Area Forecast Discussion
Issued by NWS Charleston, SC

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000
FXUS62 KCHS 201550
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
1150 AM EDT WED AUG 20 2014

.SYNOPSIS...
THE REGION WILL REMAIN SITUATED BETWEEN ATLANTIC HIGH PRESSURE
AND AN INLAND TROUGH OF LOW PRESSURE THROUGH FRIDAY. A COLD FRONT
WILL ADVANCE THROUGH THE AREA THIS WEEKEND...THEN HIGH PRESSURE
WILL BUILD FROM THE NORTH INTO THE MIDDLE OF NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
LATE WEDNESDAY MORNING...SURFACE ANALYSIS DEPICTED THAT A BROAD
SURFACE TROUGH HAD SPREAD INTO NORTHERN COUNTIES...RESULTING IN
WEAKENING WINDS AND ALLOWING EARLIER THAN EXPECTED SEA BREEZE
DEVELOPMENT. UPDATED FORECASTS TO ACCOUNT FOR THESE TRENDS.

AFTERNOON HIGHS IN THE MID TO UPPER 90S ARE STILL EXPECTED AWAY
FROM THE BEACHES. OFFSHORE WINDS COULD PUSH 90F TEMPS TO
CENTRAL/SOUTHERN BEACHES BEFORE THE SEA BREEZE DEVELOPS THIS
AFTERNOON. MAX HEAT INDEX VALUES WILL AVERAGE 100-105F AT MOST
LOCATIONS...BUT A BRIEF SURGE TO AROUND 110F IS POSSIBLE AT THE
COAST AS THE SEA BREEZE PUSHES HIGHER DEWPOINTS INLAND.

MODELS DEPICT THE UPPER RIDGE CENTERED W/SW OF THE REGION EXPANDING
THIS AFTERNOON...RESULTING IN SUBTLE MID LEVEL WARMING AND EVEN WEAK
SYNOPTIC SCALE SUBSIDENCE. 12Z SOUNDINGS ESPECIALLY AT KCHS
REVEALED LITTLE CAPPING...AND MODIFIED SOUNDINGS YIELD MLCAPES
2500-3500 J/KG AND DCAPES AS HIGH AS 1200-1400 J/KG. THUS...EVEN
THOUGH DIURNAL THUNDERSTORM COVERAGE SHOULD BE REDUCED AS COMPARED
WITH TUESDAY/JUSTIFYING ONLY SLIGHT CHANCE POPS...THE
THERMODYNAMIC ENVIRONMENT SUPPORTS AT LEAST A LOW POTENTIAL
FOR ISOLATED/BRIEF DAMAGING WIND GUSTS. THIS LIMITED POTENTIAL
FOR SEVERE WEATHER DOES NOT WARRANT A MENTION WITHIN THE HAZARDOUS
WEATHER OUTLOOK.

TROPOSPHERIC WINDS...WHILE NOT AS STRONG AS TUESDAY...ARE STILL
ELEVATED FOR THIS TIME OF YEAR...AVERAGING 15-25 KT IN THE 925-500
MB LAYER. 0-1 KM SHEAR 10-15 KT SUGGESTS THAT THUNDERSTORMS WILL
TREND TOWARD MULTICELL CLUSTERS WHICH SHOULD ADVANCE TOWARD THE
E/SE AT 20-25 KNOTS. MOST LIKELY LOCATIONS FOR THUNDERSTORMS WILL
INCLUDE THE SEA BREEZE NEAR THE COAST...NEAR THE ALTAMAHA RIVER
WHERE STRONGEST INSTABILITY/LOW-LEVEL SHEAR WILL EXIST AND ACROSS
NORTHERN COUNTIES WHERE A FEW THUNDERSTORMS DEVELOPING ALONG A
WELL-DEFINED SURFACE TROUGH TO THE N/NW COULD DROP INTO THE
REGION.

OF SOME CONCERN...WATER VAPOR IMAGERY SUGGESTS THAT A SHORTWAVE
TROUGH ADVANCING INTO THE CENTRAL/SOUTHERN APPALACHIANS APPEARS
STRONGER/DEEPER THAN DEPICTED BY GUIDANCE...AND THE UPSTREAM
CUMULUS FIELD WAS RESPONDING TO THIS FEATURE. MODELS WEAKEN THIS
FEATURE ON THE PERIPHERY OF THE BUILDING RIDGE...BUT A STRONGER
TROUGH EMERGING EAST OF THE APPALACHIANS COULD ENHANCE MULTICELL
THUNDERSTORMS DEVELOP UPSTREAM ESPECIALLY ALONG THE AFOREMENTIONED
SURFACE TROUGH. THESE THUNDERSTORMS/ASSOCIATED HIGHER POPS AND A
SLIGHTLY GREATER CHANCE FOR DAMAGING WIND GUSTS COULD THEN PUSH
INTO THE REGION ESPECIALLY INLAND/SC COUNTIES LATER THIS AFTERNOON/
THIS EVENING. WILL CONTINUE TO ASSESS.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/...
ASSUMING THAT THUNDERSTORMS DEVELOPING UPSTREAM DO NOT COMPLICATE
THE FORECAST SCENARIO THIS EVENING...ANY ONGOING DIURNAL
CONVECTION SHOULD QUICKLY DISSIPATE AFTER DARK...LEAVING A MOSTLY
CLEAR AND QUIET NIGHT WITH LIGHT WINDS/LOWS IN THE LOWER TO MIDDLE
70S MOST AREAS.

