Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 200529
AFDGSP

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
1229 AM EST SAT DEC 20 2014

.SYNOPSIS...
A PLUME OF MOISTURE WILL ROTATE ACROSS THE REGION TONIGHT INTO
SATURDAY WITH DRY HIGH PRESSURE RETURNING ON SUNDAY. AN AREA OF LOW
PRESSURE WILL BRING RAIN CHANCES BACK BY MONDAY.

&&

.NEAR TERM /THROUGH TODAY/...
0515 UTC UPDATE...WINDS WERE UPDATED FROM THE NAM AND ADJMAV. SKY
COVER WAS INCREASED PER SATELLITE IMAGERY.

915 PM UPDATE...VERY LIGHT PRECIP IS OCCURRING OVER AL/GA...WHERE
RADAR MOSAIC INDICATES ELEVATED STRATIFORM RETURNS. UPPER DIVERGENCE
APPEARS TO BE THE MAIN DRIVER FOR THIS ACTIVITY. SAID DIVERGENCE
SPREADS OVER OUR WRN ZONES A LITTLE AFTER MIDNIGHT. REVISED POPS TO
MATCH THE EXPECTED ONSET. 00Z NAM NOT LOOKING AS WEDGY TMRW
MRNG...EVIDENTLY A RESULT OF LESS QPF. ADJUSTED TEMPS THRU DAYBREAK
PER LATEST OBS AND SHORT TERM CONSENSUS PRODUCT. OBS TRENDS INDICATE
THE HIGH CLOUDINESS IS DOING PERHAPS AN EVEN BETTER JOB OF LIMITING
COOLING THAN EXPECTED AND THE NEW MIN TEMPS ARE SLIGHTLY WARMER...IN
LINE WITH EXPECTATION FOR A WEAKER WEDGE. THIS REDUCED THE COVERAGE
OF WINTRY WX EVEN MORE.

630 PM UPDATE...18Z NAM/GFS SEEM TO HAVE TRENDED A BIT MORE IN FAVOR
OF CAD DEVELOPMENT VS THEIR COUNTERPARTS FROM 12Z. THIS IS
PARTICULARLY TRUE OF THE NAM. PTYPE IS STILL A TRICKY FCST. I DO
FEEL FAIRLY CONFIDENT THAT OVER THE MTNS/FOOTHILLS THE BEST FORCING
FROM THE SHORTWAVE WILL DEPART BEFORE THE LLVL WARM ADVECTION
CREATES ENOUGH OF A WARM NOSE TO CAUSE A CHANGEOVER TO FZRA OR
SLEET. I DID REGENERATE THE WX TYPES BASED ON THE 18Z NAM AND ON
SLIGHTLY COLDER SFC TEMPS...AND AS ONE MIGHT EXPECT A BIT MORE
WINTRY PRECIP RESULTED. HOWEVER MODEL QPF IS STILL LIGHT AND TEMPS
DO WARM ENOUGH THRU THE MRNG TO CHANGE MOST AREAS BACK OVER TO RAIN
FAIRLY QUICKLY. SNOW ACCUMS REMAIN UNDER 1 INCH...WITH JUST A FEW
HUNDREDTHS OF ICE AT MOST ON RIDGETOPS. WE WILL CONTINUE TO LET THE
SPECIAL WX STATEMENT SPEAK FOR THE TRAVEL IMPACTS.

AS OF 230 PM...A FAST-MOVING MID LVL SHORTWAVE WILL TRACK ACRS THE
TN VLY TONIGHT...THEN CROSS THE CWFA EARLY SATURDAY WITHIN QUASI-
ZONAL FLOW ALOFT. MEANWHILE...A 1025-1028 MB HIGH PRES SYSTEM WILL
REMAIN ACRS THE GREAT LAKES TO THE ST LAWRENCE VLY. A WEAK SFC WAVE
IS EXPECTED TO RIPPLE ACRS THE GULF OF MEXICO...WITH A BRIEF SHOT OF
DEEPER LAYER RH CROSSING THE AREA JUST AHEAD OF THE MID LVL TROF.
THE RESULT IS AN INCREASE IN CLOUDS THRU THIS EVENING...WITH SOME
LIGHT PRECIP DEVELOPING OVERNIGHT THRU SATURDAY MORNING. MODELS
SEEMS TO HAVE SETTLED ON A VERY LIGHT QPF EVENT...WITH GENERALLY
JUST A FEW HUNDREDTHS TO A TENTH OF AN INCH IN THE MTNS AND LWR
PIEDMONT...AND A TRACE TO FEW HUNDREDTHS ELSEWHERE. THE MAIN
CHALLENGE STILL LOOKS TO BE P-TYPE IN THE MTNS AND ADJACENT NC
FOOTHILLS...WHERE TEMPS WILL BE SUPPORTIVE OF WINTRY MIX. FCST SNDGS
SHOW MOISTENING PROFILES FROM THE TOP DOWN...WITH WETBULB EFFECTS
CREATING MOSTLY A RAIN/SNOW EVENT. HOWEVER...WITH WEAK OMEGA AND
MARGINAL ICE NUCLEI ACTIVATION...THERE MAY BE SPOTTY FREEZING RAIN
AND SLEET MIXED IN AS WELL...ESP AT ONSET OF PRECIP...PROBABLY AFTER
MIDNIGHT. USING A 50/50 BLEND OF NAM/GFS WITH THE TOP-DOWN
APPROACH...I DO GET A MIXED BAG OF PRECIP DURING THE WEE MORNING
HOURS...BUT THEN TRANSITIONING TO MOSTLY SNOW BY DAYBREAK. A LIGHT
COATING OF ICE/SLEET/SNOW WILL BE POSSIBLE...ESP ABOVE 3500 FT FROM
HAYWOOD COUNTY NORTHEAST. THIS ISN/T ENUF TO WARRANT A WINTER WX
ADVISORY...SO WE WILL HIGHLIGHT WINTRY MIX AND SLICK SPOTS ON
ROADWAYS IN A SPECIAL WEATHER STATEMENT (SPS). OUTSIDE THE
MTNS...MAINLY JUST SPRINKLES TO A FEW HUNDREDTHS OF LIGHT RAIN
EXPECTED...PRIMARILY ACRS NE GA AND THE UPSTATE.

