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Area Forecast Discussion

518
FXUS62 KGSP 201124
AFDGSP

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

.SYNOPSIS...
A WAVE OF LOW PRESSURE ALONG WITH BRIEF DEEPER MOISTURE WILL MOVE
QUICKLY ACROSS THE REGION EARLY TODAY...BEFORE MOVING OFFSHORE BY
TONIGHT. LOW LEVEL MOISTURE WILL GRADUALLY RETURN BEHIND THE WAVE
FROM THE SOUTH SUNDAY THROUGH MONDAY...WITH DEEPER MOISTURE ARRIVING
ON TUESDAY AHEAD OF A SHARP COLD FRONT. THE COLD FRONT WILL CROSS
THE AREA FROM THE WEST ON WEDNESDAY. COLD AND WINDY CONDITIONS WILL
SET UP IN THE MOUNTAINS THROUGH THURSDAY...BUT WITH DRIER LATE WEEK
CONDITIONS EAST OF THE MOUNTAINS.

&&

.NEAR TERM /THROUGH TONIGHT/...
1115 UTC UPDATE...MINIMUM TEMPERATURES HAVE BEEN ADJUSTED UP BASED
ON OBSERVATION TRENDS. THIS WILL AFFECT AREAS OF FREEZING RAIN AND
SNOW IN THE NC MOUNTAINS THIS MORNING. ENOUGH WINTRY PRECIPITATION
IS STILL EXPECTED TO WARRANT THE CURRENT SPS HOWEVER. WINDS WERE
UPDATED FROM THE 06Z NAM AND ADJMAV. AREAS OF FOG...DENSE IN SOME
PLACES...WERE INTRODUCED FOR TONIGHT BASED ON GOOD AGREEMENT BETWEEN
THE MODELS. CLOUD COVER WAS INCREASED OVERNIGHT BASED ON MODEL
AGREEMENT.

AT 230 AM EST SATURDAY...A LOW AMPLITUDE UPPER PATTERN EXISTED
ACROSS THE SOUTHEAST...WITH A SHORTWAVE WAS MOVING THROUGH THE OH
AND TN RIVER VALLEYS TOWARD OUR AREA. PRECIPITATION AHEAD OF THE
SHORTWAVE IN MODEST ISENTROPIC UPGLIDE WAS SPREADING ACROSS THE
UPPER SAVANNAH RIVER VALLEY INTO OUR ARE.

THERMAL PROFILES SUPPORT A CHANGEOVER TO FREEZING RAIN OR SNOW IN
MUCH OF THE NC MOUNTAINS AND NORTHERN FOOTHILLS THIS MORNING AS
SURFACE TEMPERATURE FALL TO NEAR OR BELOW FREEZING. AS TEMPERATURES
WARM...SNOW WILL CHANGE OVER TO RAIN AT ALL BUT THE HIGHEST
ELEVATIONS LATE THIS MORNING. ICE ACCUMULATIONS WILL BE LIGHT...AND
GENERALLY LIMITED TO THE HIGHER ELEVATIONS. SNOW ACCUMULATIONS COULD
APPROACH HALF AND INCH ON THE HIGHEST PEAKS OF THE GREAT SMOKY
MOUNTAINS...AND ALMOST ONE INCH ON THE HIGHEST PEAKS OF THE NORTHERN
NC MOUNTAINS. BASED ON THESE NUMBERS...A NEW SPS WILL BE ISSUED TO
COVER THE SITUATION. PRECIPITATION WILL DECREASE FROM THE WEST THIS
AFTERNOON IN THE WAKE OF THE PASSING SHORTWAVE AND WEAK SURFACE WAVE
OFF THE CAROLINA COAST. MAXIMUM TEMPERATURES ARE EXPECTED TO RUN
AROUND SIX DEGREES BELOW NORMAL UNDER CLOUDS COVER.

PRECIPITATION WILL END EARLY THIS EVENING. MODEL TIME HEIGHTS
SUGGEST SOME LINGERING LOW AND UPPER LEVEL MOISTURE...EVEN AS THE
MID LEVELS DRY OUT...AND GUIDANCE SUPPORTS WIDESPREAD FOG TONIGHT...
POSSIBLE DENSE...IN LIGHT WINDS. MINIMUM TEMPERATURES WILL RUN
AROUND TWO DEGREES ABOVE NORMAL.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY/...
AS OF 230 AM EST SAT...A WEAK SHORTWAVE WILL MOVE EAST FROM THE SRN
PLAINS TO THE LOWER MS RIVER VALLEY SUNDAY AND SUNDAY NIGHT. AHEAD
OF THIS FEATURE...A 130 KT JET STREAK LINGERING OVER THE FORECAST
AREA WILL GRADUALLY PULL INCREASING UPPER DIVERGENCE INTO THE REGION
FROM THE SE. ALTHOUGH THE DEEPER MOISTURE MAY STAY CONFINED TO AREAS
SE OF THE FORECAST AREA...LOW LEVEL MOISTURE WILL STEADILY RETURN AS
ISENTROPIC LIFT DEVELOPS IN RESPONSE TO THE WEAK UPPER FORCING. WILL
KEEP MAX TEMPS BELOW GUIDANCE SUN AFTN AND REINTRODUCE SCHC TO CHC
POPS FOR LIGHT RAIN FROM THE S BY LATE DAY.

