Severe Thunderstorm Warning 207, NWS Charleston, SC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
409 PM EDT MON OCT 25 2010

.SYNOPSIS...
SURFACE HIGH PRESSURE WILL WEAKEN AS IT STEADILY SHIFTS AWAY FROM
THE CAROLINA COAST THROUGH MIDWEEK. A COLD FRONT WILL MOVE THROUGH
THE AREA ON THURSDAY...FOLLOWED BY HIGH PRESSURE THROUGH THE
UPCOMING WEEKEND AND INTO EARLY NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
STRONG TO SEVERE TSTMS HAVE FINALLY DEVELOPED AFTER SEVERAL HOUR
OF WAITING. MID-AFTERNOON WATER VAPOR IMAGERY SHOWS A POTENT
SHORTWAVE CROSSING EASTERN TENNESSEE AND NORTHERN GEORGIA WITH A
BAND OF ENHANCED UVM JUST NOW ENTERING THE WESTERN EDGE OF THE
LOCAL FORECAST AREA. SURFACE OBSERVATIONS AND HIGH RESOLUTION
VISIBLE SATELLITE DATA INDICATE A MESOSCALE BOUNDARY...MOST
LIKELY A DIFFERENTIAL HEATING BOUNDARY CREATED BY A PERSISTENT
CIRRUS BAND TO THE SOUTH...STRETCHES FROM SOUTH-CENTRAL GEORGIA
THROUGH THE EASTERN MIDLANDS TO JUST NORTH OF LAKE MOULTRIE. TSTMS
HAVE RECENTLY FIRED ALONG THE BOUNDARY AND HAVE SHOWN SUSTAINED
SUPERCELLULAR CHARACTERISTICS. A PARTICULARLY NASTY CELL WENT
THROUGH ALLENDALE COUNTY WHERE UP TO BASEBALL SIZE HAIL WAS
REPORTED. THE SPECIAL 17Z CHS SOUNDING SHOWS THE ATMOSPHERE HAS
DESTABILIZED SIGNIFICANTLY SINCE MID-MORNING WITH SBCAPES NEAR
3500 J/KG...LIFTED INDICES OF -8C WITH A SHOWALTER OF -2C.
INCREASING SHEAR COUPLED WITH STRONG INSTABILITY FOR LATE OCTOBER
WILL CONTINUE TO FAVOR STRONG TO SEVERE TSTMS FOR SEVERAL MORE
HOURS WITH A POTENTIAL FOR ISOLATED TORNADOES ACROSS INLAND AREAS
INVOF THE MESOSCALE BOUNDARY.

THE EVENTUAL CONVECTIVE COVERAGE WILL BE HIGHLY DEPENDENT ON
MESOSCALE BOUNDARY COLLISIONS. SHORT TERM MESOSCALE MODELS HAVE
NOT DONE A GOOD JOB WITH THE COVERAGE NOR TIMING OF TSTMS SO FAR
TODAY...BUT CURRENT RADAR AND SATELLITE TRENDS SUGGEST KEEPING A
FAIRLY HEALTH POP REGIME IN PLACE THROUGH EARLY EVENING...DESPITE
THE RELATIVE LACK OF COVERAGE SO FAR. WILL GO WITH 60-80 PERCENT
POPS THROUGH 00Z WITH THE HIGHEST GRIDDED POPS CONFINED TO
INTERIOR AREAS. POPS MAY BE ADJUSTED AT THE LAST MINUTE PENDING
RADAR TRENDS. WILL ALSO MENTION LARGE HAIL AND DAMAGING
WINDS...ALTHOUGH AN ISOLATED TORNADO CAN NOT BE RULED OUT.

