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FXUS62 KGSP 091441 AFDGSP

Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 941 AM EST Sat Dec 9 2023

SYNOPSIS
Rain will develop ahead of an approaching cold front today through tonight, with embedded thunderstorms becoming more likely late tonight through Sunday. As the rain ends Sunday night, colder air behind the front may produce some light snow in the North Carolina Mountains. The forecast then remains quiet through the rest of next week.

NEAR TERM /THROUGH TONIGHT/
As of 930 am EST Saturday: An area of widespread rain showers associated with initial surge of moisture transport and warm advection plume is moving across the western half of the forecast area this morning. PoPs had to be increased substantially and timing sped up to account for radar trends. Likely-to-categorical PoPs are still limited to the western ~2/3 of the CWA this morning, as the better warm/moist advection is expected to lift out to the northeast...and eastward extent of the precip shield should be limited by outrunning of the better forcing.

Otherwise, a 500 mb cutoff low center continues to deepen over the upper Midwest early this morning. The longwave trough associated with this feature continues to dig southward through the Plains.
This trough will become the primary player in a forecast featuring unsettled conditions across the western Carolinas and northeast Georgia over the next 24 hours. As moisture plume associated with warm/moist advection lifts out to the NE this afternoon, showers are expected to become more spotty in nature, with mechanical lift becoming the primary source of lift...mainly near the southern Blue Ridge escarpment as rather strong S/SW flow continues. Anticipate thickening and lowering cloud cover keeping a lid on temperatures, but maxes will still likely reach the 50s in the mountains and 60s elsewhere due to the warm start this morning and the southerly winds.

Forcing and deeper moisture will increase tonight as the digging upstream trough migrates eastward over the Mississippi River Valley.
An upper jetlet east of the trough axis will improve divergence aloft sharply over the southern Appalachians from 06Z to 12Z Sunday, and the convection-allowing models exhibit a robust response to this with an uptick in coverage through the early morning hours. With no strong signal suggesting that Gulf coastal convection will rob moisture influx into the region, fairly widespread heavy rainfall should develop in the pre-dawn hours Sunday, especially in the southerly upslope preferred regions of the extreme southern Appalachians. With some wetting rainfall today through this evening, overnight QPF could be sufficient for isolated flooding to develop with any training showers in and near the mountains late tonight. In addition, the 850 mb southwesterly flow jet will see an uptick to 45 to 50 kt and a steady increase in associated bulk shear by daybreak.
Cannot rule out some surface based instability of a few hundred J/kg sneaking into the Savannah River area and the southwest NC mountains through daybreak, and a marginal severe thunderstorm risk is indicated there as well late tonight. Very warm minimum temperatures will run around 20 degrees above climo tonight.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
As of 330 AM EST Saturday: A pronounced shortwave is progged to ride thru the base of a full-latitude longwave trough across the Southeast CONUS, sharpening the overall trough and pivoting its axis from positive to negatively tilted as it crosses the forecast area (FA). This trend in the guidance is resulting in a slight slowing down of the attendant cold front, expected to be pushing into the western part of the FA at 12z Sunday. The right entrance region of a 300 mb jet should be coupled with the frontal forcing supporting a fairly broad frontal precip band. The latest CAMs show a leading edge of the convective line in the FA, but disagree on the exact placement at 12z Sunday. The NAM seems to be a slow outlier, while the HRRR is right in the middle. Based on the HRRR timing, have a chc thunder mention pushing into the center of the FA (just east of the Blue Ridge escarpment). MUCAPE of up to 500 J/kg along/ahead of this line may support some strong storms, as some of this instability may be sfc-based. Strong low-level shear should be in place to support a damaging wind and isolated tornado threat. This will continue to be highlighted in the HWO. The new Day 2 Convective Outlook has pushed back the marginal risk to include most of our Piedmont zones. Heavy rain will also be a concern, as the frontal speed has trended a little slower in the guidance. Highs will be above normal, mainly in the 50 mountains and 60s Piedmont.

A sharp gradient in thicknesses along the front should result in snow levels falling across the NC mountains fairly quickly Sunday aftn-eve. PoPs should start to cut off east of the mountains as downslope NWLY flow dries out the low-levels by late evening. The NAM is the most aggressive on turning profiles from rain to snow across the NC Foothills and NW Piedmont, with Hickory possibly seeing a few snowflakes before precip completely tapers off. The GFS shows something similar, but with less overlap with moisture. For now, have just a slight chc to low-end chc of snow mixing in with rain showers east of the mountains Sunday night. But no accums are expected. Across the mountains, elevations above 3500 ft may see 1-3" of snow, while the bigger concern in the valleys will be wet roads freezing, as temps plummet into the 20s overnight. Temps approach freezing across much of the Piedmont by daybreak Monday, but roads will have more time to dry and will start out warmer. Another concern in the mountains will be gusty NW winds with strong CAA. May see advisory-level gusts above 3500 ft, especially in the Northern Mountains.

NWLY upslope flow snow showers should taper off along the TN border by midday Monday, as dry air filters in behind the departing cold front and upper trough. Winds should gradually weaken thru the day, but it will be breezy across the mountains. Cold high pressure building in from the west will keep temps well below normal in the mountains and a couple categories below normal east.

LONG TERM /TUESDAY THROUGH FRIDAY/
As of 130 AM EST Saturday: The medium range looks pretty quiet, starting out with slightly below normal temps, but overall, seasonable conditions for mid-December. Dry high pressure will settle over the region under a confluent NWLY flow aloft, which should moderate gradually thru the end of the week. The 00z GFS develops a low pressure system in the Gulf of Mexico and tracks it northeastward, spreading moisture across the FA by early Friday. But this seems to be an outlier, with none of the other deterministic models showing this, and very few of any of the various ensemble members. Precip chances won't likely return until next weekend at the earliest.

AVIATION /15Z SATURDAY THROUGH WEDNESDAY/
At KCLT and elsewhere: After a morning of mostly lower-end VFR ceilings, conditions are expected to steadily deteriorate across the terminal forecast area through the period. Lowering VFR clouds will give way to occasional MVFR cigs from the west through the day, with IFR cigs developing late today through tonight as better coverage of the showers arrives along with deeper moisture. Some thunderstorms will be possible tonight, mainly toward daybreak Sunday. Winds will be light southerly on average, perhaps a bit more southwesterly early this morning and then more southeasterly later in the day. Any low end gusts today should be confined to KAVL given the limited mixing across the foothills and Piedmont, but gusts may creep in throughout overnight.

Outlook: Widespread showers, and embedded thunderstorms, with IFR or lower restrictions are expected for much of Sunday afternoon ahead of an advancing cold front. The front will move east of the area Sunday night, with northwest flow snow showers briefly possible at or near KAVL. Dry and VFR conditions will then return by Monday and persist through the week.

GSP WATCHES/WARNINGS/ADVISORIES
GA...None.
NC...None.
SC...None.

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