Severe Thunderstorm Warning 277, NWS Charleston, SC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
443 PM EDT WED AUG 12 2009

.SYNOPSIS...
A TROUGH OF LOW PRESSURE WILL PREVAIL OVER THE CAROLINAS AND GEORGIA
THROUGH SATURDAY. HIGH PRESSURE OVER THE WESTERN ATLANTIC WILL THEN
EXPAND OVER THE AREA AND REMAIN THE DOMINANT WEATHER FEATURE THROUGH
EARLY NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
A DEEP POSITIVELY TILTED UPPER LEVEL TROUGH IS POSITIONED JUST
WEST OF THE APPALACHIANS THIS AFTERNOON...AND MAY AMPLIFY FURTHER
TONIGHT AS THE AXIS SHIFTS SLIGHTLY EASTWARD. MEANWHILE AT THE
SURFACE...A LEE SIDE TROUGH HAS BECOME NEARLY STATIONARY ACROSS
INLAND PORTIONS OF THE FORECAST AREA.

DESPITE THE ATMOSPHERE ACROSS MUCH OF THE FORECAST AREA HAVING
BEEN WORKED OVER FROM EARLIER CONVECTION...INSTABILITY PARAMETERS
REMAIN FAIRLY IMPRESSIVE LATE THIS AFTERNOON ACROSS THE
REGION...MAINLY OUTSIDE OF THE CHARLESTON TRI-COUNTY AREA. THERE
IS STILL A THREAT FOR SEVERE WEATHER AS NUMEROUS SHOWERS AND
THUNDERSTORMS SHIFT ACROSS THE AREA INTO EARLY THIS EVENING...THUS
THE SEVERE THUNDERSTORM WATCH REMAINS IN EFFECT UNTIL 9 PM.
HOWEVER...THUNDERSTORMS SHOULD BE MORE ELEVATED ACROSS NORTHERN
PORTIONS OF THE FORECAST AREA AND THE WATCH MAY BE CANCELLED
SOONER IN SOME LOCATIONS.

ALTHOUGH THE COVERAGE OF THE SHOWERS AND THUNDERSTORMS SHOULD
DIMINISH WITH THE LOSS OF DAYTIME HEATING...WILL ONLY BRING THE
PROBABILITY OF PRECIPITATION DOWN TO A CHANCE LATE THIS EVENING
AND OVERNIGHT. THE REASON FOR THIS WILL BE PERSISTENT UPPER LEVEL
SUPPORT...ALONG WITH WEAK LOW LEVEL CONVERGENCE ASSOCIATED WITH
THE LEE SIDE TROUGH.

THERE COULD BE SOME LINGERING HIGH LEVEL DEBRIS CLOUDINESS THROUGH
THIS EVENING...THEN SOME STRATUS DEVELOPMENT LATE TONIGHT.
BOUNDARY LAYER MOISTURE WILL REMAIN RATHER ABUNDANT...MAKING FOR
ANOTHER WARM/MUGGY NIGHT WITH LOWS IN THE LOWER TO MID 70S ACROSS
THE AREA.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
THE REGION WILL REMAIN WITHIN THE RIGHT ENTRANCE REGION OF A
WEAKENING UPPER LEVEL JET ON THURSDAY...WHILE AT THE SURFACE A LEE
SIDE TROUGH WILL LINGER IN THE VICINITY. DEEP LAYERED MOISTURE
WILL CONTINUE TO STREAM ACROSS THE AREA...RESULTING IN
PRECIPITABLE WATER VALUES OF 2.00-2.25 INCHES. THESE
FACTORS...COMBINED WITH MESOSCALE BOUNDARIES INCLUDING THE SEA
BREEZE SHOULD BE SUFFICIENT TO SUPPORT WIDESPREAD SHOWER AND
THUNDERSTORMS DEVELOPMENT...MAINLY IN THE AFTERNOON. THERE IS A
LIMITED THREAT FOR STRONG THUNDERSTORMS...HOWEVER INSTABILITY
SHOULD NOT BE NEARLY AS IMPRESSIVE AS IT WAS WEDNESDAY GIVEN LESS
SOLAR INSOLATION. THERE WILL BE A HEAVY RAINFALL AND LOCALIZED
FLOODING THREAT THOUGH...AND WILL HIGHLIGHT THE RISK IN THE
HAZARDOUS WEATHER OUTLOOK. THE CONVECTION SHOULD DIMINISH WITH THE
LOSS OF DAYTIME HEATING THURSDAY EVENING...BUT WILL HOLD ONTO AT
LEAST A SLIGHT CHANCE OF SHOWERS AND THUNDERSTORMS THROUGH THE
NIGHT...WITH A LOW CHANCE ALONG THE IMMEDIATE COAST.

