474
FXUS62 KGSP 180845
AFDGSP
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
345 AM EST WED DEC 18 2024
SYNOPSIS
TEMPERATURES WILL BE SEVERAL DEGREES ABOVE NORMAL TODAY, WITH A
STRONG COLD FRONT APPROACHING FROM THE WEST AND INCREASING RAIN
CHANCES AREA-WIDE THIS AFTERNOON AND EVENING. COOLER CONDITIONS
ARE EXPECTED BEHIND THE FRONT THROUGH THE WEEKEND WITH TEMPERATURES
A CATEGORY OR TWO BELOW NORMAL BY SUNDAY AND MONDAY.
NEAR TERM /THROUGH TONIGHT/
AS OF 300 AM: STALLED FRONT APPEARS TO BE NEAR THE NC/VA
BORDER. MAIN SYNOPTIC SCALE TROUGH IS STILL MOVING THRU THE PLAINS,
BUT SOUTHWESTERLY FLOW APPEARS TO BE INDUCING SHALLOW WARM ADVECTION
AND EFFECTIVELY HAS REACTIVATED THE FRONT. A WEAK RIPPLE OF VORT
PRECEDING THE TROUGH HAS SPAWNED A FEW RADAR RETURNS BUT THESE
APPEAR TO BE OF LITTLE CONSEQUENCE. WE REMAIN IN A VERY HUMID
AIRMASS AND AREAS OF FOG HAVE FORMED, SOME OF IT HAVING BEEN
DENSE SO FAR THIS MORNIGN. IT STILL HAS NOT BECOME SOLIDLY 1/4SM
ACROSS THE AREA, THOUGH CONTIGUOUS AREAS HAVE DEVELOPED IN THE
UPPER PIEDMONT, SEEMINGLY WHERE WARM UPGLIDE HAS BEEN OCCURRING
THE LONGEST. THE MAJOR MOUNTAIN VALLEYS HAVE ALREADY SEEN AT
LEAST LOW STRATUS FORM PER SATELLITE, AND AT LEAST PATCHY VSBY
RESTRICTIONS ARE BELIEVED TO BE OCCURRING THERE. TOWARD DAWN,
VSBYS ARE LIKELY TO LOWER IN THE SOUTHERN PIEDMONT WHERE FOG HAS
REMAINED MOST PATCHY SO FAR. WITH THAT, AND FOR SIMPLICITY, HAVE
ISSUED A DENSE FOG ADVISORY TIL 10 AM FOR THE WHOLE CWA.
AFTER THE FOG DISSIPATES, TEMPERATURES SHOULD WARM NICELY
INTO THE UPPER 60S TO NEAR 70 ACROSS THE PIEDMONT AND MOUNTAIN
VALLEYS. DEWPOINTS WILL REMAIN IN THE MID TO UPPER 50S IN MOST
AREAS. AN INVERSION WILL PERSIST, AND SOME PATCHY LOW CLOUD COVER
MAY REMAIN INTO AFTERNOON BENEATH IT. THE SHORTWAVE WILL TRACK EAST
ACROSS THE LOWER OH/TN VALLEYS TODAY. MCS NOW NEAR THE MS RIVER
WILL WEAKEN DIURNALLY, ALTHOUGH THE CAMS TAKE IT ALMOST TO THE
NC BORDER. REMNANT SHOWERS HOWEVER ARE LIKELY TO REACH THAT AREA
BY 4-5 PM. EAST OF THE MOUNTAINS, A DRY LAYER ALMOST LIKE AN EML
WILL PERSIST ABOVE THE INVERSION. VARIOUS MODELS DEPICT SMALL BUT
RESPECTABLE SBCAPE ON PLAN VIEWS, BUT THE EML-LIKE FEATURE MOST
LIKELY WILL INHIBIT DEEP CONVECTION MOST OF THE DAY, AND UPGLIDE
BENEATH THE INVERSION APPEARS TOO WEAK TO EXPECT MUCH CHANCE OF
POP FROM THAT ALONE. HEIGHT FALLS WITH THE SHORTWAVE, HOWEVER,
REFLECT COOLING MIDLEVELS, AND CONVECTION LOOKS SOMEWHAT PLAUSIBLE
IN OUR SW CWA BY MID-AFTERNOON, WITH POPS INCREASING TO CHANCE
