761
FXUS62 KGSP 091150
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
650 AM EST TUE FEB 9 2010
SYNOPSIS
A MOIST LOW PRESSURE SYSTEM WILL MOVE ACROSS THE REGION TODAY AND
OFF THE MID ATLANTIC COAST TONIGHT AS A STRONG AND COLD NORTHWEST
FLOW REMAINS INTO WEDNESDAY. COOLER HIGH PRESSURE WILL THEN BUILD
OVER THE CAROLINAS AND NORTHEAST GEORGIA FOR THE REMAINDER OF THE
WEEK AS ANOTHER LOW CROSSES FLORIDA TO START THE WEEKEND.
NEAR TERM /THROUGH TONIGHT/
WATER VAPOR IMAGERY SHOWS A DEEP VORTEX SINKING SWD ALONG THE NE/IA
BORDER EARLY THIS MORNING. A STRONG MOISTURE FETCH HAS DEVELOPED
AHEAD OF THIS FEATURE FROM THE CENTRAL GULF COAST THROUGH MIDDLE TN.
RADAR COVERAGE HAS BEEN STEADILY INCREASING THIS MORNING...AND LIGHT
RAIN AND SNOW ARE STARTING TO APPEAR IN OBS ACROSS ERN TN AND IN THE
NRN NC MTNS. PRECIP WILL STEADILY APPEAR IN THE LOWER MTN VALLEYS
THROUGH DAYBREAK AND THEN SPREAD EASTWARD AS UPPER DIVERGENCE AND
ISENTROPIC LIFT INCREASE THROUGH MIDDAY...WITH BETTER DYNAMICS
ARRIVING LATER THIS AFTN. THE MAIN SFC WAVE WILL MOVE WEST OF THE
SRN APPALACHIANS TODAY...AS A SECONDARY LOW DEVELOPS ALONG THE SE
COASTLINE.
MODEL SOUNDINGS ARE A BIT WARMER WITH THE LATEST FORECAST
CYCLE...WHICH WILL MAKE PRECIPITATION TYPE MORE QUESTIONABLE TODAY.
MAINLY SNOW...MIXED WITH SOME SLEET AT TIMES...IS EXPECTED ACROSS
MUCH OF THE NC MTNS AND WRN FOOTHILLS. ALTHOUGH WARNING CRITERIA
SNOWFALL MAY ONLY BE REACHED ACROSS THE NRN MTNS AND HIGHER TERRAIN
OF THE SRN AND CENTRAL MTNS THROUGH THE NEAR TERM...THE EXISTING
WARNING WILL BE MAINTAINED AS A PRELUDE TO THE UPCOMING NORTHWEST
FLOW SNOWFALL. ELSEWHERE...MORE OF A WINTRY MIXED BAG IS EXPECTED
FARTHER EAST ALONG THE INTERSTATE 40 CORRIDOR WHERE MID LEVEL WARM
NOSING WILL COMBINE WITH COLD SFC TEMPS IN WEAK CAD TO CREATE LIGHT
ICING CONDITIONS. ADVISORY CONDITIONS MAY BE MARGINAL FOR PARTS OF
THE NE GA MTNS AND UPSTATE SC MTNS. LIGHT ACCUMULATING SNOW MIXED
WITH SLEET MAY STILL BE SUFFICIENT TO KEEP THE MENTION GOING
EVERYWHERE EXCEPT HABERSHAM COUNTY.
THE DEEP UPPER LOW WILL MOVE INTO THE UPPER OH RIVER VALLEY
TONIGHT...DRAGGING CONTINUED STRONG DPVA ACROSS THE NRN TIER OF THE
FORECAST AREA THIS EVENING. HOWEVER...A DEEP MID LEVEL DRY SLOT WILL
WRAP INTO THE AREA QUICKLY FROM THE SW THROUGH THE EVENING
HOURS...ALONG WITH THE COLD FROPA...TO LARGELY SHUT PRECIPITATION
OFF IN MOST AREAS. NW FLOW SNOW WILL INCREASE DURING THE OVERNIGHT
HOURS NEAR THE TN BORDER WITH VERY WINDY CONDITIONS DEVELOPING
RAPIDLY TOWARD DAYBREAK ACROSS THE HIGH TERRAIN.
SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/
AS OF 315 AM TUESDAY...LOW PRESSURE WILL RAPIDLY DEEPEN OFF THE MID
ATLANTIC COAST EARLY WEDNESDAY. CIRCULATION AROUND THE LOW WILL
DRIVE A STRONG COLD FRONT EAST OF THE CAROLINA COAST BY DAY BREAK.
WEST OF THE FRONT...SHORT RANGE MODELS AGREE WELL WITH THE TIMING
AND PLACEMENT OF THE BULLS EYE OF BITTERLY COLD H85 TEMPERATURES
OVER THE SOUTHERN APPALACHIAN REGION 12Z WED. H85 TEMPS WILL LIKELY
BEGIN THE DAY -10C ACROSS THE PIEDMONT TO -16C ACROSS THE NRN
MTNS...RESULTING IN TEMPS WELL BELOW NORMAL ON WED. STRONG
WEST-NORTHWEST FLOW ACROSS THE MTNS WILL VEER THROUGHOUT THE
DAY...BECOMING NORTHWEST BY MID DAY. BOTH THE NAM AND GFS SHOW H85
WINDS 50 TO 60 KTS ACROSS THE CWA...ESPECIALLY ACROSS THE NC MTNS
AND ADJACENT FOOTHILLS. WRAP AROUND MOISTURE AND STRONG LLVL
NORTHWEST WINDS WILL INCREASE OVER THE NC MTNS WEDNESDAY
MORNING...PEAKING DURING THE LATE AFTERNOON AND EVENING...THEN BEGIN TO
WEAKEN OVERNIGHT. NORTHWEST FLOW SNOW SHOWERS COVERAGE WILL REMAIN
CATEGORICAL THROUGH THE DAY OVER THE TN BORDER COUNTIES. EAST OF THE
MTNS...DOWNSLOPE FLOW WILL LIMIT THE EASTWARD TRACK OF THE SHSN TO
THE ADJACENT FOOTHILLS. WED NIGHT...MOIST AND COLD NORTHWEST WINDS
WILL CONTINUE TO SUPPORT NUM TO SCT SHSN. QPF WILL REMAIN LIMITED TO
THE MTNS...RANGING FROM A FEW HUNDREDTHS ACROSS THE EAST FACING
SLOPES TO .15 TO .2 OF AN INCH ACROSS THE TN BORDER COUNTIES. THE
QPF WILL FALL ENTIRELY IN THE FORM OF SNOW.
DUE TO THE STRONG GUSTY CONDITIONS...I WILL KEEP THE SNOW RATIO
LIMITED TO 15:1 ACROSS THE MTNS THROUGH WED NIGHT. THIS APPROACH
YIELDS 2 TO 4 INCHES ACROSS THE NRN MTNS AND 1 TO 2 ACROSS THE REST
OF THE TN BORDER COUNTIES. AREAS WITHIN THE MTN VALLEYS WILL SEE
AMOUNTS RANGE FROM .5 TO 1 INCH. THE CURRENT WINTER STORM WARNING
LOOKS GOOD. EAST OF THE WARNING...SNOW SHOWER WILL TRACK
EAST...LIKELY NOT SURVIVING THE DOWNSLOPE FLOW IN THE LEE OF THE
MTNS. UPSLOPE MOISTURE WILL BECOME LIMITED LATE WED NIGHT...ENDING
SHOWER ACTIVITY.