TEMPERATURES ON THURSDAY WILL BE SLIGHTLY BELOW NORMAL...WITH
HIGHS IN THE UPPER 80S. HIGH DEWPOINT VALUES WILL MAKE FOR RATHER
HUMID CONDITIONS HOWEVER.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
MEDIUM RANGE GUIDANCE IS IN FAIRLY GOOD AGREEMENT WITH LARGE SCALE
SYNOPTIC FEATURES...ALTHOUGH THERE IS SOME DIFFERENCE ON FRIDAY
REGARDING A SURFACE TROUGH OF LOW PRESSURE NEAR THE COAST AS WELL AS
THE STRENGTH/LOCATION OF THE UPPER RIDGE BUILDING IN EARLY NEXT
WEEK.

THE NAM12 /WRF/ INDICATES TROUGHING LINGERING JUST OFFSHORE WHILE
THE GFS KEEPS THIS FEATURE FARTHER INLAND. HAVE FOLLOWED THE GFS
SOLUTION FOR NOW FOR PERSISTENCE/CLIMATOLOGICAL PURPOSES. THE TROUGH
OF LOW PRESSURE AT THE SURFACE SHOULD WEAKEN AND THEN DISSIPATE OVER
THE WEEKEND AS ATLANTIC HIGH PRESSURE IS THEN EXPECTED TO BUILD BACK
INTO THE MIDDLE OF NEXT WEEK. OTHERWISE...UPPER TROUGHING OVER THE
SOUTHEAST STATES FRIDAY WILL BEGIN TO BREAK DOWN OVER THE WEEKEND AS
UPPER LEVEL RIDGING BUILDS BACK INTO THE REGION. UPPER LEVEL RIDGING
IS THEN EXPECTED TO HOLD IN PLACE ACROSS THE SOUTHEAST CONUS THROUGH
EARLY NEXT WEEK WHILE ATLANTIC HIGH PRESSURE EXPANDS OVER THE AREA.

AS FOR FORECAST DETAILS...UPPER LEVEL TROUGHING...PLENTY OF DEEP
MOISTURE...AND ADEQUATE INSTABILITY SHOULD SUPPORT SCATTERED TO
NUMEROUS SHOWERS/THUNDERSTORMS FRIDAY. EXPECT SHOWER/THUNDERSTORM
COVERAGE TO SLOWLY DECREASE THROUGH THE WEEKEND INTO EARLY NEXT WEEK
AS RIDGING TAKES HOLD AT THE SURFACE AND ALOFT...SO WILL KEEP
SCATTERED POPS GOING SATURDAY AND SUNDAY BEFORE TAPERING TO SLIGHT
CHANCE POPS MONDAY AND TUESDAY.

TEMPERATURES AREA EXPECTED TO RUN NEAR TO SLIGHTLY BELOW NORMAL
THROUGH SATURDAY...THEN NEAR TO SLIGHTLY ABOVE NORMAL SUNDAY THROUGH
WEDNESDAY.

&&

.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
STRONG INSTABILITY COMBINED WITH DEEP LAYERED MOISTURE...NUMEROUS
SURFACE BOUNDARIES AND GOOD UPPER LEVEL SUPPORT WILL RESULT IN
NUMEROUS SHOWERS AND THUNDERSTORMS INTO EARLY THIS EVENING. AS
THUNDERSTORMS IMPACT THE TERMINALS...MVFR OR LOWER CONDITIONS ARE
EXPECTED TO OCCUR...ALONG WITH GUSTY AND ERRATIC WINDS. THE
GREATEST THREAT FOR THESE CONDITIONS APPEARS TO BE AT KSAV WHERE
INSTABILITY IS MORE PREVALENT. THERE WILL BE A DIMINISHING TREND
TO THE CONVECTION WITH THE LOSS OF DAYTIME HEATING...HOWEVER COULD
NOT RULE OUT ADDITIONAL CONVECTION THROUGH THE NIGHT. HIGH LEVEL
DEBRIS CLOUDINESS COULD PERSIST INTO THE OVERNIGHT HOURS...WITH
SOME GUIDANCE SHOWING THE POTENTIAL FOR LOWER CEILINGS TO DEVELOP
AROUND DAYBREAK.