1026 MB SFC HIGH PRESSURE WILL SETTLE OVER TO THE N SUN NIGHT
THROUGH MONDAY. ISENTROPIC LIFT WILL GRADUALLY IMPROVE OVER THE
DEVELOPING HYBRID SFC CAD LAYER...WITH WIDESPREAD CLOUDS AND MUCH
MORE LIKELY CHANCES OF LIGHT RAIN AND DRIZZLE. TEMPERATURE PROFILES
ALONG THE NRN BLUE RIDGE EARLY MONDAY MAY BRIEFLY BE JUST COLD
ENOUGH FOR WINTRY MIXED PTYPES...BUT THIS WOULD BE MAINLY SPOTTY
LIGHT FREEZING RAIN OR FREEZING DRIZZLE GIVEN THE SHALLOW NATURE OF
THE MOISTURE. WILL KEEP TEMPS WELL BELOW GUIDANCE AGAIN MON AFTN
GIVEN THE ANTICIPATED CAD...ALBEIT WITH FAIRLY LIGHT PRECIPITATION
INTO THE WEDGE.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
AS OF 230 PM SAT...A FULL LATITUDE TROUGH WILL DEVELOP FROM THE
PLAINS TO THE MS RIVER VALLEY MON NIGHT TO TUE NIGHT. MOISTURE WILL
STEADILY INCREASE AHEAD OF THIS SYSTEM ACROSS THE SOUTHEAST
TUESDAY...WITH THE DEEPEST LAYER FORCING LIKELY CROSSING THE
FORECAST AREA TUE NIGHT INTO WED MORNING JUST AHEAD OF THE
ASSOCIATED SHARP COLD FRONT. THE RESIDUAL CAD LAYER OVER THE REGION
WILL STEADILY ERODE TUE THROUGH TUE NIGHT...WITH NON DIURNAL WARMING
TEMPS IN THE PIEDMONT LIKELY LATE TUE NIGHT. A 40+ KT SRLY LOW LEVEL
JET IS EXPECTED JUST AHEAD OF THE COLD FRONT...BUT SFC BASED
INSTABILITY AND MARGINAL LAPSE RATES ALOFT SHOULD BE STRONG LIMITING
FACTORS FOR SEVERE CONVECTION WED MORNING. A LOWER PIEDMONT ISOLD
THUNDER MENTION COULD BE WARRANTED. ANTICIPATE COLD FROPA FROM THE
WEST 12Z TO 18Z WED...WITH A DRY SLOT ARRIVING QUICKLY WED AFTN.

FAIRLY DEEP MOISTURE IN THE COLD ADVECTION NW FLOW IS EXPECTED WED
NIGHT. THE SFC TO 850 FLOW MAY BE SUFFICIENTLY BACKED THAT THE
UPSLOPE COMPONENT IS NOT TREMENDOUS...BUT MOISTURE WILL BE DEEP AND
850 MB TEMPS MAY FALL TO NEAR MINUS 8 C OVER THE WRN MTNS BY
DAYBREAK. LIGHT SNOW ACCUMULATIONS LOOK QUITE POSSIBLE IN THE WRN NC
MOUNTAINS EARLY CHRISTMAS MORNING. ANTICIPATE A STRONG PRES GRADIENT
BEHIND THE TROUGH THU...WITH WINDY CONDITIONS POSSIBLE. THE UPPER
FLOW SHOULD BECOME MORE ZONAL THU NIGHT INTO FRIDAY...WITH A WEAK
FRONT APPROACHING FROM THE WEST IN RELATIVELY DRY PROFILES.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
AT KCLT...GUIDANCE SUPPORTS PROLONGED LOW CLOUDS OVER THE
AREA...AND LOWERS THE CIG FROM LOW VFR TO MVFR BY MIDDAY. THE CIG
BRIEFLY RISES TO LOW VFR LATE IN THE DAY...BEFORE DROPPING BACK TO
MVFR THIS EVENING...WHERE IT REMAINS TIL DAWN SUNDAY. PRECIPITATION
ASSOCIATED WITH A MID LEVEL SYSTEM IS EXPECTED TO TAPER OFF THIS
AFTERNOON. GUIDANCE IS IN GOOD AGREEMENT OF FOG OVERNIGHT...WITH IFR
VSBY BEFORE DAWN. WINDS WILL FAVOR THE NE.

ELSEWHERE...LOW VFR CIGS ARE EXPECTED TO LOWER TO MVFR IN SC THIS
MORNING...THEN RETURN TO LOW VFR THIS AFTERNOON. GUIDANCE THEN TAKES
SC CIGS BACK TO MVFR THIS EVENING...WHERE THEY REMAIN TIL DAWN.
GUIDANCE KEEPS NC CIGS LOW VFR DURING THIS TIME...BUT WITH GUIDANCE
HITTING FOG VERY HARD TOWARD DAYBREAK SUNDAY...WITH IFR OR LOWER
VSBY EXPECTED...CONFIDENCE IS LOW THAT CIGS WILL STAY UP OVERNIGHT.
LIGHT PRECIPITATION WILL TAPER OFF LATE TODAY. WINDS WILL FAVOR THE
NE...EXCEPT AT KAVL WHERE THEY WILL CHANNEL DOWNVALLEY FORM THE SE.

OUTLOOK...LINGERING LOW LVL MOISTURE MAY KEEP LOW STRATUS AND NIGHT
FOG AROUND THRU SUNDAY NIGHT. 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...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       HIGH 100%     MED   66%     HIGH  83%     MED   73%
KGSP       HIGH 100%     HIGH  91%     HIGH  83%     HIGH 100%
KAVL       HIGH  90%     HIGH  83%     MED   75%     LOW   53%
KHKY       HIGH 100%     HIGH 100%     HIGH 100%     LOW   53%
KGMU       HIGH 100%     HIGH  91%     HIGH  83%     HIGH 100%
KAND       HIGH 100%     HIGH 100%     MED   66%     MED   73%

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...HG
NEAR TERM...JAT
SHORT TERM...HG
LONG TERM...HG
AVIATION...JAT



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