THE OTHER MAIN FORECAST CHALLENGE WILL BE THE WIND FORECAST FOR
WEDNESDAY AND WEDNESDAY NIGHT. FORECAST SOUNDINGS INDICATE THAT THE
MIXING HEIGHT WILL PEAK DURING THE MID AFTERNOON...WITH MIXING FROM
THE SFC TO AROUND 5 KFT. WINDS AT THE TOP OF THE MIXED LAYER RANGE
FROM 50 TO 65 KTS. MOMENTUM TRANSFER SHOULD EASILY OCCUR WITH DRY
ADIABATIC LAPSE RATES AND DOWNSLOPE FLOW EAST OF THE HIGH TERRAIN.
PRESSURE GRADIENT WILL RANGE FROM 13 TO 16 MBS...SUPPORTING
SUSTAINED WINDS OF 30 TO 40 MPH...ESPECIALLY ACROSS THE MTNS. PEAK
GUSTS AROUND 60 MPH APPEAR POSSIBLE ANYWHERE ACROSS THE CWA.
HOWEVER...I WILL ISSUE THE WATCH FROM 6 AM WED UNTIL 6 AM THURS FOR
ALL ZONES...SAVE THE EASTERN PIEDMONT OF SC AND GA. IT IS ASSUMED
THAT SOIL CONDITIONS WILL REMAIN FAIRLY UNSTABLE ON WED AFTER
TODAY/S PRECIPITATION AND TONIGHT/S HARD FREEZE. THE SOIL CONDITIONS
COMBINED WITH TREE DAMAGE FROM RECENT WINTER STORMS MAY PROVIDE
ADDITIONAL POTENTIAL FOR WIND DAMAGE. IN ADDITION...GUSTY WINDS WILL
RESULT IN AREAS OF BLOWING SNOW ACROSS THE HIGH TERRAIN AND WIND
CHILL VALUES SLIGHTLY BELOW ZERO ON WEDS.
THURSDAY AND THURSDAY NIGHT...DRY HIGH PRESSURE WITH ZONAL FLOW
SHOULD PROVIDE MOSTLY CLEAR CONDITIONS. TEMPERATURES WILL REMAIN
BELOW NORMAL.
LONG TERM /FRIDAY THROUGH MONDAY/
AS OF 245 AM TUESDAY...TO START THE MEDIUM RANGE ON FRIDAY...THE
SOUTHERN STREAM LOW IS SHOWN BY THE GFS TO PASS WELL SOUTH OF OUR
FORECAST AREA WHILE OTHER MODELS BRING IT MORE NORTH. THE NEW ECMWF
NOW TAKES THE PRECIP SHIELD INTO THE ASHEVILLE AREA AND IS NOW IN
CLOSE AGREEMENT WITH THE GEM. THE SREF ENDS AT 12Z FRIDAY...BUT
FOLLOWING CLOSELY THE TRACK OF THE GEM FOR THIS LOW. THE MODELS
TEND TO AGREE ON THE MAIN EFFECTS OF THIS LOW...ALTHOUGH GENERALLY
ACROSS NE GEORGIA AND UPSTATE...BEING FRIDAY NIGHT CENTERED AROUND
06Z. ONLY CARRYING SLIGHT CHANCES. BASED ON THE LOW THICKNESSES OF
THE MODELS BEING PERSISTENT WITH THE 500MB TROUGH AND THE POSITION
OF THE SURFACE HIGH BRINGING COLD AIR ADVECTION THROUGH A LONG
PERIOD...GOING COLDER THAN THE CURRENT GUIDANCE SEEMS THE RIGHT
THING TO DO.
THE MODELS ARE NOT CONSISTENT ON NORTHWEST FLOW PRECIP FOR DAYS 6
AND 7. HOWEVER...THERE IS ENOUGH AGREEMENT IN PRINCIPLE TO KEEP THE
PREVIOUS PROJECTION OF SNOW SHOWERS ALONG THE TENNESSEE BORDER FROM
SATURDAY NIGHT THROUGH MONDAY NIGHT. SHORTWAVES ROTATE AROUND THE
TROUGH BUT TIMING IS DIFFERENT WITH EACH MODEL.