EXTENDED AVIATION OUTLOOK...VFR CONDITIONS WILL GENERALLY
PREVAIL. HOWEVER...MVFR/IFR CONDITIONS ARE POSSIBLE IN SCATTERED
AFTERNOON AND EVENING SHOWERS/THUNDERSTORMS THROUGH MONDAY.

&&

.MARINE...
WIND FIELDS WILL BE DISRUPTED EARLY THIS EVENING GIVEN CONVECTION
OVER THE COASTAL WATERS. HOWEVER...A SOUTHERLY SURGE SHOULD
DEVELOP LATE THIS EVENING INTO THE OVERNIGHT HOURS...BUT WILL
GENERALLY CAP WINDS AROUND 15 KT.

A LEE SIDE TROUGH WILL PERSIST INLAND ON THURSDAY...WITH A
SOUTHERLY FLOW CONTINUING OVER THE COASTAL WATERS. A WEAK SEA
BREEZE CIRCULATION COULD ELEVATE WINDS ALONG THE IMMEDIATE
COAST...BUT DO NOT EXPECT WINDS TO GET ANY GREATER THAN ABOUT 15
KT ANYWHERE IN THE COASTAL WATERS. ANOTHER NOCTURNAL SURGE COULD
DEVELOP THURSDAY NIGHT...BUT AGAIN WIND FIELDS MAY BECOME
DISRUPTED IN THE LATE AFTERNOON/EARLY EVENING DUE TO CONVECTION
AND WILL HOLD WINDS AT 15 KT.

FRIDAY THROUGH MONDAY...A TROUGH OF LOW PRESSURE WILL LINGER OVER OR
NEAR THE AREA INTO SATURDAY BEFORE ATLANTIC HIGH PRESSURE EXPANDS
OVER THE AREA OVER THE WEEKEND INTO EARLY NEXT WEEK. THIS WILL
RESULT IN A FAIRLY TYPICAL SUMMERTIME PATTERN OVER THE COASTAL
WATERS...CHARACTERIZED BY AFTERNOON SEA BREEZES AND NOCTURNAL
SURGES...ESPECIALLY BEYOND FRI. WINDS AND SEAS ARE EXPECTED TO
REMAIN BELOW SMALL CRAFT ADVISORY CRITERIA THROUGH THE PERIOD.

&&

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

&&

JAQ/RJB

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE CHARLESTON SC
457 PM EDT WED AUG 12 2009

AMZ330-GAZ087-088-099>101-114>119-137>141-SCZ040-042>045-047>051-
131000-
CHARLESTON HARBOR-JENKINS-SCREVEN-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-
457 PM EDT WED AUG 12 2009

...SEVERE THUNDERSTORM WATCH 700 IN EFFECT UNTIL 9 PM EDT THIS
EVENING...

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

.DAY ONE...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...THURSDAY THROUGH TUESDAY.

HEAVY RAINFALL...SHOWERS AND THUNDERSTORMS ARE EXPECTED TO BECOME
WIDESPREAD ACROSS THE AREA THURSDAY AFTERNOON. THERE IS THE
POTENTIAL FOR HEAVY RAINFALL ACROSS THE AREA...WHICH MAY RESULT IN
LOCALIZED FLOOD OF LOW LYING...URBANIZED AND OTHER POOR DRAINAGE
AREAS.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION IS REQUESTED INTO THIS EVENING.

AMZ350-352-354-374-131000-
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-
WATERS FROM SAVANNAH GA TO ALTAMAHA SOUND GA EXTENDING FROM 20 TO
60 NM-
457 PM EDT WED AUG 12 2009

THIS HAZARDOUS WEATHER OUTLOOK IS FOR ATLANTIC COASTAL WATERS.

.DAY ONE...TONIGHT.

SEVERE THUNDERSTORMS...SCATTERED SHOWERS AND THUNDERSTORMS COULD
PRODUCE DAMAGING WINDS AND LARGE HAIL INTO THIS EVENING ACROSS THE
COASTAL WATERS. IN ADDITION...HEAVY RAINFALL AND FREQUENT CLOUD TO
WATER LIGHTNING IS EXPECTED.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY.

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION IS REQUESTED INTO THIS EVENING.

JAQ

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