Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 210512
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
112 AM EDT SUN SEP 21 2014

.SYNOPSIS...
A WEAK RIDGE OF HIGH PRESSURE WILL LINGER ATOP THE REGION THROUGH
TONIGHT. TEMPERATURES WILL WARM ON SUNDAY BEFORE A COLD FRONT
CROSSES THE CAROLINAS LATER THAT NIGHT. COOL AND DRY HIGH PRESSURE
WILL THEN SPREAD ACROSS THE REGION AND REMAIN IN PLACE THROUGH AT
LEAST MID WEEK.

&&

.NEAR TERM /THROUGH TODAY/...
0515 UTC UPDATE...SKY COVER HAS BEEN ADJUSTED PER INFRARED
SATELLITE IMAGERY...RESULTING IN MORE UPPER LEVEL CLOUD COVER OVER
THE INTERSTATE 77 CORRIDOR. WINDS WERE UPDATED WITH A BLEND OF THE
LATEST NAM AND ADJMET...FAVORING THE FORMER BY TWO TO ONE FOR
GREATER WIND GUST POTENTIAL AT HIGHER ELEVATIONS AFTER DAWN.

AS OF 930 PM...FCST STILL IN REALLY GOOD SHAPE. THE HIGH CLOUD
SHIELD ON THE WRN PERIPHERY OF THE COASTAL LOW WILL CONTINUE TO
SPREAD NW AND STREAM OVERHEAD ACROSS THE PIEDMONT OF THE CAROLINAS
THRU THE EARLY MORNING HOURS. THIS SHOULD HOPEFULLY THIN OUT BY THE
EARLY MORNING HOURS AND NOT AFFECT MIN TEMPS...YET. AS IT IS...TEMPS
WERE RUNNING A BIT WARMER THAN FCST IN SOME PLACES OVER THE NC
PIEDMONT...AND COOLER THAN FCST WHERE IT REMAINS CLEAR. NO CHANGES.

REMAINDER OF PREVIOUS DISCUSSION...

GUIDANCE SUGGESTS A GOOD NITE FOR AT LEAST PATCHY DENSE MTN VALLEY
FOG... WHILE MOST AREAS OUTSIDE OF THE MTNS GENERALLY TOO DRY. SKIES
SHOULD BE MOSTLY CLEAR WITH LOWS NEAR TO SLIGHTLY BELOW NORMAL.

AN UPPER TROF DIGS TOWARD THE AREA SUNDAY PUSHING A FRONT INTO THE
MTNS BY THE END OF THE PERIOD. LITTLE IN THE WAY OF PRECIP IS
EXPECTED UNTIL VERY LATE IN THE PERIOD...THEN ONLY ACROSS THE MTNS
WHERE MOISTURE...FORCING AND INSTABILITY WILL BE BETTER. LOW LEVEL
FLOW REMAINS WLY KEEPING AREAS OUTSIDE OF THE MTNS DRIER WITH A LOW
LEVEL WARM NOSE KEEPING THE ATMOS GENERALLY CAPPED WITH ONLY WEAK
ELEVATED INSTABILITY AT BEST. BRING CHC POP INTO THE TN BORDER
COUNTIES BY LATE AFTERNOON...WITH SLIGHT CHC ACROSS THE REST OF THE
MTNS...AND A DRY FCST ELSEWHERE. CANNOT RULE OUT AN ISOLATED SEVERE
WIND GUST ACROSS THE MTNS GIVEN THE INCREASING SHEAR AND DRY MID
LEVELS...BUT CHC IS LOW. HIGHS WILL BE 5 TO 10 DEGREES ABOVE NORMAL
AS THICKNESSES INCREASE SIGNIFICANTLY AHEAD OF THE FRONT ALONG WITH
THE INCREASING WLY LOW LEVEL FLOW.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...
AS OF 145 PM EDT SATURDAY...THE SHORT TERM FORECAST PERIOD
INITIALIZES SUNDAY EVENING AMIDST AN UPPER TROF EJECTING TO THE EAST
ACROSS THE GREAT LAKES REGION WITH A WEAK SURFACE LOW SLIDING
THROUGH SOUTHERN ONTARIO.  A COLD FRONT WILL BE DRAPED FROM THIS
PARENT SURFACE LOW SOUTHWEST ALONG THE APPALACHIAN MOUNTAINS.
CLOSER TO HOME...A WEAK UPPER/SURFACE LOW OFF THE CAROLINA COASTLINE
WILL MIGRATE NORTHWARD WITH NO SIGNIFICANT IMPACTS EXPECTED.  THERE
REMAINS SOME UNCERTAINTY REGARDING THE AMOUNT OF ONGOING CONVECTION
AT FORECAST INITIALIZATION.  THE USUALLY MORE AGGRESSIVE/UNSTABLE
NAM INDICATES NEARLY 1000J/KG SBCAPE ACROSS THE WARM SECTOR WITH
CONVECTION SPREAD OUT ACROSS THE HIGH TERRAIN AND NC
PIEDMONT...LIKELY TIED TO THE PREFRONTAL SURFACE TROF AS A FOCUSING
MECHANISM.  THE GFS/ECMWF FAVOR A MORE STABLE SOLUTION WITH LITTLE
TO NO PREFRONTAL TROF FOCUSED ACTIVITY.  ALTHOUGH SHEAR WILL BE
INCREASING...THE BEST UPPER JET DYNAMICS WILL PASS BY TO THE
NORTH...THEREFORE DECIDED TO LEAN TOWARD THE MORE SUBTLE SOLUTIONS
OF THE GFS/ECMWF.  THE FRONT WILL SWEEP THROUGH THE REGION DURING
THE OVERNIGHT HOURS WITH OVERALL CONVECTIVE COVERAGE EXPECTED TO
DISSIPATE WITH LOSS OF HEATING.  THE FRONT LOOKS TO CLEAR THE
FORECAST AREA BY NLT NOON ON MONDAY BEFORE ANY DIURNAL HEATING
INDUCED INSTABILITY CAN PLAY A FACTOR.