SATURDAY...AS THE MID LVL WAVE EXITS...TAKING THE MOISTURE AND LIFT
WITH IT...POPS QUICKLY TAPER OFF. FCST SNDGS SHOW THE SFC LAYER
WARMING...RESULTING IN ANY LINGERING LIGHT PRECIP CHANGING BACK TO
RAIN. COLD AIR DAMMING LOOKS TO BE WEAK OR EVENT MORE OF A
"LOOK-ALIKE" EVENT...AS PRECIP JUST TOO LIGHT TO HOLD A WEDGE
IN...GIVEN PARENT HIGH WEAK AND WELL TO THE NE OF THE FAVORED AREA.
TEMPS WILL BE TRICKY...BUT EXPECTING PLENTY OF CLOUD COVER...SO I
STILL UNDERCUT MOS BY A COUPLE CATEGORIES...GENERALLY MID-UPR 30S
ABOVE 3500 FT...AND MID-UPR 40S BELOW 3500 FT...INCLUDING PIEDMONT.
IF CLOUDS SCATTER OUT...THEN TEMPS COULD BUST BY A COUPLE CATEGORIES.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...
AS OF 230 PM...SUNDAY SHOULD BEGIN DRY AS S/W RIDGING BUILDS OVER
THE SOUTHERN APPALACHIAN REGION. HOWEVER...SHORT RANGE GUIDANCE
INDICATES THAT 290K ISENTROPIC LIFT WILL BEGIN TO INCREASE FROM THE
SOUTH SUNDAY AFTERNOON. THE COMBINATION OF INCREASING
MOISTURE...SOUTHEAST LLVL FLOW...AND INCREASING UPGLIDE SHOULD
RESULT SCT LIGHT RAINFALL BY LATE AFTERNOON ACROSS THE REGION. THE
BUILDING CLOUD COVER AND SCT RAIN SHOULD LIMIT HEATING...WITH HIGHS
RANGING FROM THE U40S TO LOW 50S.

SUNDAY NIGHT...COLD AIR DAMMING APPEARS TO INCREASE ACROSS THE
FOOTHILLS AND PIEDMONT. SHORT RANGE MODELS INDICATE INCREASING NE
SFC WINDS WITH 4 MB NE-SW PRESSURE GRADIENT. ISENTROPIC LIFT IS
EXPECTED TO PEAK EAST OF THE MTNS AROUND DAWN ON MONDAY. THE PERIOD
FROM 6Z-15Z SHOULD SEE THE GREATEST RAINFALL RATES...GENERALLY
AROUND 0.25/3 HRS. SUNDAY NIGHT TEMPERATURES ARE FORECAST TO REMAIN
IN THE U30S TO L40S. HOWEVER...AREAS ACROSS THE NRN NC MTNS COULD
DIP TO FREEZING DURING THE PRE DAWN HOURS. SPOTTY FZRA IS POSSIBLE
ACROSS PORTIONS OF THE NRN MTNS...LITTLE TO NO ICE ACCUMULATIONS.
FOR MONDAY...COLD AIR DAMMING WILL RULE THE WEATHER. SHALLOW
ISENTROPIC LIFT SHOULD CONTINUE THROUGH THE DAY...SUPPORTING LIGHT
RAINFALL THROUGH THE DAYLIGHT HOURS. STEADY NE WINDS AND THICK
CLOUDS WILL LIMIT DAYTIME HEATING. I WILL FORECAST HIGHS RISING OVER
MORNING LOWS BY 5 DEGREES OR LESS. THIS APPROACH WILL PLACE FORECAST
HIGHS BETWEEN THE U30S TO L40S...PERFECTLY PLACED COOLER THAN WARM
BIAS GUIDANCE DURING CAD.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
AS OF 220 PM FRIDAY...THE TRANSITION TO A HIGHLY AMPLIFIED...YET
SOMEWHAT PROGRESSIVE UPPER AIR PATTERN WILL BE COMPLETE BY THE
BEGINNING OF THE MEDIUM RANGE PERIOD...WITH A DEEP CYCLONE/UPPER
TROUGH EXPECTED TO TRANSLATE ACROSS THE EASTERN CONUS EARLY IN THE
PERIOD. LIGHT RAIN WILL BE POSSIBLE MONDAY NIGHT THROUGH TUE IN
WEAK/MOIST ISENTROPIC/UPSLOPE LIFT REGIME. RAIN CHANCES WILL THEN
INCREASE SUBSTANTIALLY TUE NIGHT/EARLY WED...AS COLD FRONT AND
POTENT SHORT WAVE TROUGH APPROACH THE SOUTHEAST. A CONSENSUS OF
GLOBAL MODEL SOLUTIONS WOULD SEE SURFACE LOW PRESSURE PASS WEST OF
THE APPALACHIANS. THUS...P-TYPE SHOULD BE ALL-LIQUID. THERE IS A LOW
CHANCE FOR THUNDER ACROSS THE PIEDMONT WED MORNING...ALTHOUGH IF
ANYTHING THE LATEST GUIDANCE SUGGESTS LESS OF A POTENTIAL FOR THIS.

