREEZE. SPEEDS WILL COME DOWN FROM 10-15 KT
EARLY TO NEAR 10 KT BEFORE A LATE DAY INCREASE BACK INTO THE
10-15 KT RANGE. SEAS MAINLY 2-3 FT. TONIGHT...A COLD FRONT WILL BE
MAKING SOME LATE NIGHT HEADWAY INTO THE WATERS AND OTHER THAN THE
LIGHT LATE NIGHT WIND SHIFT...WINDS AND SEAS ON THE LIGHT SIDE. AS
MENTIONED IN THE SHORT TERM DISCUSSION... MARINE INTERESTS SHOULD
KEEP AN EYE OUT FOR A POTENTIAL OF SOME STRONG EVENING TSTMS.

ATLANTIC HIGH PRESSURE WILL REBUILD OVER THE SOUTHEAST UNITED STATES
SATURDAY THROUGH TUESDAY...RESULTING IN TRANQUIL CONDITIONS OVER THE
SOUTH CAROLINA AND GEORGIA COASTAL WATERS. WINDS ARE EXPECTED TO
REMAIN AT OR BELOW 15 KNOTS...WITH SEAS AT OR BELOW 3 FEET SATURDAY
THROUGH TUESDAY.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...NONE.

&&

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE CHARLESTON SC
322 PM EDT FRI MAY 29 2009

AMZ330-350-352-354-GAZ099>101-114>119-137>141-SCZ040-042>045-047>051-
301100-
CHARLESTON HARBOR-
WATERS FROM SOUTH SANTEE RIVER TO EDISTO BEACH SC OUT 20 NM-
WATERS FROM EDISTO BEACH SC TO SAVANNAH GA OUT 20 NM-
WATERS FROM SAVANNAH GA TO ALTAMAHA SOUND GA OUT 20 NM...INCLUDING
GRAYS REEF NATIONAL MARINE SANCTUARY-CANDLER-BULLOCH-EFFINGHAM-
TATTNALL-EVANS-INLAND BRYAN-COASTAL BRYAN-INLAND CHATHAM-
COASTAL CHATHAM-LONG-INLAND LIBERTY-COASTAL LIBERTY-INLAND MCINTOSH-
COASTAL MCINTOSH-ALLENDALE-HAMPTON-INLAND COLLETON-DORCHESTER-
BERKELEY-INLAND JASPER-BEAUFORT-COASTAL COLLETON-CHARLESTON-
COASTAL JASPER-
322 PM EDT FRI MAY 29 2009

...SEVERE THUNDERSTORM WATCH 315 IN EFFECT UNTIL 10 PM EDT THIS
EVENING...

THIS HAZARDOUS WEATHER OUTLOOK IS FOR ATLANTIC COASTAL
WATERS...SOUTHEAST GEORGIA AND SOUTHEAST SOUTH CAROLINA.

.DAY ONE...THIS AFTERNOON AND TONIGHT.

PLEASE LISTEN TO NOAA WEATHER RADIO OR GO TO WEATHER.GOV ON THE
INTERNET FOR MORE INFORMATION ABOUT THE FOLLOWING HAZARDS.

   SEVERE THUNDERSTORM WATCH.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY.

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.SPOTTER INFORMATION STATEMENT...

WEATHER SPOTTERS ARE ENCOURAGED TO REPORT SIGNIFICANT WEATHER
CONDITIONS ACCORDING TO STANDARD OPERATING PROCEDURES.

AMZ374-GAZ087-088-301100-
WATERS FROM SAVANNAH GA TO ALTAMAHA SOUND GA EXTENDING FROM 20 TO
60 NM-JENKINS-SCREVEN-
322 PM EDT FRI MAY 29 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR ATLANTIC COASTAL WATERS AND
SOUTHEAST GEORGIA.

.DAY ONE...THIS AFTERNOON AND TONIGHT.

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY.

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.SPOTTER INFORMATION STATEMENT...

WEATHER SPOTTERS ARE ENCOURAGED TO REPORT SIGNIFICANT WEATHER
CONDITIONS ACCORDING TO STANDARD OPERATING PROCEDURES.

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