FXUS62 KGSP 241530
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
1030 AM EST THU DEC 24 2009

.SYNOPSIS...
HIGH PRESSURE WILL CONTINUE TO RIDGE DOWN THE EASTERN SEABOARD TODAY 
AS A VIGOROUS LOW PRESSURE SYSTEM DEVELOPS ACROSS THE MISSISSIPPI 
VALLEY. A COLD FRONT WILL THEN MOVE ACROSS THE WESTERN CAROLINAS AND 
NORTHEAST GEORGIA ON FRIDAY WITH A WEAK AREA OF HIGH PRESSURE 
BUILDING IN BEHIND IT FOR THE WEEKEND.

&&

.NEAR TERM /THROUGH TONIGHT/...
INCREASING WAA FLOW AND THE INFLUX OF ATLANTIC MOISTURE WILL
INCREASE LOWER CLOUDS LATER THIS AFTERNOON...BUT UNTIL THEN JUST
A NOMINAL INCREASE IN HIGHER LEVEL CLOUDS. HAVE NUDGED MAXES UPWARD
ACRS THE PIEDMONT AWAY FROM CORE OF DEVELOPING SFC WEDGE. NOT
SURPRISINGLY...THE 12 UTC OP NAM IS STILL DRY THRU SUNSET AND
THE 09 UTC SREF PROGGING SLIGHT CHANCE LATE DAY POPS ACRS THE
SW UPSLOPE AREAS. 

A MAJOR STORM SYSTEM TO DEEPEN INVOF THE ARKLATEX...THEN SLOWLY 
TURNS NWD TO A POSITION INVOF THE QUAD CITIES IA/IL BY 12Z FRI. A 
CLASSIC COMMA HEAD CONFIGURATION IN LAYER RH FIELDS IN ALL THE 
MODELS...WITH A DRY SLOT AND ATTENDANT COLD/OCCLUDING FRONT RACING 
ACRS THE SOUTHEAST STATES...REACHING THE WRN EDGE OF THE CWFA IN THE 
09Z-15Z TIME FRAME FRI. AS THE FNT APPROACHES...LLVL FLOW 
STRENGTHENS OUT OF THE S/SE WITH AN H85 JET OF 70+ KTS TRAVERSING 
W-E ACRS THE FA OVRNGT TNGT.  STRONG FGEN/QG FORCING ALONG WEDGE FNT 
AND UPSLOPE FLOW ALONG SE-FACING BLUE RIDGE ESCARPMENT WILL BE 
LIFTING RATHER MOIST AIR (PWATS 1-1.25" EXPECTED ALONG AND AHEAD OF 
THE FNT). MODELS IN DECENT AGREEMENT ON TIMING OF THE LLJ...BEST 
FORCING...AND FROPA...BUT DISAGREE ON DETAILS OF THERMAL PROFILES 
AND PRECIP ONSET. (SO A LOT OF MIX AND MATCH OF GUIDANCE WITH THIS 
FCST).

GUIDANCE IN AGREEMENT ON BEST FORCING IN THE 06Z-12Z TIME FRAME. 
NEAR 100 PCT POPS LOOKS GOOD. FOR THE OVERNIGHT THRU FRI 
MORNING...THERE WILL BE FOUR MAIN ISSUES/CONCERNS. 

FIRST...PRECIP TYPE IN THE NRN MTNS AND NRN NC FOOTHILLS...AS FCST 
SNDGS AND P-TYPE NOMOGRAMS ARE IN AGREEMENT ON FREEZING RAIN LATER 
TONIGHT ALONG THE EAST FACING SLOPES OF THE NC MTNS MAINLY ALONG AND 
N OF I-40...AND W OF A MARION..TABLE ROCK...PATTERSON LINE. 
BASICALLY IT LOOKS LIKE AN ICE STORM FOR THESE AREAS. A COMBINATION 
OF HPC QPF...NAM SFC TEMPS...AND A 50/50 NAM/GFS BLEND OF 
TEMP/MOISTURE PROFILES YIELDS ICE ACCUMS OF 0.25" TO 0.50" FROM THIS 
EVENING TO NOON FRIDAY. FURTHER SOUTH ALONG THE ESCARPMENT AND 
FURTHER EAST AWAY FROM THE ESCARPMENT DO HAVE POTENTIAL FOR WARNING 
CRITERIA ICE BUT THE CONFIDENCE IS MUCH LOWER DUE TO THE MARGINAL 
COLD NOSE AND WARM NOSE TEMPS IN THE GUIDANCE. I HAVE DECIDED TO GO 
WITH AN ICE STORM WARNING FOR THE NRN MTNS AND NRN HALF OF THE 
FOOTHILLS (INCLUDING ALL OF BURKE...CALDWELL AND ALEXANDER COUNTY). 
A FREEZING RAIN ADVISORY WILL BE POSTED ALONG THE PERIPHERY OF THE 
WARNING AS A BUFFER. THE MAIN TIME FRAME FOR APPRECIABLE ICING WILL 
BE AFTER MIDNIGHT TONIGHT UNTIL AFTER DAYBREAK FRI.