SHOWERS/TSTMS WILL STEADILY DIMINISH THIS EVENING AS THE SHORTWAVE
PUSHES EAST AND THE STRONGEST UVM SHIFTS OFFSHORE. SUSPECT MOST
ACTIVITY WILL BE OVER BY MID-EVENING...IF NOT SOONER...BUT A FEW
SHOWERS COULD LINGER ACROSS PARTS OF THE CHARLESTON METRO AREA AS
LATE AS MIDNIGHT. ONCE THE RAIN ENDS...THE RISK FOR LOW CLOUDS
AND FOG WILL INCREASE AS DRIER MID-LEVEL AIR ADVECTS IN FROM THE
SOUTHWEST. SIMILAR TO YESTERDAY...GUIDANCE IS ALL OVER THE PAST
WITH RESPECT TO BOTH CEILINGS AND VISIBILITIES. SREF VISIBILITIES
AND CEILING PLUMES ARE OFFERING LITTLE HELP AS WELL. SOME SOME
AREAS LIKELY SEEING SOME FAIRLY HEALTHY RAINS...WILL CONTINUE WITH
OUR PREVIOUS FORECAST OF /AREAS OF FOG/ THROUGH SUNRISE TUESDAY.

IT WILL REMAIN A RATHER WARM NIGHT WITH FOG AND LOW CLOUDS IN
PLACE...RANGING FROM THE MID 60S WELL FAR INTERIOR SOUTHEAST
GEORGIA TO THE LOWER 70S AT THE BEACHES.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
MODEL GUIDANCE IS IN GOOD AGREEMENT KEEPING ATLANTIC HIGH PRESSURE
IN PLACE OVER THE REGION TUESDAY THROUGH WEDNESDAY. WILL CARRY
SLIGHT CHANCE POPS EACH AFTERNOON TUESDAY AND WEDNESDAY IN MOST
AREAS...AS THE SEA BREEZE MAY BE ENOUGH TO POP AN ISOLATED SHOWER OR
THUNDERSTORM GIVEN CAPE VALUES BETWEEN 1500 AND 2000 J/KG.
ADDITIONALLY...COULD SEE A FEW ISOLATED SHOWERS ROLL IN OFF THE
ATLANTIC AFTER MIDNIGHT TUESDAY NIGHT BEFORE LOW LEVEL STEERING FLOW
SHIFTS OFFSHORE WEDNESDAY MORNING.

MODEL SOLUTIONS THEN AGREE THAT A COLD FRONT WILL APPROACH FROM THE
NORTHWEST WEDNESDAY NIGHT...BUT DIFFER SLIGHTLY ON THE TIMING OF THE
COLD FRONTAL PASSAGE ON THURSDAY. GIVEN THE UNCERTAINTY IN FRONTAL
TIMING OPTED TO KEEP CHANCE POPS IN THE FORECAST WEDNESDAY NIGHT AND
THURSDAY...THEN TAPER POPS INTO THE SLIGHT CHANCE RANGE FOR THURSDAY
NIGHT.

TEMPERATURES WILL REMAIN WELL ABOVE NORMAL THROUGH THE SHORT TERM
PERIODS...WITH HIGHS IN THE 80S AWAY FROM THE BEACHES AND LOWS
RANGING FROM THE MID 60S TO LOWER 70S TUESDAY NIGHT AND WEDNESDAY
NIGHT...THEN DROPPING BACK INTO THE 50S FOR THURSDAY NIGHT.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
GUIDANCE IS IN FAIRLY GOOD AGREEMENT SHOWING THE FLOW ALOFT BECOMING
FAIRLY PROGRESSIVE DURING THE LONG TERM PERIOD. SURFACE HIGH
PRESSURE WILL INITIALLY BUILD FROM THE MISSISSIPPI VALLEY THURSDAY
NIGHT AND FRIDAY...BEFORE SHIFTING ACROSS THE CAROLINAS ON SATURDAY
THEN INTO THE ATLANTIC LATE IN THE WEEKEND INTO EARLY NEXT WEEK.