AVIATION /12Z TUESDAY THROUGH SATURDAY/
AT KCLT...PRECIPITATION AND THE ONSET OF DEEPER MOISTURE HAVE BEEN
SLOWER TO DEVELOP THAN PREVIOUSLY ANTICIPATED. WILL STILL FORECAST A
STEADY DECLINE IN CIGS THROUGH THE MVFR RANGE THIS MORNING...WITH
IFR NOW EXPECTED ALONG WITH THE BEST RAINFALL CIRCA 20Z. WINDS WILL
BE MAINLY NE THROUGH THE DAY...EXCEPT FOR SOME ADJUSTMENT TO SE LATE
IN THE AFTN AS WEAK COLD AIR DAMMING RELAXES. A SWIFT TURN TO
WESTERLY FLOW IS EXPECTED AFTER 01Z WITH GUSTS DEVELOPING IN THE
POST FROPA COLD ADVECTION BY 06Z.
AT KAVL...CIGS WILL LOWER THROUGH THE MVFR RANGE THIS MORNING TO
REACH IFR CONDITIONS CIRCA 16Z AS STEADIER SNOW AND SLEET DEVELOPS.
WARMING PROFILES WILL ALLOW A MIXOVER TO RAIN THROUGH THE DAY. ANY
LINGERING PRECIPITATION THROUGH THE NIGHT WOULD SWITCH BACK TO SNOW
SHOWERS AS STRONG NORTHWEST FLOW DEVELOPS IN THE COLD ADVECTION.
WILL FEATURE GUSTS TO 28 KT FOR NOW...BUT CONSIDERABLY STRONGER
GUSTS COULD DEVELOP TOWARD 12Z WED.
ELSEWHERE...VFR CIGS WILL LOWER TO MVFR AROUND 15Z THIS MORNING AS
MOIST UPGLIDE DEVELOPS. IFR CIGS SHOULD STILL DEVELOP AFTER 18Z AS
THE DEEPER MOISTURE AND FORCING ARRIVES. PRECIPITATION SHOULD REMAIN
LIQUID AT ALL FOOTHILL/PIEDMONT TAF SITES EXCEPT KHKY...WHERE SN/PL
EARLY WILL TRANSITION TO RA/PL THIS AFTERNOON. WINDS WILL TURN
SHARPLY WESTERLY WITH FROPA THIS EVENING...WITH GUSTS DEVELOPING
OVERNIGHT...AS LOW CLOUDS SCATTER OUT.
OUTLOOK...LOW PRESSURE WILL PULL AWAY TO THE NE WED THROUGH WED
NIGHT. VERY WINDY CONDITIONS WILL DEVELOP IN THE NW FLOW ON
WED...WITH LINGERING SNOW SHOWERS AND ASSOCIATED RESTRICTIONS
POSSIBLE AT KAVL. DRIER HIGH PRESSURE...WITH WEAKENING WINDS...WILL
DEVELOP THROUGH THE REMAINDER OF THE OUTLOOK PERIOD.
GSP WATCHES/WARNINGS/ADVISORIES
GA...WINTER WEATHER ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR GAZ010.
HIGH WIND WATCH FROM WEDNESDAY MORNING THROUGH LATE WEDNESDAY
NIGHT FOR GAZ010-017-018-026-028.
NC...WINTER STORM WARNING UNTIL 7 AM EST THURSDAY FOR NCZ033-
048>053-059-501-503-505.
WINTER WEATHER ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR
NCZ035>037-056>058-062>065-502-504-506>510.
HIGH WIND WATCH FROM WEDNESDAY MORNING THROUGH LATE WEDNESDAY
NIGHT FOR NCZ035>037-056>058-062>065-068>072-082-502-504-
506>510.
SC...WINTER WEATHER ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR
SCZ001>003.
HIGH WIND WATCH FROM WEDNESDAY MORNING THROUGH LATE WEDNESDAY
NIGHT FOR SCZ001>010.
SYNOPSIS...DEO
NEAR TERM...HG
SHORT TERM...NED
LONG TERM...DEO
AVIATION...HG
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