BEHIND THE FRONT ON MONDAY AFTERNOON...UPPER TROF AXIS AND
ASSOCIATED VORTICITY MAXIMA WILL SLIDE THROUGH THE SOUTHERN/CENTRAL
APPS.  MODERATE TO STRONG NORTHWEST FLOW THROUGH THE PROFILE WILL
LEAD TO THE POSSIBILITY OF A FEW UPSLOPE INDUCED SHOWERS ALONG THE
SPINE OF THE APPS.  THUS THE FORECAST WILL FEATURE CHANCE LEVEL POPS
ACROSS THE HIGH TERRAIN SUNDAY NIGHT TAPERING TO SLIGHT CHANCE POPS
ACROSS THE REMAINDER OF THE FORECAST AREA THROUGH MONDAY MORNING TO
ACCOUNT FOR PASSAGE OF THE FRONT AND WEAK NORTHWEST UPSLOPE FLOW
SHOWERS ALONG THE TN LINE.  THE ABOVE MENTIONED UPPER FEATURES WILL
SLIDE EAST LATE IN THE DAY MAKING WAY FOR RISING HEIGHTS AND
CANADIAN HIGH PRESSURE AT THE SURFACE THAT WILL LAST THROUGH THE
PERIOD.  THIS WILL YIELD MORE FALL LIKE CONDITIONS WITH TEMPERATURES
GENERALLY 8-10 DEGREES BELOW NORMAL AMONGST DRIER NORTHERLY FLOW.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
AS OF 2 PM SATURDAY...THE MEDIUM RANGE FCST PICKS UP AT 00Z ON
WEDNESDAY WITH HEIGHTS STEADILY RISING AS A BROAD AND STEEP UPPER
RIDGE SPREADS ACROSS THE REGION. THIS IMPRESSIVE RIDGE IS EXPECTED
TO REMAIN IN PLACE THRU NEW DAY 7 OVER THE EASTERN HALF OF THE CONUS
WHILE ANOTHER STEEP UPPER TROF DIGS DOWN OVER THE WEST COAST BY
THE END OF THE MEDIUM RANGE.

AT THE SFC...STRONG CANADIAN HIGH PRESSURE WILL BE CENTERED TO OUR
NORTH WITH COOL AND DRY AIR OVER THE FCST AREA. OVER THE NEXT COUPLE
OF DAYS...THE LONG RANGE GUIDANCE SUGGESTS THAT THIS HIGH WILL
GRADUALLY DRIFT EASTWARD WITH SOME DEEPER LYR MOISTURE ENCROACHING
UPON THE CWFA FROM THE SOUTH AND EAST. THE MODELS DIFFER WITH
REGARDS TO HOW FAR NW THIS DEEPER MOISTURE WILL EXPAND. THE ECMWF
CONTINUES TO SHOW GREATER COVERAGE OVER THE CWFA ON FRI AND MOST OF
SAT WHILE THE NEWER RUN OF THE GFS KEEPS THE MOISTURE PLUME MORE TO
THE SOUTH ON FRI AND THEN EXPANDS IT FARTHER NORTH ON SAT.
INTERESTINGLY...THE LATEST 12Z RUN OF THE CMC MODEL KEEPS THE DEEPER
MOISTURE EVEN FARTHER SOUTH THAN THE GFS THRU ITS FCST ENDING TIME
OF 12Z FRI. BASED ON THE MIXED MODEL SPREAD TOWARDS THE END OF THE
FCST PERIOD...I DID NOT MAKE ANY MAJOR CHANGES TO THE SENSIBLE FCST.
I GRADUALLY INCREASED POPS TO SLIGHT CHANCE DURING THE LATTER PART
OF THURS AND KEEP THEM THERE FOR FRI. I DID INCLUDE AN AREA OF SOLID
CHANCE VALUES OVER THE FAR SOUTHERN ZONES FOR SAT AFTERNOON INTO SAT
EVENING...OTHERWISE MOST OTHER AREAS STILL ONLY CARRY SLIGHT CHANCE.
TEMPS WERE CHANGED LITTLE WITH VALUES REMAINING A FEW DEGREES BELOW
CLIMATOLOGY FOR LATE SEPT.