RAPIDLY DROPPING SNOW LEVELS MAY RESULT IN A TRANSITION TO A
NORTHWEST FLOW SNOW EVENT ACROSS THE MOUNTAINS ON WEDNESDAY.
HOWEVER...A STRONG SPEED MAX UPSTREAM OF THE UPPER TROUGH AXIS WILL
KEEP LOW LEVEL FLOW MORE WESTERLY THAN NW THROUGH MUCH OF WED.
MEANWHILE...MOISTURE WILL ALSO RETREAT DURING THIS TIME...AS THE
UPPER TROUGH AXIS PROGRESSES RATHER QUICKLY OFF THE EAST COAST.
THEREFORE...ANY NORTHWEST FLOW SNOW EVENT WOULD BE QUITE LOW-IMPACT.

THE REMAINDER OF THE MEDIUM RANGE WILL SEE A RETURN TO A LESS
AMPLIFIED PATTERN ACROSS THE CONUS...SPELLING MODERATING TEMPS BY
DAY 7...AFTER SEVERAL DAYS OF COOL WEATHER IN CLOUDY/PERIODIC IN-
SITU COLD AIR DAMMING/LOW UPPER HEIGHTS REGIME.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
AT KCLT...CIG WILL LOWER TO LOW VFR OVERNIGHT AS LOW LEVEL MOISTURE
AND PRECIPITATION ARRIVE FROM THE SW. GUIDANCE TAKES THE CIG DOWN TO
MVFR IN THE AFTERNOON...LEAVING IT THERE UNTIL CLOUDS AND ERODE AND
PRECIPITATION ENDS LATE SATURDAY EVENING IN THE WAKE OF A DEPARTING
COASTAL LOW. A GUIDANCE BLEND SUPPORTS MVFR VSBY SATURDAY EVENING.
NE WINDS WILL PERSIST.

ELSEWHERE...LOW VFR CIGS AND LIGHT PRECIPITATION ARE EXPECTED BEFORE
DAWN AS LOW LEVEL MOISTURE OVERSPREADS THE AREA...WITH SNOW AND
FREEZING RAIN POSSIBLE AT KAVL A FEW HOURS EITHER SIDE OF
DAYBREAK...AND SNOW AT KHKY AROUND THE SAME TIME. BY MORNING
GUIDANCE TAKES THE KAVL CIG DOWN TO MVFR...WITH SC SITES FOLLOWING
BY MIDDAY. GUIDANCE SUGGESTS A BRIEF LOW IFR CIG AT KAVL EARLY
FRIDAY EVENING. CIGS ERODE IN THE FOOTHILLS SATURDAY EVENING AS
CLOUDS ERODE BEHIND A DEPARTING COASTAL LOW...WHILE KAVL IMPROVES TO
MVFR LATE.

OUTLOOK...LINGERING LOW LVL MOISTURE MAY KEEP SOME FOG AND LOW
STRATUS AROUND SATURDAY NIGHT THRU SUNDAY. A WETTER SYSTEM WILL THEN
TRACK INTO THE AREA FOR MONDAY LIKELY BRINGING WIDESPREAD CLOUDS AND
PRECIP RESTRICTIONS. YET ANOTHER STORM SYSTEM IS EXPECTED TO CROSS
THE AREA LATE TUESDAY OR EARLY WEDNESDAY...WITH ANOTHER ROUND OF
PRECIP.

CONFIDENCE TABLE...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KGSP       HIGH 100%     HIGH 100%     HIGH 100%     HIGH 100%
KAVL       HIGH 100%     HIGH 100%     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...CSH
NEAR TERM...ARK/JAT/WIMBERLEY
SHORT TERM...NED
LONG TERM...JDL
AVIATION...JAT


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