VERY DRY AND COOL CONDITIONS WILL BE IN PLACE THROUGH SATURDAY...
ALONG WITH BELOW NORMAL TEMPERATURES. A SOUTHERLY RETURN FLOW WILL
DEVELOP BY LATE IN THE WEEKEND...RESULTING IN AN INCREASE IN
MOISTURE. HOWEVER...STRONG SUBSIDENCE WILL KEEP RAIN-FREE CONDITIONS
IN PLACE. TEMPERATURES WILL ALSO RETURN TO NEAR NORMAL BY LATE IN
THE WEEKEND.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
BOTH KCHS AND KSAV HAVE POTENTIAL TO SEE DETERIORATING CONDITIONS AS
A LINE OF SHOWERS AND THUNDERSTORMS TRACK NORTHEASTWARD ACROSS THE
FORECAST AREA. HAVE INCLUDED CBS FOR BOTH SITES THIS AFTERNOON...BUT
ACCORDING TO CURRENT RADAR TRENDS...PRECIP CHANCES APPEAR HIGHER AT
KCHS THAN KSAV FOR THIS AFTERNOON. HOWEVER...STILL EXPECT ADDITIONAL
DEVELOPMENT FURTHER WEST AND SOUTH WHICH COULD EVENTUALLY LEAD TO
AMENDMENTS TO BOTH TAFS. EVEN ONCE PRECIP PASSES THE AREA...PLENTY
OF LINGERING LOW LEVEL MOISTURE WILL RESIDE AT BOTH SITES. THIS WILL
LEAD TO FAVORABLE CONDITIONS FOR A LOW STRATUS LAYER EARLY TUESDAY
MORNING. HAVE ALSO INCLUDE SOME MVFR VISIBILITIES...BUT EXPECT WINDS TO
REMAIN 5 KNOTS OR GREATER THROUGH THE OVERNIGHT PERIOD WHICH
SHOULD HINDER SIGNIFICANT FOG DEVELOPMENT.

EXTENDED AVIATION OUTLOOK...LOW STRATUS AND/OR FOG WILL LIKELY
RESULT IN MVFR OR LOWER CONDITIONS DURING THE EARLY MORNING HOURS ON
TUESDAY AND WEDNESDAY. SHOWERS ASSOCIATED WITH A PASSING COLD FRONT
COULD ALSO BRIEFLY REDUCE CEILINGS AND/OR VISIBILITIES TO MVFR
LEVELS ON THURSDAY.

&&

.MARINE...
SEAS HAVE BREACHED 6 FT AT 41004 AND SEAS WILL CONTINUE TO BUILD
TONIGHT. WW3 OUTPUT HAS DONE REASONABLY WELL TODAY AND SHOWS 6 FT
SEAS REACHING INTO THE SOUTH CAROLINA NEARSHORE WATERS THIS
EVENING. WILL MAINTAIN FLAGS FOR THE GEORGIA OFFSHORE WATERS AND
EXPAND IT INTO THE SOUTH CAROLINA NEARSHORE WATERS. COULD SEE A
BIT OF NOCTURNAL SURGING...BUT THINK WINDS WILL REMAIN 15 KT
NEARSHORE AND 15-20 KT OFFSHORE.

ATLANTIC HIGH PRESSURE WILL HOLD IN PLACE THROUGH WEDNESDAY BEFORE
SHIFTING EASTWARD IN ADVANCE OF AN APPROACHING COLD FRONT WEDNESDAY
NIGHT. THE COLD FRONT WILL MOVE THROUGH THE WATERS ON
THURSDAY...WITH HIGH PRESSURE THEN BUILDING BACK INTO THE AREA FOR
THE END OF THE WEEK INTO THE UPCOMING WEEKEND.

SEAS ARE EXPECTED TO SUBSIDE BELOW ADVISORY LEVELS ACROSS THE
NEARSHORE SOUTH CAROLINA WATERS TUESDAY MORNING...BUT WILL LIKELY
BUILD TO ADVISORY LEVELS ONCE AGAIN TUESDAY NIGHT. ACROSS THE OUTER
GEORGIA WATERS SEAS ARE EXPECTED TO REMAIN AT SMALL CRAFT LEVELS
THROUGH WEDNESDAY MORNING. WINDS AND SEAS ARE THEN EXPECTED TO HOLD
BELOW HEADLINE THRESHOLDS WEDNESDAY AFTERNOON THROUGH THURSDAY.
SMALL CRAFT ADVISORIES MAY THEN BE NEEDED ONCE AGAIN THURSDAY NIGHT
INTO FRIDAY MORNING BEHIND THE COLD FRONT ACROSS PORTIONS OF THE
WATERS...AND SEAS MAY LINGER WITHIN THE SMALL CRAFT RANGE ACROSS THE
OUTER GEORGIA WATERS THROUGH SATURDAY.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 11 AM EDT TUESDAY FOR AMZ350-352.
     SMALL CRAFT ADVISORY UNTIL NOON EDT WEDNESDAY FOR AMZ374.

&&

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