&&

.AVIATION /05Z SUNDAY THROUGH THURSDAY/...
AT KCLT...VFR. HIGH CLOUDS ASSOCIATED WITH A COASTAL LOW ARE PASSING
OVER THE FIELD...CREATING A CEILING JUST OT THE EAST. WINDS...DEW
POINT DEPRESSIONS AND GUIDANCE ARE NOT ESPECIALLY FAVORABLE FOR
DAYBREAK FOG. CLOUD COVER WILL INCREASE THIS AFTERNOON AND EVENING
AHEAD OF AN APPROACHING COLD FRONT...BUT CONFIDENCE IS LOW FOR
ENOUGH CLOUDS SURVIVING TO CREATE A LOW VFR CIG...SO A HIGH LEVEL
CIG WILL BE CARRIED INSTEAD. THE CHANCE OF CONVECTION IS NOT ENOUGH
FOR A MENTION. LIGHT N WINDS WILL BACK TO SW AND INCREASE TODAY.
WINDS WILL STAY UP AHEAD OF THE FRONT THIS EVENING.

FOOTHILLS SITES EXCEPT KAVL...VFR. HIGH LEVEL CIRRUS RELATED TO A
COASTAL LOW SHOULD REMAIN EAST OF THE FOOTHILLS. GUIDANCE DOES NOT
FAVOR MORNING FOG RESTRICTIONS...DESPITE LIGHT WINDS AND LOW DEW
POINT DEPRESSIONS. A LOW VFR VSBY IN FOG WILL BE CARRIED AT MOST
SITES AROUND DAWN. CLOUDS WILL INCREASE FROM THE WEST LATER TODAY
AHEAD OF A COLD FRONT...BUT AT THIS TIME IT APPEARS LOW CLOUD WILL
BE TOO LIMITED EAST OTHER MOUNTAINS FOR A LOW VFR CIG...AND
OVERLYING CIRRUS WILL FORM A HIGH LEVEL CIG INSTEAD. CONVECTIVE
CHANCES WILL BE TOO LOW TO MENTION. LIGHT NW WINDS WILL BACK TO SW
AND INCREASE TODAY...REMAINING UP AHEAD OF THE FRONT.

AT KAVL...GUIDANCE SUPPORTS A DAYBREAK FOG RESTRICTION...BUT
CONFIDENCE WAS NOT GREAT ENOUGH YET TO GO VLIFR...SO IFR WILL BE
CARRIED FOR NOW. FOG SHOULD BURN OFF QUICKLY AFTER DAWN. AN
APPROACHING COLD FRONT WILL BRING INCREASING CLOUDS AND
PRECIPITATION...BUT CONFIDENCE IS TOO LOW FOR RESTRICTIONS. NW WIND
WILL INCREASE TODAY AND STAY UP AS THE FRONT ARRIVES.

OUTLOOK...HIGH PRESSURE BUILDS BACK IN EARLY NEXT WEEK WITH VFR
CONDITIONS. MOISTURE MAY RETURN FROM THE COAST BY LATE IN THE WEEK.

CONFIDENCE TABLE...

            05-11Z        11-17Z        17-23Z        23-05Z
KCLT       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAVL       MED   79%     MED   67%     HIGH 100%     HIGH 100%
KHKY       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGMU       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAND       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%

THE PERCENTAGE REFLECTS THE NUMBER OF GUIDANCE MEMBERS AGREEING
WITH THE SCHEDULED TAF ISSUANCE FLIGHT RULE CATEGORY. COMPLETE
HOURLY EXPERIMENTAL AVIATION FORECAST CONSISTENCY TABLES AND ENSEMBLE
FORECASTS ARE AVAILABLE AT THE FOLLOWING LINK: (MUST BE LOWER CASE)

WWW.WEATHER.GOV/GSP/AVIATION_TABLES

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
NC...NONE.
SC...NONE.

&&

$$
SYNOPSIS...JPT
NEAR TERM...JAT/PM/RWH
SHORT TERM...CDG
LONG TERM...JPT
AVIATION...JAT





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