FXUS62 KGSP 251908
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
208 PM EST WED NOV 25 2009
.SYNOPSIS...
HIGH PRESSURE WILL GIVE AWAY TO AN ADVANCING COLD FRONT WHICH WILL
ARRIVE ON THURSDAY. HIGH PRESSURE WILL BE IN CONTROL DURING THE
WEEKEND AND INTO EARLY NEXT WEEK. ANOTHER COLD FRONT AND LOW
PRESSURE WILL ARRIVE MONDAY NIGHT AND TUESDAY.
&&
.NEAR TERM /THROUGH TONIGHT/...
A COLD FRONT WILL STALL AND WASH OUT JUST EAST OF THE
APPALACHIANS THIS EVENING...AS A LONGWAVE TROF IS REINFORCED BY
DIVING SHORTWAVE ENERGY INVOF NE/IA. SATELLITE IMAGERY SHOWS A
WEST-TO-EAST CLEARING OF THE STRATOCU...AS DRY WLY FLOW BEHIND THE
FRONT HELPS THE CLOUDS SCATTER OUT. OVERALL...IT SHOULD BE CLEAR
TONIGHT AND WINDS ARE EXPECTED TO DECOUPLE ACRS THE LOWER ELEVATIONS
OF THE CWFA. THE MAIN QUESTION IS HOW MUCH WILL THE BL DRY OUT
BEHIND THE FROPA TONIGHT...AND WILL IT BE ENUF TO PRECLUDE FOG
DEVELOPMENT. THE MAV/MET/LAV MOS GUIDANCE ARE IN DISAGREEMENT ON HOW
MUCH AND WHERE THE FOG WILL DEVELOP ACRS THE CWFA. THE NAM AND GFS
SEEM TO AGREE ON THE MOST PROLONGED DECOUPLING TO OCCUR IN THE MTN
VALLEYS AND ACROSS THE NC FOOTHILLS AND PIEDMONT. THE SREF MVFR VSBY
PROB ARE IN LINE WITH THIS...AND THAT WAS USED TO PAINT PATCHY FOG
IN THOSE AREAS. TEMPS WILL BE SEASONABLE TONIGHT...WITH MIN TEMPS
RANGING FROM THE LOWER 30S IN THE HIGH MTN VALLEYS...TO THE LOWER
40S OVR THE THERMAL BELT OF THE UPSTATE.
THANKSGIVING DAY...THE AFOREMENTIONED DIVING SHORTWAVE WILL ADVECT
ACRS THE CENTRAL-SOUTHERN APPALACHIANS DURING THE AFTN HOURS. AN
ASSOCIATED COLD FRONT ARRIVES JUST AHEAD OF THE UPPER
TROF...BRINGING WITH IT SOME LLVL MOISTURE. H85 WINDS REMAIN
RELATIVELY BACKED THRU THE AFTN (270-300 DEG)...AND ONLY 15-20 KTS.
SO THERE WILL BE LITTLE UPSLOPE FORCE SHOWERS. A SLGT CHC POP FOR
THE NRN MTNS STILL LOOKS GOOD. ANY SNOW MIXING IN WITH THE RAIN IS
NOT EXPECTED TO AMOUNT TO ANY SIGNIFICANT ACCUMULATION THRU 6 PM.
TEMPS WILL BE MAINLY IN THE 40S IN THE HIGH ELEVATIONS...50S IN THE
VALLEYS...AND LOWER 60S ACROSS THE PIEDMONT.
&&
.SHORT TERM /THURSDAY/...
AS OF 130 PM WED...MAIN WEATHER IMPACT IN THE SHORT TERM WILL BE A
RATHER QUICK HITTING NW FLOW SNOW EVENT FOR THE NC MOUNTAINS THU
NIGHT INTO EARLY FRI. 12Z MODEL RUNS OFFER NO SURPRISES...AS A DEEP
UPPER LOW IS FORECAST TO MOVE THROUGH THE SOUTHERN GREAT LAKES THU
EVENING/NIGHT...WITH THE UPPER TROUGH AXIS MOVING ACROSS THE NC
MOUNTAINS IN THE 00-03Z FRI TIMEFRAME. BEST DPVA IS A LITTLE AHEAD OF
THE DEEPER MOISTURE AND BEST UPSLOPE...BUT NEVERTHELESS THERE SHOULD
BE SUFFICIENT SUPPORT TO GENERATE 1-2 INCHES OF SNOW ACROSS THE WEST
FACING SLOPES NEAR THE TENNESSEE LINE THU NIGHT INTO EARLY FRI. SNOW
LEVELS WILL QUICKLY DROP THU EVENING FROM AROUND 3500FT TO THE
VALLEY FLOORS BY MIDNIGHT. WITH GOOD COLD ADVECTION AND TIGHTENING
PRESSURE GRADIENT...EXPECT GUSTY NW WINDS WILL ALSO DEVELOP. GUSTS
TO 30-40 MPH ARE A GOOD BET IN THE MOUNTAINS ABOVE 3500FT AND 20-30
MPH BELOW. EXPECT THAT THE OVERNIGHT CREW WILL HOIST A WINTER
WEATHER ADVISORY FOR THU NIGHT FOR THE COUNTIES ADJACENT TO THE TN
LINE TO ADDRESS THE SNOW AND WIND SITUATION. DEPTH OF MOISTURE QUICKLY
ERODES BY FRI AFTERNOON...SO EXPECT THAT SNOW SHOWERS WILL DIMINISH
TO FLURRIES BY EARLY FRI AFTERNOON IN THE UPSLOPE AREAS AND END
CERTAINLY BY LATE AFTERNOON. OUTSIDE OF THE MOUNTAINS...DOWNSLOPE
FLOW WILL PREVAIL WITH GENERALLY CLEAR CONDITIONS ALONG WITH COLDER
TEMPERATURES THU NIGHT. SOME GUSTY WINDS ON FRI WILL MAKE FOR A
CHILLY DAY. HIGH PRESSURE WILL BUILD OVER THE REGION FRI NIGHT AND
CREST OVER THE AREA ON SAT. EXPECT GENRALLY CLEAR CONDITIONS TO
PREVAIL THROUGH SAT NIGHT WITH SOME MODERATION IN TEMPERATURES FOR
SAT. USED A MOS BLEND FOR TEMPS THROUGH FRI NIGHT. HOWEVER...I
UNDERCUT GMOS/MEX BY ABOUT A CATEGORY ON SAT PER 1000-850 THICKNESS
SCHEME. USED HPC GUIDANCE FOR SAT NIGHT TEMPS.