RANGEE THERE ACCORDINGLY. THERE LIKELY WILL BE A SHARP INSTABILITY
GRADIENT NEAR OUR SE BORDER OR PERHAPS IN THE MIDLANDS--CAMS
ARE SPLIT AS TO WHERE THIS DEVELOPS. SAID GRADIENT ALSO MAY BE A
FOCUS FOR CONVECTION. HENCE SMALL CHANCE POPS ALSO CREEP IN FROM
THE SOUTH. WITH SO LITTLE GUIDANCE SUPPORT FOR PRECIP UNDER THE
INVERSION, CONSIDERED KEEPING THE NC PIEDMONT/FOOTHILLS DRY THRU
THE DAYTIME PERIOD, BUT GIVEN APPROACHING SHORTWAVE AND PREVIOUS
FCST CONTINUITY, ALLOWED A SLIGHT-CHANCE MENTION THERE LATE IN THE
DAY. SMALL THUNDER CHANCES ARE INCLUDED IN THE SOUTH AND FAR WEST
EDGES OF THE CWA; EFFECTIVE SHEAR OF 30 KT COULD ALLOW SOME MORE
VIGOROUS STORMS ALTHOUGH PROFILES ARE UNIDIRECTIONAL. EXCLUDING
THE EXCITABLE HRW-FV3, CAMS REALLY ARE NOT ALL THAT IMPRESSED
WITH THE SETUP FOR CONVECTION IN OUR SOUTHERN CWA. THAT SAID,
IF A STORM FORMS, IT WOULD NOT BE ALL THAT SURPRISING IF IT POSED
A VERY SMALL RISK OF DAMAGING WIND. PARTICULARLY WITH RELATIVELY
SMALL POP OVERALL, THE LACK OF AN SPC RISK AREA APPEARS APPROPRIATE.
COLD FRONT WILL SWEEP ACROSS THE MOUNTAINS THIS EVENING AND POPS
BEGIN TO DECLINE AFTER SUNSET. LAPSE RATES IMPROVE IN OUR EAST
AS THE SHORTWAVE ENTERS, SO THE BEST OVERALL CHANCE FOR THE I-77
CORRIDOR IS AFTER 00Z, BUT STILL ONLY AROUND 40%. ANY SHOWERS SHOULD
END BY MIDNIGHT, BEFORE TEMPS COOL ENOUGH IN THE TN BORDER AREA
TO BE CONCERNED FOR A CHANGE TO SNOW. MINS DIP TO AROUND FREEZING
IN THE HIGH ELEVATIONS AND GENERALLY IN LOWER 40S IN THE VALLEYS,
AND MID 40S FOR THE PIEDMONT.
SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/
AS OF 200 AM EST WEDNESDAY: IN BETWEEN TWO SHORTWAVES WITH THE
NEAR-TERM TROUGH SLIPPING EAST OF THE CFWA AND A POTENT SHORTWAVE
DIGGING INTO THE NORTH-CENTRAL CONUS WITH A SURFACE LOW FOLLOWING
UNDERNEATH WILL LEAD TO A QUIET WEATHER PERIOD ON THURSDAY. WEAK
HIGH PRESSURE WILL SETTLE OVER THE REGION DURING THE DAY THURSDAY
WITH ONLY SUBTLE HEIGHT FALLS. AFTERNOON HIGHS ARE EXPECTED TO RUN
AT OR SLIGHTLY ABOVE NORMAL AS A RESULT. WINDS ATOP THE BOUNDARY
LAYER AND ALOFT WILL VEER MORE SOUTHWESTERLY THURSDAY NIGHT, WHILE
A NOTICEABLE DRY SLOT IS EVIDENT IN THE MID-LEVELS, CREATING A
LOW-LEVEL INVERSION LAYER. WITH A SLIGHT UPTICK IN 850MB WINDS
(15-20 KTS) OUT OF THE SOUTHWEST, LOW-LEVEL MOISTURE WILL BECOME
TRAPPED UNDERNEATH THE INVERSION AND MAY CREATE A LOW STRATUS
DECK OVERNIGHT THURSDAY. IN THIS CASE, OVERNIGHT LOWS THURSDAY
WILL RUN A FEW DEGREES ABOVE NORMAL, ESPECIALLY IF THE BOUNDARY
LAYER STRUGGLES TO FULLY DECOUPLE.
