169
FXUS62 KGSP 181941
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
241 PM EST WED DEC 18 2024
SYNOPSIS
MUCH COOLER CONDITIONS ARE EXPECTED TO FILTER INTO OUR REGION
LATE IN THE WEEK. A NORTHWEST FLOW SNOW EVENT WILL PRODUCE LIGHT
SNOW ALONG THE TENNESSEE BORDER AREAS FROM LATE FRIDAY THROUGH
SATURDAY MORNING. THE COLDEST DAYS ARE EXPECTED TO BE SUNDAY AND
MONDAY THEN A LITTLE WARMER FOR CHRISTMAS.
NEAR TERM /THROUGH THURSDAY/
AS OF 230 PM: A POSITIVELY TILTED UPPER TROUGH WILL SWING THRU
THE AREA THIS EVENING, PROVIDING SOME GOOD UPPER-LEVEL DPVA. THIS
FEATURE IS HELPING SUSTAIN A SOLID LINE OF CONVECTION ASSOCIATED
WITH AN APPROACHING COLD FRONT, WHICH HAS BEEN OVERALL WEAKENING
AS IT CROSSES EASTERN TN. LOW-LEVEL CONVERGENCE WILL BE RATHER
WEAK ALONG THE COLD FRONT, AND THE CAMS ARE IN GOOD AGREEMENT THAT
THIS ACTIVITY WILL BREAK UP INTO SCATTERED SHOWERS AS IT TRIES TO
CROSS THE MOUNTAINS LATE AFTN THRU THE EVENING. THE COMBINATION
OF MODEST EFFECTIVE SHEAR (30-40 KT) AND WEAK SBCAPE (<1000 J/KG)
WILL LIMIT ANY SEVERE TSTM POTENTIAL TO NEAR ZERO. MEANWHILE, SWLY
UPGLIDE FLOW MAY SUPPORT ISOLATED TO WIDELY SCATTERED SHOWERS EAST
OF THE MOUNTAINS LATE AFTN INTO EVENING, BUT THE ACTUAL FRONTAL
LINE DOESN'T LOOK TO SURVIVE OVER THE MOUNTAINS. THE COLD FRONT
SHOULD PUSH THRU THE FA OVERNIGHT (ALTHOUGH OVERNIGHT FRONTS OFTEN
GET HUNG UP ALONG THE BLUE RIDGE). VIRTUALLY NO GUIDANCE DOES THAT,
HOWEVER, SO CONFIDENCE IS FAIRLY HIGH THAT THE FROPA SHOULD CLEAR
THE PIEDMONT EARLY ENOUGH TO STAVE OFF ANOTHER FOGGY NIGHT. THE
EXCEPTION MAY BE THE FAR SOUTHERN AND SOUTHEASTERN PORTIONS OF THE
FA, WHERE PATCHY DENSE FOG CANNOT BE RULED OUT. ANY FOG THAT DOES
DEVELOP SHOULD MIX OUT QUICKLY THURSDAY MORNING, AS DRY AIR AND
LOW-LEVEL CAA RAMPS UP BEING THE FRONT. LOWS WILL BE ABOVE NORMAL,
MAINLY IN THE 40S (EXCEPT 30S IN THE HIGHER ELEVATIONS).
A DEEP TROUGH WILL DIG INTO THE MIDWEST ASSOCIATED WITH A STRONG
CLIPPER LOW PRES SYSTEM. WEAK SFC HIGH PRES WILL CROSS THE
EASTERN GREAT LAKES, PROVIDING A WEAK WEDGE-LIKE PATTERN ACROSS
THE FA. LOW-LEVEL FLOW WILL BEGIN TO SHIFT OUT OF THE SW AGAIN BY
MIDDAY, WHICH MAY HAVE ENOUGH MOISTURE FOR INCREASING STRATOCU. THE
COMBINATION OF NELY SFC FLOW AND THE POSSIBLE CLOUDS WILL RESULT
IN HIGH TEMPS ABOUT 10-15 DEG COOLER THAN TODAY'S READINGS.
SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/
AS OF 200 PM EST WEDNESDAY: A RELATIVELY STRONG SHORT WAVE WILL DIVE
ACROSS THE AREA FRIDAY BRINGING AN ASSOCIATED ALBERTA CLIPPER
SURFACE LOW JUST NORTH OF THE AREA. THERE IS GOOD FORCING WITH THE
WAVE AND AN ASSOCIATED UPPER JET. THE RELATIVELY DEEP MOISTURE,
STRONG FORCING, AND DROPPING TEMPS WILL LEAD TO RAIN CHANGING TO
SNOW, MAINLY IN THE COUNTIES ALONG THE TN BORDER. THE SNOW CONTINUES
FRIDAY NIGHT INTO EARLY SATURDAY MORNING AS NW UPSLOPE FLOW REMAINS
OVER THE AREA. RIGHT NOW, IT LOOKS LIKE SNOW OF 1 TO 3 INCHES WILL
BE POSSIBLE ACROSS THE HIGHER ELEVATIONS WITH AROUND AN INCH OR SO
IN THE VALLEYS. A WINTER WEATHER ADVISORY MAY BE NEEDED ONCE THE
EVENT GETS CLOSER IN TIME. WINDY CONDITIONS ARE ALSO EXPECTED ACROSS
THE MOUNTAINS THROUGH THE PERIOD.
THE DEEPEST MOISTURE REMAINS ACROSS THE MOUNTAINS, BUT SOME WILL
SPILL OUT ACROSS THE FOOTHILLS AND I-77 CORRIDOR. EXPECT ISOLATED
SHOWERS OUTSIDE OF THE MOUNTAINS, BUT TEMPS AND DON'T FALL AS
QUICKLY THERE, SO JUST RAIN SHOWERS ARE EXPECTED. THAT SAID, IF THE
FREEZING LEVEL FALLS FAST ENOUGH OR THE WEAK INSTABILITY IS DEEP
ENOUGH, THEN GRAUPEL OR MIXED RAIN/SNOW IS POSSIBLE. NE GA AND MOST
OF THE UPSTATE REMAIN DRY, BUT SOME CLOUDS ARE POSSIBLE. BREEZY TO
WINDY CONDITIONS ARE EXPECTED.
THE PRESSURE GRADIENT ACROSS THE AREA RELAXES SATURDAY WITH SNOW
SHOWERS ENDING ACROSS THE MOUNTAINS AND WINDS DIMINISHING ACROSS THE
AREA. LOWS THURSDAY NIGHT WILL BE AROUND 5 DEGREES ABOVE NORMAL.
HIGHS FRIDAY WILL BE NEAR NORMAL BUT WITH AN EARLY HIGH ACROSS THE
HIGHER ELEVATIONS NEAR THE TN BORDER. LOWS FRIDAY NIGHT AROUND 5
DEGREES BELOW NORMAL. HIGHS SATURDAY AROUND 10 DEGREES BELOW NORMAL
AS A VERY COLD AIR MASS MOVES INTO THE AREA.
LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/
AS OF 230 PM EST WEDNESDAY: A WEAK SHORT WAVE DROPS INTO THE AREA
SUNDAY KEEPING THE DEEP TROUGH OVER THE AREA. HEIGHTS THEN RISE
SLOWLY THROUGH TUESDAY. HIGH PRESSURE BUILDS INTO THE AREA IN A COLD
AIR DAMMING CONFIGURATION SUNDAY AND REMAINS IN PLACE THROUGH
TUESDAY. THE AIR MASS IS STILL VERY COLD ON SUNDAY THEN SLOWLY
MODERATES BY TUESDAY. TEMPS WILL BE 10 TO 15 DEGREES BELOW NORMAL
THROUGH MONDAY RISING BUT STILL BELOW NORMAL TUESDAY. THE GUIDANCE
DIFFERS ON CHRISTMAS DAY WITH A LOW PRESSURE SYSTEM AND PRECIP
APPROACHING FROM THE WEST ON THE GFS AND CANADIAN. THE ECMWF IS
SLOWER WITH THE SYSTEM, KEEPING THE AREA DRY. HAVE FOLLOWED THE
GUIDANCE BLEND WHICH BRINGS A CHANCE OF RAIN INTO THE AREA DURING
THE DAY. SURFACE TEMPS COULD BE TRICKY, ESPECIALLY OF PRECIP STARTED
EARLY ENOUGH, MEANING A CHANCE OF FREEZING RAIN WITH A STRONG WARM
NOSE DEVELOPING. THAT SAID, THE OPERATIONAL AND ENSEMBLE GUIDANCE
INDICATE TEMPS WOULD BE WARM ENOUGH FOR ALL RAIN, SO WENT THAT WAY
AS WELL.