SECOND...WIND ABV 3500 FT...AS SELY LLJ OF 70KT+ WILL BE SCREAMING 
ACRS THE MTNS OVRNGT AHD OF THE FNT. IT SEEMS PRUDENT TO ISSUE A 
WIND ADVY FOR THE MTN ZONES (THAT ARE NOT IN A WINTER HAZARD)...AS 
ELEVATIONS ESP ABV 3500 FT WILL LIKELY SEEM 40-55 MPH GUSTS.

THIRD...FLOODING POTENTIAL IN THE MTNS. MODEL CONSENSUS ON QPF 
GENERATES 1-1.5" WITHIN ABOUT A 6-HR PERIOD. THE RIVERS ARE PRIMED 
FOR SHARP RISES...AS LEVELS HAVE BEEN RELATIVELY FLAT...AND THERE IS 
PLENTY OF SNOW COVER REMAINING ACRS THE NC MTNS THAT WILL MELT WITH 
THE RAIN. FOR NOW...CONFIDENCE OF WIDESPREAD FLOODING IS TOO LOW TO 
WARRANT ANY WATCH. HOWEVER...SOME RIVERS MAY REACH FLOOD STAGE AFTER 
THIS EVENT.

FOURTH...SVR THREAT ALONG THE SRN FRINGE OF THE CWFA 
(ELBERT-ABBEVILLE-GREENWOOD COUNTIES AND PERHAPS SRN PORTIONS OF 
UNION SC-CHESTER-UNION NC LATER FRI MORNING). SOME OF THE 0-3 KM 
HELICITY PROGGED BY THE NAM/GFS WELL OVER 1000 M2/S2 ALONG A 
POTENTIAL TRIPLE POINT LOW COULD BE REALIZED BY SHALLOW 
CONVECTION...AS BOTH THE NAM AND GFS BRING A LTL SFC-BASED CAPE INTO 
THE AFOREMENTIONED COUNTIES ALONG THE COLD/OCCLUDED FNT. DAMAGING 
WINDS AND ISOLD TORNADOES WILL BE POSSIBLE WITH ANY STORMS THAT 
ORGANIZE ALONG THE FNT. I WILL ADD A MENTION OF ISOLD THUNDER IN 
THOSE COUNTIES STARTING AT 09Z FRI.

&&

.SHORT TERM /FRIDAY THROUGH SUNDAY/...
AS OF 300 AM THU...A DEEP VORTEX WILL BE IN PLACE ACROSS THE MIDWEST 
ON FRI. THE ASSOCIATED OCCLUDING/COLD FRONT SHOULD BE ENTERING THE 
SW PORTION OF THE CWFA EARLY IN THE DAY...WITH WEAK LOW PRESSURE 
FORMING ALONG THE BOUNDARY NEAR THE SRN PIEDMONT. A MYRIAD OF 
HAZARDS WILL THUS CONTINUE INTO THE EARLY PART OF THE SHORT RANGE 
FORECAST...WITH VERY WINDY CONDITIONS ALONG THE HIGHER TERRAIN OF 
THE NC MTNS IN SOUTHERLY FLOW...LINGERING ICING IN THE NORTHERN NC 
MOUNTAINS AND FOOTHILLS...AND AN EMERGING SEVERE WEATHER THREAT IN 
THE EXTREME SE PIEDMONT.