THE AFOREMENTIONED SHORTWAVE WILL SWING ACROSS THE MIDWEST AND OH
VALLEY THURSDAY AND SET UP SHOP OVER THE NORTHEAST AND MID-ATLANTIC
BY FRIDAY. THE ASSOCIATED SURFACE LOW WILL EJECT IN CLIPPER FASHION
NORTH OF THE CFWA. HOWEVER, ON THE BACKSIDE OF THE LOW, LOW-LEVEL
NORTHWEST FLOW FILTERS IN FRIDAY AFTERNOON/EVENING AS A VORT MAX
ROUNDS THE BASE OF THE SHORTWAVE AND CRASHES INTO THE CFWA. RECENT
MODEL GUIDANCE PRODUCES SLIGHTLY BETTER SATURATION IN LOW-LEVEL
PROFILES AND SUGGESTS THAT THE PRECIPITATION WILL BE COLD ENOUGH
FOR WINTRY P-TYPES, WHILE THE AVAILABLE MOISTURE SHOULD BE ABLE TO
TAP INTO THE DENDRITIC GROWTH ZONE. EXPECT A PERIOD OF NORTHWEST
FLOW SNOW ALONG THE NC/TN BORDER FRIDAY AFTERNOON THROUGH SATURDAY
MORNING. THE POSITION OF THE VORT MAX COULD FAVOR A FEW NORTHWEST
FLOW SNOW SHOWERS BREAKING CONTAINMENT FURTHER EAST INTO THE NORTH
CAROLINA MOUNTAINS, POTENTIALLY IN THE PARTS OF THE MAJOR MOUNTAIN
VALLEYS. A SECOND AREA OF LIGHT PRECIP IS NOW BEING PICKED UP BY THE
LATEST MODEL GUIDANCE ACROSS THE CLT METRO AND LOCATIONS ALONG AND
EAST OF I-77 FRIDAY AFTERNOON/EVENING IN ASSOCIATION WITH THE VORT
MAX AS IT TRAVERSES ACROSS THE CAROLINAS. PROFILES ARE TOO WARM
FOR WINTRY P-TYPES AND A VERY DRY SUB-CLOUD LAYER SUGGESTS THAT
ANY PRECIP THAT FALLS WILL HAVE TO INDUCE EVAPORATIONAL COOLING
BEFORE REACHING THE SURFACE. INTRODUCING SLIGHT CHANCE POPS IN
THE SPECIFIED LOCATIONS EAST OF THE MOUNTAINS FOR MEASURABLE
PRECIP, BUT THE DEVELOPMENT OF VIRGA WILL BE EVIDENT IF ANYTHING
ELSE. AFTERNOON HIGHS FRIDAY WILL BE NEAR-NORMAL FOR LOCATIONS
EAST OF THE MOUNTAINS AND MAJOR MOUNTAIN VALLEYS, WHILE INCREASING
CLOUD COVER AND CAA WILL KEEP TEMPERATURES IN THE 30S OVER THE
HIGHER ELEVATIONS.
ONGOING NORTHWEST FLOW SNOW ALONG THE NC/TN BORDER WILL GRADUALLY
DWINDLE IN COVERAGE OVERNIGHT FRIDAY AS THE VORT MAX SHIFTS
NORTHEAST OF THE CFWA. LINGERING LOW-LEVEL MOISTURE AND SCATTERED
VORT ENERGY ALOFT BEHIND THE MAIN VORT MAX WILL KEEP SNOW SHOWERS
GOING THROUGH AT LEAST DAYBREAK SATURDAY. SNOW AMOUNTS WILL VARY A
LOT, BUT CAN'T RULE OUT A COUPLE OF INCHES IN THE USUAL NORTHWEST
FACING SLOPES ALONG THE NC/TN BORDER, ESPECIALLY IN THE NORTHERN
MOUNTAINS. WITH SIGNIFICANT HEIGHT FALLS RESULTING IN COLDER AIR
ALOFT MOVING INTO THE AREA FRIDAY NIGHT, OVERNIGHT LOWS WILL DIP
A FEW TICKS BELOW-NORMAL FOR MOST LOCATIONS.