THURSDAY THROUGH FRIDAY...A STRONG MID AND UPPER LEVEL RIDGE
CENTERED OVER THE DEEP SOUTH WILL EXPAND EASTWARD OVER THE REGION
ON THURSDAY...THEN ERODE A BIT ALONG ITS EASTERN PERIPHERY ON
FRIDAY AS AN UPPER LOW CROSSES NEW ENGLAND AND CAUSES A TROUGH TO
DIG SOUTHWARD JUST OFF THE MID-ATLANTIC COAST. MEANWHILE AT THE
SURFACE...HIGH PRESSURE WILL PREVAIL SOUTHEAST OF THE REGION WHILE
A TROUGH PERSISTS INLAND.

THIS PERIOD WILL BE CHARACTERIZED BY NORTHWEST FLOW ALOFT GIVEN THE
POSITION OF THE UPPER RIDGE...THEREFORE EXPECT HOT AND RELATIVELY
DRY CONDITIONS ACROSS THE FORECAST AREA. A PINNED SEA BREEZE...
DOWNSLOPE FLOW ALOFT AND STRONG SUBSIDENCE WILL SUPPORT HIGHS AROUND
100 IN MOST PLACES INLAND FROM THE IMMEDIATE COAST BOTH DAYS. HEAT
INDICES WILL EASILY REACH 105-110 DEGREES...WITH THE LOWEST VALUES
WELL INLAND WHERE DEWPOINTS WILL MIX OUT BEST. IT IS VERY POSSIBLE
THAT HEAT ADVISORIES COULD BE REQUIRED FOR AT LEAST PORTIONS OF THE
FORECAST AREA. LOWS WILL BE IN THE MID AND UPPER 70S BOTH THURSDAY
AND FRIDAY NIGHT. EXPECT ONLY ISOLATED CONVECTION TO DEVELOP ALONG
THE IMMEDIATE COAST AND PERHAPS INTO PARTS OF THE CHARLESTON
TRI-COUNTY AREA THURSDAY IN VICINITY OF THE PINNED SEA BREEZE...BUT
IT SHOULD BE TOO DRY AND SUBSIDENCE TOO STRONG FOR ANY CONVECTION
ELSEWHERE. THEN ON FRIDAY WITH SOME WEAKENING OF THE UPPER RIDGE
WILL FORECAST A SLIGHT CHANCE OF SHOWERS AND THUNDERSTORMS ACROSS
THE AREA IN THE AFTERNOON/EVENING.

SATURDAY...A BACKDOOR COLD FRONT WILL BEGIN TO DROP SOUTHWARD ACROSS
THE AREA. THE FORECAST INDICATES A CHANCE OF SHOWERS AND
THUNDERSTORMS ACROSS THE CHARLESTON TRI-COUNTY AREA AND A SLIGHT
CHANCE ELSEWHERE ACCOMPANYING THE FRONTAL PASSAGE. TEMPERATURES WILL
COOL A BIT WITH INCREASED CLOUD COVER AND WEAKER SUBSIDENCE
ALOFT...WITH HIGHS IN THE MID TO UPPER 90S AWAY FROM THE IMMEDIATE
COAST.

&&

.LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
MEDIUM RANGE GUIDANCE IS IN FAIRLY GOOD AGREEMENT THROUGH MOST OF
THE LONG TERM PERIOD WITH THE OVERALL SYNOPTIC PATTERN. A BACKDOOR
COLD FRONT WILL DROP SOUTH OF THE REGION LATE IN THE WEEKEND...
RESULTING IN A MOIST NORTHEAST FLOW WHICH WILL PREVAIL INTO AT LEAST
EARLY NEXT WEEK AS HIGH PRESSURE BUILDS FROM THE NORTH. THIS WILL
BRING UNSETTLED CONDITIONS TO THE AREA...WITH A SOLID CHANCE OF
SHOWERS AND THUNDERSTORMS EXPECTED ON SUNDAY WITH PROBABILITIES THEN
GRADUALLY LOWERING FROM NORTH TO SOUTH EACH DAY THROUGH WEDNESDAY.
MUCH COOLER TEMPERATURES WILL ALSO OCCUR ONCE THE FRONT MOVES
THROUGH...WITH TEMPERATURES MODERATING CLOSER TO SEASONAL NORMALS.