THE SURFACE BASED COLD NOSE IN THE HEART OF THE COLD AIR DAMMING 
SHOULD CONTINUE TO GET PINCHED E OF THE BLUE RIDGE FRI 
MORNING...WITH FREEZING RAIN GRADUALLY MIXING BACK TO RAIN. 
ADDITIONAL ACCUMULATIONS OF ONE TENTH TO ONE QUARTER INCH OF ICE 
WILL BE POSSIBLE IN THE WARNING AREA...TO GO WITH THAT ALREADY 
ACRRUED THROUGH THE EARLY MORNING HOURS.

MEANWHILE...SHEAR WILL BE VERY HIGH ALONG THE SE FRINGE OF THE CWFA 
FRIDAY MORNING...AND THE NAM IS UNCOVERING SOME CAPE AS THE WARM 
SECTOR WRAPS IN AHEAD OF A LOW ALONG THE PASSING FRONTAL ZONE. WILL 
ADD AN ISOLATED THUNDER MENTION TO THE EXTREME SE PIEDMONT AND 
CONTINUE TO ADVERTISE AN ISOLATED SEVERE WIND/TORNADO THREAT FROM 
ELBERTON TO GREENWOOD EARLY...POSSIBLY SHIFTING NE TO TOWARD CHESTER 
AND UNION NC MIDDAY.

DEEP DRYING WILL WRAP IN FROM THE SW THROUGH THE AFTERNOON HOURS 
FRI. SLIGHTLY BETTER MOISTURE IN CONTINUED SW FLOW ALOFT COULD KEEP 
PRECIP CHANCES LINGERING ACROSS THE NORTH CAROLINA MOUNTAINS INTO 
THE EVENING HOURS. COOLING PROFILES MAY BRIEFLY ALLOW SOME SLEET OR 
WET SNOW TO MIX BACK IN ALONG THE RIDGE TOPS BEFORE THE PRECIP 
ENDS...BUT NO ACCUMS ARE EXPECTED.

THE DEEP CLOSED LOW WILL THEN SLOWLY MEANDER EWD ACROSS THE SRN 
SHORES OF THE GREAT LAKES THROUGH THE WEEKEND. A LULL IN 
PRECIPITATION WILL DEVELOP ACROSS THE AREA ON SAT...WITH DRY 
PROFILES AND WSW FLOW THROUGH A DEEP LAYER SE OF THE VORTEX. 850 MB 
MOISTURE THEM MOVES BACK INTO THE NC MTNS SAT NIGHT THROUGH 
SUN...BUT WITH WESTERLY UPSLOPE FLOW NOT PRODUCING MUCH OF AN 
ORTHOGONAL COMPONENT TO THE HIGH TERRAIN. QPF AND ANY ASSOCIATED 
SNOW AMOUNTS WILL THUS BE VERY LIGHT...AND CONFINED MAINLY TO THE TN 
BORDER/HIGHER ELEVATIONS LATE IN THE WEEKEND.

&&

.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
AS OF 245 AM THURSDAY...NO SIGNIFICANT CHANGES WERE MADE TO THE 
MEDIUM RANGE FORECAST. THE FIRST PART OF THE PERIOD STILL LOOKS LIKE 
IT WILL BE DOMINATED BY LOW LEVEL NW FLOW UNDERNEATH THE CYCLONIC 
FLOW AROUND THE LARGE CLOSED LOW CENTERED JUST SOUTH OF THE GREAT 
LAKES. THE GFS...ECMWF...AND GEM AGREE THAT A TROF AXIS WILL ROTATE 
AROUND THE CIRCULATION AND ACROSS THE SOUTHERN APPALACHIANS ON 
MONDAY WITH A PERIOD OF ENHANCED CAA. I HAVE INCREASED POPS A BIT 
SUNDAY NIGHT AND MONDAY ALONG THE TN LINE ALTHOUGH THE STRENGTH OF 
THE FLOW ISN/T EXCEPTIONAL AND THE MOISTURE ISN/T EXTREMELY DEEP. 
STILL...ISOLATED TO SCATTERED SNOW SHOWERS ARE A FAIRLY SAFE BET IN 
THE USUAL LOCATIONS. FROM THEN ON FORECAST CONFIDENCE PLUMMETS AS 
THE MODELS DIVERGE GREATLY. WHATEVER DOES HAPPEN IN THE MID WEEK 
TIME PERIOD WILL CENTER AROUND SOUTHERN STREAM TROF ENERGY MOVING 
OUT OF THE DESERT SW WHICH SPARKS SOME DEGREE OF GULF CYCLOGENESIS. 
ON ONE HAND THE GFS SHEARS THE MID AND UPPER LEVEL TROF OUT AND 
KEEPS THE SURFACE LOW AND ASSOCIATED PRECIP WELL TO THE SOUTH. ON 
THE OTHER HAND THE 00Z ECMWF CONTINUES ITS TREND OF BRINGING THE 
SURFACE LOW ACROSS THE NORTHERN GULF COAST...AND EVENTUALLY HUGS THE 
CAROLINAS COASTLINE. THE HIGH PRESSURE THAT BUILDS IN NORTH OF THE 
FA IS OF POLAR ORIGINS AND SHOULD SET THE TABLE WITH SOME 
SUBSTANTIAL COLD AIR. JUST FOR KICKS...LOW LEVEL THICKNESSES OFF THE 
NEW ECMWF WOULD SUPPORT SNOW OR A WINTRY MIX ACROSS THE ENTIRE FA 
WITH A NEAR ISOTHERMAL STRUCTURE EVIDENT IN SOUNDINGS. AFTER ALL THIS 
DISCUSSION...I HAVE LEFT THE END OF THE FORECAST AS IT IS SINCE 
CONFIDENCE IS SO PUTRID AT THIS POINT.