&&
.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
AS OF 200 PM WEDNESDAY...THE 12 UTC GFS AND LAST NIGHT/S RUN OF THE
ECMWF BOTH DEVELOP A SPLIT FLOW PATTERN OVER THE CONUS EARLY NEXT
WEEK. HPC FAVORS THE EC SOLUTION WHICH IS WETTER OVER OUR AREA
THROUGH MUCH OF THE EXTENDED. THE EC HAS MORE PHASING BETWEEN THE
STREAMS AS IT SLOWLY MOVES THE POSITIVELY TILTED TROUGH INTO THE ERN
US. THIS RESULTS IN LARGE BAND OF PCPN THAT CROSSES THE REGION MON
AND MON NGHT. THE GFS HAS A MORE PROGRESSIVE NRN STREAM TROUGH AND
CONSIDERABLY MORE SHORTWAVE RIDGING BETWEEN IT AND THE SRN STREAM
CUT OFF LOW. THIS RESULTS IN A DRIER PATTERN AND A FASTER MOVING
INITIAL AREA OF PCPN. RIGHT NOW ALL THE PCPN LOOKS TO BE LIQUID
DURING THIS TIME PERIOD.
THE TRAILING SRN STREAM LOW WILL THEN EJECT QUICKLY TO THE NE IN
RESPONSE TO A BOAT-LOAD OF ENERGY FALLING INTO THE NRN TIER OF THE
US LATER TUE INTO WED. AS THIS SRN STREAM LOW LIFTS TO THE
NORTH...PCPN SHOULD REDEVELOP AHEAD OF IT. THE RETREATING NRN STREAM
TROUGH WILL BRING HIGH PRESSURE INTO THE REGION AHEAD OF THE
LOW...AND THIS LOOKS LIKE A GOOD COLD AIR DAMMING SETUP.
ATTM...WHILE THERE SHOULD BE A PLENTY OF PCPN...THE PREDOMINATE
P-TYPE STILL LOOKS TO BE LIQUID. IT/S POSSIBLE...HOWEVER...THAT A
STRONGER RIDGE MAY RESULT OF PARTIAL THICKNESSES LOW ENOUGH FOR SOME
TYPE OF MIX OVER PARTS OF THE NC FOOTHILLS AND MTNS. IT/S A LONG
WAYS OFF...BUT IT/S CERTAINLY SOMETHING TO KEEP AN EYE ON.
&&
.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
AT KCLT...SATELLITE IMAGERY AND METARS TO THE WEST OF THE TERMINAL
INDICATE THAT THE MVFR CIGS SHOULD SCATTER OUT BY 19Z...LEAVING SOME
LINGERING LIFTING CUMULUS AND BKN CIRRUS. WINDS WILL REMAIN LIGHT
OUT OF THE SW THRU THIS EVENING...AND WILL LIKELY GO CALM OVERNIGHT.
GUIDANCE IS IN DISAGREEMENT ON WHETHER THERE WILL BE GOOD ENUF
CONDITIONS TO SUPPORT FOG DEVELOPMENT TOWARD DAYBREAK. FOR NOW...I
HAVE ADDED A 6SM BR MENTION TO REFLECT A POTENTIAL IMPACT.
OTHERWISE...VFR WITH A LIGHT NW WIND AFTER SUNRISE. WIND WILL SHIFT
BACK TO SW BY AROUND 18Z THU.
ELSEWHERE...STRATOCU IS SCATTERING OUT ALONG A LINE EXTENDING FROM
KHKY...KEHO...KSPA. SO VFR CONDITIONS ARE EXPECTED THE REST OF THE
AFTN AND INTO TONIGHT. BASED ON THE MODEL CONSENSUS...KAVL AND KHKY
HAVE THE HIGHEST PROB OF FOG DEVELOPMENT LATE TONIGHT. SOME OF THE
GUIDANCE DOES HAVE FOG DEVELOPMENT ACROSS THE UPSTATE. BUT WINDS ARE
NOT EXPECTED TO DECOUPLE THERE. ANY FOG THAT DEVELOPS SHOULD QUICKLY
BURN OFF BY LATE THU MORNING.
OUTLOOK...ANOTHER COLD FRONT WILL BRING BREEZY NORTHWESTERLY WINDS
ON THURSDAY NIGHT AND FRIDAY. THERE COULD BE BRIEF PERIODS OF
RESTRICTIONS IN SHOWERS AT KAVL...MAINLY EARLY FRIDAY MORNING.
OTHERWISE...VFR THROUGH SUNDAY.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
NC...NONE.
SC...NONE.
&&
$$
SYNOPSIS...DEO
NEAR TERM...ARK
SHORT TERM...LG
LONG TERM...MCAVOY
AVIATION...ARK