AVIATION /20Z WEDNESDAY THROUGH MONDAY/
AT KCLT AND ELSEWHERE: CONFIDENCE IS BELOW AVERAGE FOR THE CIGS
THIS AFTN INTO TONIGHT, AS A LIFR CLOUD DECK DRIFTS SLOWLY NORTH
FROM THE MIDLANDS TOWARD THE UPSTATE SITES AND KCLT. THE EDGE SEEMS
TO BE SLOWLY DOWN ON SATELLITE IMAGERY, BUT COULD BRIEFLY IMPACT
KAND. AN MVFR CU FIELD IS FORMING IN THE CLEAR AREAS OVER THE
TAF SITES, SO EXPECT THAT TO BE THE MAIN CIGS. A FEW SHOWERS MAY
DEVELOP, WITH CHANCES JUST HIGH ENOUGH TO WARRANT PROB30S AND/OR
TEMPOS. CONFIDENCE IS HIGHEST AT KAND AND KCLT, WHICH ARE CLOSE
TO A DIFFERENTIAL HEATING BOUNDARY ASSOCIATED WITH THE EDGE OF
THE LOW CLOUD DECK. MEANWHILE, A LINE OF WEAKENING CONVECTION IS
EXPECTED TO ENTER THE NC MOUNTAINS LATER THIS AFTN, AND CONTINUE
WEAKENING AS THE ACTIVITY TRIES TO CROSS THE MOUNTAINS TO THE
EAST. KAVL WILL LIKELY HAVE SOME SHRA AROUND THIS EVENING, SO WILL
CONTINUE A PROB30 THERE. SOME OF THE ACTIVITY MAY EVEN REACH KHKY,
BUT NOT EXPECTED TO IMPACT THE UPSTATE OR KCLT. A COLD FRONT WILL
PUSH THRU OVERNIGHT, TOGGLING WINDS FROM SW TO N/NE. GUIDANCE IS
MIXED ON HOW MUCH LOW STRATUS AND/OR FOG MAY BE ABLE TO FORM, AS
THE FRONT SLIPS THRU, SO WILL HANDLE WITH FEW/SCT LOW CLOUDS LATE
TONIGHT THRU THU MORNING (MAINLY THE UPSTATE). THERE IS EXPECTED
TO BE ENOUGH MIXING TO LIMIT FOG DEVELOPMENT, BUT SOME FOG CANNOT
BE RULED OUT IN THE PIEDMONT. AT KAVL, GUSTY N/NW WINDS EXPECTED
BEHIND THE FRONT, WITH POSSIBLY A BRIEF PERIOD OF MVFR CIGS. VFR
CONDITIONS WITH NE WINDS EXPECTED DURING THE DAY THU, BUT SOME
GUIDANCE SHOWS MVFR CLOUDS FORMING ACROSS PORTIONS OF THE UPSTATE
AND NC PIEDMONT TOWARD THE END OF THE 18Z TAF PERIOD.
OUTLOOK: BEHIND THE COLD FRONT, HIGH PRESSURE AND DRY CONDITIONS
LAST THROUGH THE WEEKEND, WITH THE EXCEPTION BEING A CHANCE FOR
SNOW ALONG THE TN BORDER FRI NIGHT INTO SAT WITH A REINFORCING
COLD FRONT. PERIODIC CIG RESTRICTIONS CAN'T BE RULED OUT UNTIL
THAT FRONT PASSES.
GSP WATCHES/WARNINGS/ADVISORIES
GA...NONE.
NC...NONE.
SC...NONE.
SYNOPSIS...DEO
NEAR TERM...ARK
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...ARK
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