LONG TERM /SATURDAY THROUGH TUESDAY/
AS OF 310 AM EST WEDNESDAY: ANY LINGERING NORTHWEST FLOW SNOW WILL
END SATURDAY MORNING FOLLOWED BY BREEZY CONDITIONS THANKS TO A TIGHT
PRESSURE GRADIENT WITH CAA PRESENT. BROAD CYCLONIC FLOW ALOFT AND
COLDER THICKNESSES WILL ALLOW TEMPERATURES TO DIP 5-10 DEGREES BELOW
NORMAL SATURDAY, WHILE A SHORTWAVE ROUNDS THE PERIPHERAL OF THE
BROAD UPPER TROUGHINESS IN PLACE TO HELP REINFORCE THE COLDER AIR
ALOFT ON SUNDAY, WHILE A LARGE AND STRONG SURFACE HIGH (~1040MB)
SLIPS INTO THE GREAT LAKES AND EVENTUALLY THE NORTHEASTERN CONUS
SUNDAY INTO SUNDAY NIGHT. THE LOCATION OF THE HIGH WILL FILTER IN A
CAD-LIKE CONFIGURATION INTO THE CFWA AND REMAIN IN PLACE GOING INTO
THE EARLY PART OF NEXT WEEK. A FLAT SHORTWAVE RIDGE BUILDS SHIFTS
OVER THE AREA MONDAY INTO TUESDAY AND THE HIGH SLIPS OFFSHORE BY
TUESDAY IN RESPONSE, AHEAD OF A CENTRAL CONUS UPPER TROUGH. AS A
RESULT, TEMPERATURES ARE LIKELY TO DROP 10-15 DEGREES BELOW NORMAL
SUNDAY AND MONDAY, WITH ONLY GRADUAL MODIFICATION BY TUESDAY. MUCH
COLDER CONDITIONS EXPECTED DURING THE MEDIUM RANGE, BUT THE SENSIBLE
WEATHER SHOULD REMAIN QUIET THROUGH THE FORECAST PERIOD.
AVIATION /09Z WEDNESDAY THROUGH SUNDAY/
AT KCLT AND ELSEWHERE: PATCHY/VARIABLE DENSE FOG AROUND THE TERMINAL
AREA TO START THE PERIOD. RAPID CHANGES IN FLIGHT CATEGORY GENERALLY
LOOK TO CONTINUE IN THE FIRST FEW HRS, ALTHOUGH AS HIGHER CLOUD
DECKS ADVECT EAST CLOSER TO DAWN, THE FOG PROBABLY WILL SOLIDIFY
AND MOST LIKELY LAST UNTIL 14-16Z. ALL SITES WILL BEAR SOME RISK
OF FALLING BELOW MINIMA UNTIL THE FOG LAYER DISSIPATES IN THE EARLY
DAYTIME. INVERSION CONTINUES SUCH THAT PATCHY MVFR CIGS WILL LINGER
MOST OF THE REMAINDER OF THE DAY. WINDS START OFF MOSTLY CALM/VRB
BUT ARE EXPECTED TO PICK UP FROM THE SW AFTER DAYBREAK. APPROACH
OF COLD FRONT THIS AFTN/EVENING WILL BRING A CHANCE OF -SHRA AND
MAYBE EVEN A FEW TSRA, BUT GIVEN SHALLOW MOIST LAYER AND INHIBITING
INVERSION, THIS IS MENTIONED ONLY AS VCSH FOR NOW; WILL CONSIDER
PROB30S FOR NEXT ISSUANCE. WINDS TURN TO NW BEHIND THE FRONT,
WITH SHIFT TIMED AROUND 00Z AT KAVL AND AFTER 06Z ELSEWHERE.
OUTLOOK: BEHIND THE COLD FRONT, HIGH PRESSURE AND DRY CONDITIONS
RETURN THURSDAY AND LAST THROUGH THE WEEKEND, WITH THE EXCEPTION
BEING A CHANCE FOR SNOW ALONG THE TN BORDER FRI NIGHT INTO SAT
(BUT NOT LIKELY AFFECTING ANY OF OUR TERMINALS). PERIODIC CIG
RESTRICTIONS CAN'T BE RULED OUT OVER THE WEEKEND BUT SHOULD BE
TOO DRY FOR FOG.
GSP WATCHES/WARNINGS/ADVISORIES
GA...DENSE FOG ADVISORY UNTIL 10 AM EST THIS MORNING FOR GAZ010-017-
018-026-028-029.
NC...DENSE FOG ADVISORY UNTIL 10 AM EST THIS MORNING FOR NCZ033-
035>037-048>053-056>059-062>065-068>072-082-501>510.
SC...DENSE FOG ADVISORY UNTIL 10 AM EST THIS MORNING FOR SCZ008>014-
019-101>109.
SYNOPSIS...CAC/WIMBERLEY
NEAR TERM...WIMBERLEY
SHORT TERM...CAC
LONG TERM...CAC
AVIATION...WIMBERLEY
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