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FXUS62 KGSP 100803 AFDGSP

Area Forecast Discussion National Weather Service Greenville-Spartanburg SC 303 AM EST Sun Dec 10 2023

SYNOPSIS
Periods of rain will persist ahead of an approaching cold front today, with embedded thunderstorms developing at times. The cold front will pass through the area by early evening and gradually end the precipitation chances from the west for most of the area tonight. The main exception will be the North Carolina mountains, where colder air spilling in behind the front will produce high elevation snow and windy conditions into Monday morning. Dry high pressure then settles over the region on Monday and persists through the work week.

NEAR TERM /THROUGH TONIGHT/
As of 255 am EST Sunday: Water vapor imagery shows a prominent upstream trough axis stretching from Lake Michigan to Arkansas early this morning, with the base of the trough sharpening up near the Mississippi River Valley. An associated surface cold front was analyzed just west of the southern Appalachians and this boundary continues to show slow but steady progress eastward. Regional radars show showers and embedded thunderstorms becoming more widespread across the mountains and poised to move east across the foothills and Piedmont through the rest of the morning hours as upper jetlet divergence, increasing deep-layer DPVA, and frontal circulations promote strong forcing. Instability ahead of the main frontal band remains the primary limiting factor for more widespread severe thunderstorm potential, with little to no SBCAPE indicated across the forecast area at present, but with 100 to 500 J/kg of MUCAPE encroaching on the southern half. The convection should increase in strength through late morning as dewpoints recover into the lower 60s from the southeast. In addition, surface-3km bulk shear values are on the upswing and should reach 45 to 50 kt for most of the area as the southwesterly low-level jet intensifies as it translates east across the foothills and then the Piedmont through late morning. All convective cells and lines developing in this regime will have to be watched very closely for isolated wind/tornado potential, mainly closer to the escarpment areas early and then tending toward I-77 late this morning and this afternoon. In addition, a surface wave will likely ride northward along the frontal zone this afternoon, and this feature could focus both the tornado and the heavy rainfall threat nearer the I-77 corridor as the day progresses.

The very low stream and river levels across the region do not support enough of a hydro threat for any Flood Watches. However, the additional 1 to 2 inch amounts expected today could still yield isolated hydro problems, especially in the increasingly worked-over southern escarpment areas and also in flood prone locations nearer I- 77 in any heavier rainfall rates.

The advancing upper trough should go negative tilt this evening, slightly slowing the back edge of the precipitation through the evening hours. Still anticipate most areas drying out after 06Z Monday. However, snow levels will crash down across the mountains quickly tonight behind the departing front, and even some I-40 locations could see a quick mix over to snow before the precipitation ends in the early morning hours. The latest mesoscale models depict slightly more QPF in the NW flow upslope prone locations overnight and this will warrant posting a Winter Advisory for the northern NC mountains tonight. Will also wrap some high elevation 40 to 50 mph wind gusts in the WSW product. Breezy to windy conditions will occur elsewhere, but with gusts below advisory criteria at all but the highest elevations.

SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/
As of 300 am EST Sunday: NW flow moisture will dry up quickly in the northern mountains Monday morning, but cold temperatures in the 20s will support continued slippery road conditions. The WSW will be maintained for the northern mountains through noon. An SPS could be warranted in other parts of the NC mountains for Monday morning travel and black ice issues, but will hold off on this for now until the cold air gets closer. Wind issues will linger through the morning and into the early afternoon, but with the pressure gradually relaxing later in the day.

Upper flow quickly goes zonal on Monday and persists through Tuesday night. Temperatures will remain below climo through the period as dry surface high pressure builds over the region. Another cold front will likely lay over to the north of the area Tuesday night, but with little to no attendant precipitation.

LONG TERM /WEDNESDAY THROUGH SATURDAY/
As of 1230 AM Sunday...Most of the medium range remains quiet, as dry sfc high pressure dominates the region under a NWLY confluent flow aloft. A reinforcing dry cold front will drop south thru the forecast area late Wednesday, keeping temps a little below normal, despite plenty of sunshine. An upper ridge axis will begin to shift east from the Lower-MS Valley to the Southeast Friday, allowing temps to start a slow warming trend. The latest guidance is in good agreement on the fcst remaining dry thru Friday, but then diverge quickly on what's going to happen over the weekend. Given the lack of agreement in the model guidance, have gone with the NBM for next weekend's fcst. This results PoPs ramping up to chance by Saturday night, with temps mainly in the upper 30s to mid 40s. Right now going with a rain fcst, except for possibly some snow in the highest elevations.

AVIATION /08Z SUNDAY THROUGH THURSDAY/
At KCLT and elsewhere: Widespread showers and thunderstorms ahead of a cold front are poised to arrive from the west across the terminal forecast area through the rest of the morning hours, with associated IFR visibility and IFR to LIFR cigs expected. Given the potential for rogue lightning strikes just about anywhere at any time, will ride mainly on a VCTS mentions along with the prevailing SHRA for the mountains and foothills, but then feature a PROB30 window of TSRA for KCLT starting at 17Z. Foothill and Piedmont precipitation will taper off from the west this evening. Winds will increase from the south ahead of the advancing front and then shift to westerly with fropa late day, with NW flow through the evening and overnight hours. Gusts may be quite robust, above 30 kt, at KAVL tonight in the peak of the cold advection flow. KAVL could also see some snow showers mix in at times overnight before the NW flow moisture diminishes by daybreak Monday.

Outlook: Dry high pressure will return from the west on Monday and persist through the week.

GSP WATCHES/WARNINGS/ADVISORIES
GA...None.
NC...Winter Weather Advisory from 6 PM this evening to noon EST Monday for NCZ033-049-050.
SC...None.

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