textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Snow totals have increased again in the high elevations of the NC mountains near the TN border, but remain very low elsewhere in the mountains. A Winter Weather Advisory was issued for elevations above 3500 feet in Swain and Haywood counties.

Minor changes were made for the weekend forecast, which remains low-confidence.

KEY MESSAGES

1. Accumulating snowfall is expected to occur at high elevations along the NC-TN border this afternoon and into tonight. Significant impacts are still expected to be confined to elevations above 3500ft, and a Winter Weather Advisory has been issued for elevations above 3500ft in Swain County and Haywood County. 2. Possibly the coldest air mass of the season will settle over the region Thursday night, but low temperatures will NOT reach anywhere close to records. Dangerously cold wind chills will be possible above 3500 ft early Friday morning. 3. Some light snow potential over the mountains Friday night and early Saturday could result in minor travel issues near the TN border. 4. We will be keeping an eye on the potential for winter precip east of the mountains on Sunday, but confidence right now remains relatively low.

DISCUSSION

Key message 1: Accumulating snowfall is expected to occur at high elevations along the NC-TN border this afternoon and into tonight. Significant impacts are still expected to be confined to elevations above 3500ft, and a Winter Weather Advisory has been issued for elevations above 3500ft in Swain County and Haywood County.

Just now seeing the first round of cirrus roll into the SC Upstate ahead of a compact shortwave trough analyzed over the ArkLaTex region. Over the next 12 hours, this feature will phrase with a more robust trough over the Midwest, resulting in the development of a high-amplitude z500 trough extending from the upper Midwest all the way into the lower Mississippi Valley by daybreak. Associated with this, a round of synoptic forcing and associated low-level moist upglide will develop across the US Southeast over the coming hours. Given substantially-dry air upstream as depicted in the 00z FFC RAOB, it seems unlikely that much will result from this, at least initially, besides a steady increase in cloud cover, but most of the CAMs do indicate that shortly after daybreak, a round of drizzle may develop, mainly along and south of I-85, as profiles moisten and isentropic ascent peaks ahead of the shortwave.

Our big weather-maker, however, will not materialize until mid-day, as a surface cold front associated with the deepening trough arrives in the Appalachians. Precipitation will generally start off as rain, except at the highest elevations, but changeover to snow at successively lower elevations will take place throughout the evening and overnight...with Tw cold enough for snow in valleys by 9-10pm based on BUFKIT soundings from the NAMnest and RAP. Soundings also depict a shallow surface-based unstable layer that could boost snowfall rates, contributing to a trend toward greater accumulations at high elevations as compared to previous forecast cycles. Reverse trajectories from the 18z GFS also indicate a weak Great Lakes connection in the arriving air mass...further supporting snowfall totals of 10" or more at high elevations in the Smokies...as depicted in the new NBM and recent runs of the HRRR.

Having said all this...still think impacts will be quite elevation-dependent, with 4-7" possible on some favored, west-facing slopes, and isolated high-elevation locations receiving up to 12" of snow...mostly between midnight tonight and daybreak Thursday morning. Based on this, will hoist a Winter Weather Advisory for elevations above 3500ft in Swain and Haywood Counties for tonight. After 12z Thursday, CAA will steadily ramp down, and hi-res guidance depicts valley moisture tapering off by 18z or so...and so the thinking remains that anything other than brief, isolated flurries will come to an end by Thursday afternoon. The latest guidance depicts somewhat slower onset to CAA and thus less overlap between the cold postfrontal air mass and remaining moisture over the I-77 corridor and eastern Upstate...which translates to less potential for any flurries to develop there as compared with the previous forecast cycle.

Key message 2: Possibly the coldest air mass of the season will settle over the region Thursday night, but low temperatures will NOT reach anywhere close to records. Dangerously cold wind chills will be possible above 3500 ft early Friday morning.