&&

.AVIATION /16Z WEDNESDAY THROUGH SUNDAY/...
VFR THROUGH TONIGHT. THE POTENTIAL FOR THUNDERSTORMS REMAINS LOW
THIS AFTERNOON/EVENING...BUT REMAIN ALERT FOR CHANGES/AMENDMENTS.

EXTENDED AVIATION OUTLOOK...MAINLY VFR CONDITIONS EXPECTED
THROUGH FRIDAY. INCREASING CHANCES FOR FLIGHT RESTRICTIONS
SATURDAY INTO SUNDAY AS A FRONT DROPS THROUGH THE REGION. LOW
CEILINGS AND/OR VSBYS ARE POSSIBLE WITH THE FRONT...AS WELL AS
INCREASING CHANCES FOR SHOWERS AND TSTMS.

&&

.MARINE...
OFFSHORE WINDS 10-20 KT WILL TURN TOWARD THE SW AS THE SEA BREEZE
DEVELOPS THIS AFTERNOON. TONIGHT...W/SW WINDS WILL AVERAGE LESS
THAN 20 KT MOST AREAS. SEAS WILL RANGE FROM 2-3 FT WITHIN
NEARSHORE WATERS TO 3-5 FT BEYOND 20 NM THROUGH TONIGHT.

THURSDAY THROUGH MONDAY...A SOUTH/SOUTHWEST FLOW WILL PREVAIL
THROUGH FRIDAY OVER THE WATERS WITH SPEEDS AROUND 15 KT OR LESS AS
HIGH PRESSURE PREVAILS SOUTHEAST OF THE REGION WITH A TROUGH OF LOW
PRESSURE INLAND. A LOW LEVEL JET COULD STRENGTHEN FRIDAY NIGHT AHEAD
OF AN APPROACHING COLD FRONT AND COULD PUSH WINDS UPWARDS OF 15-20
KT OVER MAINLY THE OUTER GEORGIA WATERS. THE COLD FRONT WILL
GRADUALLY PUSH SOUTHWARD OVER THE WEEKEND...WITH WINDS VEERING
NORTHEAST BY SUNDAY. A STRONGER EAST/NORTHEAST FLOW WILL THEN
PREVAIL THROUGH MONDAY WITH WINDS REACHING UPWARDS OF 15-20 KT WITH
A FEW HIGHER GUSTS. SMALL CRAFT ADVISORY CONDITIONS COULD DEVELOP
OVER MAINLY THE OUTER GEORGIA WATERS AS SEAS BUILD UPWARDS OF 6 FT
WITH THE GOOD ONSHORE FETCH LATE MONDAY.

&&

.CLIMATE...
RECORD MAX TEMPS...

WED 8/20...
KCHS 97 SET IN 2005 AND PREVIOUS
KCXM 99 SET IN 1902
KSAV 102 SET IN 1900

THU 8/21...
KCHS 100 SET IN 1983
KCXM 99 SET IN 1900
KSAV 101 SET IN 1902

FRI 8/22...
KCHS 100 SET IN 1983
KCXM 98 SET IN 1983
KSAV 100 SET IN 1983

LAST TIME 100 DEGREES OR HIGHER...
KCHS JUNE 21 2011
KCXM JUNE 20 2011
KSAV JULY 26 2012

&&

.EQUIPMENT...
THE CHARLESTON SC WSR-88D /KCLX/ REMAINS OUT OF SERVICE DUE TO A
SIGNIFICANT MECHANICAL FAILURE. REPAIR WORK CONTINUES AND THE
RADAR COULD BECOME OPERATIONAL BY LATE THIS WEEK OR EARLY NEXT
WEEK. UNTIL THE CHARLESTON SC RADAR RETURNS TO SERVICE...FOR
DOPPLER RADAR COVERAGE OF SOUTHEAST GEORGIA AND SOUTH COASTAL
SOUTH CAROLINA UTILIZE SURROUNDING RADARS INCLUDING KJAX...KJGX...
KVAX...KCAE AND KLTX.

&&

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

&&

$$

NEAR TERM...SPR
SHORT TERM...JAQ
LONG TERM...JAQ
AVIATION...JAQ/SPR
MARINE...JAQ/JRL/RJB
CLIMATE...
EQUIPMENT...






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