FXUS62 KGSP 241149
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
649 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/...
PLENTY OF WEATHER TO DISCUSS FOR TNGT...AS A MAJOR STORM SYSTEM
DEEPENS 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).
TODAY...ONE LAST RELATIVELY QUIET DAY BEFORE PRECIP BREAKS OUT LATE
AFTN/EARLY EVENING. THERE SHUD BE A GENERAL INCR IN CLOUDINESS THRU
THE DAY...AS LLVLS MOISTEN. THE 00Z NAM AND GFS STILL DISAGREE ON
TIMING OF ONSET OF PRECIP...WITH THE GFS BEING MUCH FASTER (18Z THIS
AFTN) THAN THE NAM (03Z TNGT). I TOOK A BLEND...AS OTHER GUIDANCE
ALSO IS SPLIT ON THIS. SO PERHAPS SOME LIGHT RAIN MAY BEGIN BEFORE 6
PM (MAINLY IN THE SW CORNER OF THE CWFA). THE MAV AND MET DISAGREE
ON TEMPS (PROBABLY DUE MAINLY TO PRECIP ONSET DIFFERENCES). LLVL
THICKNESSES WILL BE ABOUT 15 M LOWER THAN YDA DUE TO STRENGTHENING
CAD...AND WITH MORE CLOUD COVER...I THINK THE MET NUMBERS ARE A BIT
TOO WARM AND WILL GO CLOSE TO THE MAV.
TONIGHT...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 /12Z 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
SHORT TERM...HG
LONG TERM...BSH
AVIATION...ARK/BSH