Looking like good old fashioned winter around some parts Thursday night as the sfc high pressure ridge moves overhead and provides for excellent radiational cooling. The low temp fcst continues to creep downward and now has plenty of single digits over the mtns and what would be the lowest min temp so far this season at AVL, but not quite so at GSP and CLT. These low temps are on the order of 15 degrees below normal, but not anywhere close to record lows. The low temps at the high elevations are so cold that we could flirt with Cold Weather Advisory criteria, but not enough of the area reaches criteria as it stands at this time. It won't take much more of a downward trend to require an Advisory. However, our forecast is already down in the colder end of the guidance envelope.

Key message 3: Some light snow potential over the mountains Friday night and early Saturday could result in minor travel issues near the TN border.

By Friday, a new upper low is expected to dive down from Canada and move across the Great Lakes region, with a broad but deep mid/upper trof whose axis remains to our west through the better part of the weekend. A lead short wave coming around the bottom of the trof to our west may bring a shot of DPVA and mid-level isentropic lift across the mtns Friday evening and early Saturday morning, but the moisture return ahead of the trof will be minimal at best. Low level flow will remain W to WSW because of the trof axis hanging back closer to the MS River. In this situation, some light precip generation from higher-based cloud cover will probably happen, but will take some time to reach the ground and when it does, it should be light snow over the mtns. The QPF is for less than a tenth of liquid though, without much potential for higher amounts owing to a lack of deep moisture and low level forcing. The upshot is a chance of maybe up to an inch of light snow accum mainly over the TN border Zones and southwest NC by daybreak Saturday. A few flakes could be seen over the mtns of northeast GA and the nrn Upstate Friday evening as well, but the possibility remains low.

The pattern favors temps rebounding toward normal for the first half of the weekend as the sfc high pressure ridge gets pushed quickly offshore Friday, then the improving westerly flow on Saturday provides some downslope warming over the area east of the mtns.

Key message 4: We will be keeping an eye on the potential for winter precip east of the mountains on Sunday, but confidence right now remains relatively low.

We note some interesting developments in the operational GFS during the latter half of the weekend as the aforementioned upper trof amplifies Saturday and Saturday night with a strong short wave diving down from the northern Plains. The deepening trof will push a cold front across the region late on Saturday and the cold air should return Saturday night, dropping temps back below normal. The short wave depiction on the GFS moving across the Southeast on Sunday looks quite dynamic and would induce some good frontogenesis and isentropic upglide back over the cold air east of the mtns Sunday morning. If precip were to develop over our eastern zones on Sunday it would be snow. Right now, our forecast will remain essentially dry for this day along the lines of the National Blend, which has by far the most support for no precip. The point here is not to wish-cast, but to point out this alternate scenario. We would be remiss to not mention that the operational GFS has some support from its ensemble showing the western edge of a band of wintry precip over the eastern zones, so this will be worth keeping an eye on going forward while managing expectations.

AVIATION /06Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Dry and VFR conditions expected to continue through tonight, with steadily increasing cloud cover, and ceilings above 10kft expected by daybreak. Potential for a round of light -RA at the Upstate terminals Wednesday morning was handled with TEMPOs, before a return to dry conditions in the afternoon. Expect ceilings to slowly lower through the day but remain VFR. A steady 8-12kt SW wind should persist through Wednesday, with increasingly frequent gusts as high as 20kts during the afternoon. Come this evening, another round of -SHRA could develop along and south of I-85, this time potentially resulting in MVFR restrictions developing...and this was handled with PROB30s. For the mountain terminals, more respectable precipitation is expected to develop overnight. At KAVL, there's limited potential for some very light SN to develop, but confidence was too low for a mention in the TAF at this time. Winds should toggle around to the NW overnight and remain gusty through the end of the period.

Outlook: Dry and VFR conditions return on Thursday morning, and linger through at least Friday. Precipitation and associated restrictions could return as early as Saturday, but confidence remains low at this time.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Winter Weather Advisory from 3 PM this afternoon to noon EST Thursday for NCZ051-052. SC...None.