textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Updated Aviation discussion. No significant changes to the public forecast since this afternoon.

KEY MESSAGES

1. A wintry mix of snow, sleet, and freezing rain will affect the North Carolina mountains and possibly the mountains of northeast Georgia and the Upstate tonight through Saturday morning. Only minor accumulations are expected, but could result in minor travel issues, mainly above 3500 ft. 2. A low pressure system moves into the area Sat night and Sunday with a chance of rain or snow, mainly outside of the NC mountains. Guidance continues to trend toward minimal impacts. 3. Another cold air mass could settle over the region by the middle of next week. A Cold Weather Advisory may be needed for parts of the mountains Monday night into Tuesday. 4. A low-pressure system may move over or near the area Thursday or Friday. Sensible weather and any impacts are highly uncertain and confidence is very low at this time.

DISCUSSION

Key message 1: A wintry mix of snow, sleet, and freezing rain will affect the North Carolina mountains and possibly the mountains of northeast Georgia and the Upstate tonight through Saturday morning. Only minor accumulations are expected, but could result in minor travel issues, mainly above 3500 ft.

An upper trough will continue to dig over the central CONUS, as a series of shortwaves dive south thru its axis over the Plains to the Mid-MS Valley. Backing deep-layer SWLY flow will set up atop the CWFA tonight. Height falls ahead of this trough will provide some mid and upper forcing for precip. The latest guidance is in good agreement on a narrow and relatively meager band of moisture associated with the trough crossing the TN Valley and pushing into the Southern Appalachians around midnight tonight. Other than some SLY to SWLY upslope lift within a strong 50-60 kt LLJ, low-level forcing looks fairly weak, as frontogenesis bands are strongest to our west and become weak and noisy as they interact with the mountains. Overall, QPF looks light with this event, and models have trended drier for all but the upslope areas west of the French Broad Valley. Forecast soundings show a warm nose in place as profiles moisten, and should support a mixed bag of snow, sleet, and freezing rain at the onset (but mostly just snow on the highest peaks).

Precip falling into an initially very dry boundary layer will support temps locking in around or just below freezing in some sheltered mountain valleys and along the Escarpment overnight. Fortunately, these areas will also likely see the least precip due to downsloping and evaporation. I did not deviate much from the NBM for the temps, QPF and snow amounts, as they looked reasonable. A little more sleet and ice mixing in compared to previous forecasts looks to eat into snow totals, and freezing rain still looks spotty and very light (mainly a couple hundredths of an inch on mostly elevated surfaces). Roads should start out above freezing, given today's relatively warm temps and sunny skies. So at this point, I think the potential wintry precip hazards can be handled with a mention in the HWO and an SPS. Precip outside the mountains should be all liquid (except perhaps the northern NC foothills, which could see trace amounts of wintry mix). Temps will hover in the 20s to lower 30s in the mountains and mainly mid 30s elsewhere. The strong SWLY low-level jet around 50-60 kt just ahead of and leading edge of the moisture may result in strong wind gusts along the ridgetops of the Smokies and Balsams. But with most of that jet advecting warm air, not expecting gusts to warrant an advisory.

Longwave upper trough will continue to deepen and sharpen up across the Mid-MS Valley to the Southern Plains, but a dry slot will punch in by daybreak Saturday and cut off precip across the forecast area. Clearing skies and warm SWLY flow should allow temps to rebound into the upper 30s to upper 40s in the mountains and lower to mid 50s in the Piedmont. Thus, any slippery road conditions should start improving by late morning.

Key message 2: A low pressure system moves into the area Sat night and Sunday with a chance of rain or snow, mainly outside of the NC mountains. Guidance continues to trend toward minimal impacts.

A strong short wave swings through the deep upper trough over the eastern CONUS on Sunday. This causes cyclogenesis to take place along a frontal boundary off the Carolina coast. Deep moisture increases in the developing southwesterly flow ahead of the short wave. However, the guidance continues to show the axis of this moisture remaining west and south of the area. Frontogenetical forcing takes place in this area producing precip. With the axis of deep moisture and strong forcing to our south and east, the heaviest precip amounts will be there as well. That said, there may be enough moisture and forcing west of the axis for light precip to develop over our area Saturday night and continue into Sunday morning, especially along and south/east of the I-85 and I-77 corridors. Thermal profiles and forecast soundings show a rain/snow scenario. With the light QPF and short precip window, snow accums look to be less than an inch where snow does occur. There are still a couple of guidance members that do show around an inch or so of snow for those areas; however, guidance probabilities of greater than an inch are very low. Therefore, at this time any impacts are expected to be minor.

Key message 3: Another cold air mass could settle over the region by the middle of next week. A Cold Weather Advisory may be needed for parts of the mountains Monday night into Tuesday.

A dry cold front crosses the area Monday ushering in a very cold air mass by Tuesday. Lows Monday night will be 10 to 20 degrees below normal. With breezy conditions across the mountains wind chills look to fall into the Cold Weather Advisory range, possibly even for the NE GA and Upstate mountains. Lows Tuesday night will be a little warmer, especially across the mountains. Much lighter winds will significantly lower the chance of significantly cold wind chills.

Key message 4: A low-pressure system may move over or near the area Thursday or Friday. Sensible weather and any impacts are highly uncertain and confidence is very low at this time.

Operational and ensemble guidance show some version of a Miller A type low moving near the area Thursday or Friday. Given that they all depict various timings, precip amounts, and thermal profiles, confidence in any deterministic solution is very low. As a result, have gone with the model blend which has temps near or even a little above normal and low precip chances.

AVIATION /00Z SATURDAY THROUGH WEDNESDAY/

At KCLT and elsewhere: VFR/mostly SKC conditions this evening will give way to increasing cloud cover and probable restrictions as a cold front sweeps across the Terminal Forecast Area from the west. Very dry air ahead of the front will limit the precipitation potential at the TAF sites, but in light of a narrow band of moisture that will accompany the front, Prob30s are warranted at the upstate SC terminals and KAVL, mainly between 09- 12Z. While mostly -RA is expected, the potential for -SN is high enough to include in KAVL's Prob30. Also can't rule out a brief period of -FZRA at KAVL. Otherwise, MVFR cigs are expected to fill in at all sites in the 09-13Z time frame. IFR cigs are possible, but confidence isn't high, and the mention is carried only in Prob30 groups. Cigs should scatter throughout the morning...with VFR/mostly SKC expected at all sites by afternoon. SSW winds of 5-10 kts expected at most sites through much of the period, although an increase to a little over 10 kts is expected at the upstate SC terminals and at KCLT by late Sat morning, with some higher gusts possible during the afternoon.

Outlook: A cold front may bring a quick shot of precipitation and associated restrictions Saturday night into Sunday, mainly across the far southeastern parts of the forecast area, but confidence remains low. Dry and VFR conditions return early next week.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...None. SC...None.