TEMPS WILL BE COOL AS THE FIRST HALF OF THE FORECAST WILL BE 
DOMINATED BY LOW LEVEL CAA. OVERALL FORECAST TEMPS ARE ON THE ORDER 
OF 3 TO 5 DEGREES BELOW NORMAL THROUGH MID WEEK.

&&

.AVIATION /16Z THURSDAY THROUGH MONDAY/...
AT KCLT AND ELSEWHERE...BASED ON THE TRENDS OF THE MODELS...I HAVE 
DELAYED THE TIMING OF LOWERING CIGS AND ONSET OF PRECIP ACRS THE 
WESTERN CAROLINAS WITH THE 12Z TAFS. OVERALL...THE TRENDS ARE THE 
SAME...JUST A FEW HOURS LATER. AFTER 06Z...NELY/ELY WINDS 
INCR...WITH SOME GUIDANCE SHOWING GUSTS APPROACHING 30 KTS AT KCLT 
AND THE UPSTATE SITES. I DO MENTION 20 OR SO KT GUSTS FOR NOW. AT 
KAVL AND KHKY...THERE IS POTENTIAL FOR TEMPS TO BE ABLE TO REACH 
FREEZING AS RAIN MOVES IN TOWARD DAYBREAK CHRISTMAS MORNING. FOR 
NOW...THE ODDS ARE STILL LOW ENUF TO LEAVE OUT ANY MENTION OF FZRA. 
OVERALL...VFR WITH GRADUALLY LOWERING CIGS TODAY. MVFR CIG 
RESTRICTIONS ARE EXPECTED SOMETIME IN THE 00Z-06Z TIME FRAME...THEN 
POSSIBLY TO IFR BY DAYBREAK. THE WORST OF THE RAIN/LOW CLOUDS WILL 
PROBABLY OCCUR 09Z-18Z FRI.

OUTLOOK...WIDESPREAD MVFR/IFR RESTRICTIONS CAN BE EXPECTED ON FRIDAY 
AS A COLD FRONT CROSSES THE AREA. CONDITIONS SHOULD IMPROVE FRIDAY 
NIGHT THROUGH SAT AS DRY AIR COMES IN FROM THE W. VFR CONDITIONS 
EXPECTED SUN AND MON.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...WIND ADVISORY FROM 6 PM THIS EVENING TO NOON EST FRIDAY FOR 
     GAZ010-017.
NC...WIND ADVISORY FROM 6 PM THIS EVENING TO NOON EST FRIDAY FOR 
     NCZ048-051-052-058-059-062-063.
     FREEZING RAIN ADVISORY FROM 6 PM THIS EVENING TO NOON EST 
     FRIDAY FOR NCZ036-053-056-064-065-068-507>510.
     ICE STORM WARNING FROM 6 PM THIS EVENING TO NOON EST FRIDAY 
     FOR NCZ033-035-049-050-501>506.
SC...NONE.

&&

$$
SYNOPSIS...BSH
NEAR TERM...ARK/CSH
SHORT TERM...HG
LONG TERM...BSH
AVIATION...ARK/BSH