textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Patchy black ice is possible in locations that received accumulating snow from this morning, especially on untreated bridges and secondary roads.

A Winter Weather Advisory was issued for Avery County and elevations above 3500 feet for Madison, Yancey, and Mitchell counties from 18Z Friday through 12Z Saturday. Snow amounts have increased sufficiently at high elevations along the TN border to warrant an Advisory.

A High Wind Watch was issued for the NC mountain zones on the east side of the spine, as well as the NC foothills/NW Piedmont, for the period from 00Z Saturday through 00Z Sunday. Damaging wind gust potential in those areas continues to increase.

Red Flag Warning conditions are looking like a greater possibility for Saturday east of the mountains, especially across the NC foothills.

KEY MESSAGES

1. Lingering light snow will continue into the early afternoon hours before dissipating. Patchy black ice possible overnight into Friday morning, especially in locations with light snow accumulations from today. 2. A Winter Weather Advisory will be issued for Avery County and elevations above 3500 feet in Madison, Yancey, and Mitchell counties from 18Z Friday through 12Z Saturday for snow accumulation that will result in some travel impacts. 3. A High Wind Watch will be issued for the east side of the NC mountains and the NC Foothills from 00Z Saturday through 00Z Sunday. Wind gusts of 45 to 55 mph will be possible, especially near the Blue Ridge Escarpment on Saturday morning, which may knock down tree limbs and cause power outages. Some locations east of the Blue Ridge could get near Red Flag Warning conditions due to frequent strong gusts and afternoon RH dropping down to 25% or less, leading to dangerous fire behavior. 4. Cold again Saturday night across the mountains, possibly cold enough for a Cold Weather Advisory, but a warming trend is still expected next week.

DISCUSSION

Key message 1: Lingering light snow will continue into the early afternoon hours before dissipating. Patchy black ice possible overnight into Friday morning, especially in locations with light snow accumulations from today.

Upper trough axis in the midst of slipping across the CWFA as it tilts more neutral and should provide enough dry air entrainment later this afternoon to help the band of snow to dissipate, while scattering clouds as well. The band of snow has been stubborn, but current radar shows the higher returns diminishing as it moves across the I-85 corridor. Cloud cover and lingering precipitation will keep afternoon highs today 5-10 degrees below normal. Scattering clouds and weak CAA will allow temperatures to drop 6-12 degrees below normal for overnight lows, closer to 12 than 6 in locations with a snowpack. Any meltwater or slush leftover on the roadways will refreeze tonight, creating black ice and slick spots in the NC foothills and Northwest Piedmont for the Friday morning commute. Use extra caution on the roadways tonight into Friday morning, especially on untreated bridges and secondary roads.

Key message 2: A Winter Weather Advisory will be issued for Avery County and elevations above 3500 feet in Madison, Yancey, and Mitchell counties from 18Z Friday through 12Z Saturday for snow accumulation that will result in some travel impacts.

The model guidance continues to slowly trend upward on the snow potential beginning Friday afternoon and continuing overnight through daybreak Saturday over the mountains, mainly near the TN border, to the point where the forecast supports issuing a Winter Weather Advisory for parts of the northern mountains. Wintry precip may come in two waves. The first round of precip will be associated with the passage of a reinforcing cold front that trails an Alberta Clipper low moving across the Great Lakes and southern Ontario on Friday. The precip should arrive after noon with the cold front, but this initial burst will have support of a compact short wave that dives down from the NW with a quick shot of DPVA to enhance the snow production. The short wave should force some light precip beyond the NW flow upslope zones, and there's a chance of precip out over the nrn foothills and NW Piedmont in the afternoon, but it should be too warm for snow east of the mountains. In the wake of the wave, there could be lull where the precip tapers off to something more like rime icing or maybe even freezing drizzle in some spots Friday evening as the moisture becomes more shallow briefly, but this detail will not be included yet because of low confidence. Another round of precip will take place late Friday night and early Saturday morning as deeper moisture drops down from the NW and saturates the profile up into the bottom of the dendritic snow growth zone. The two waves combined would be just enough to push Avery County and the other high elevations of the nrn mountains into the Advisory range on average, though amounts will be highly variable. The snow threat should end by daybreak Saturday as the moisture pulls out while the wind picks up. More on that below...

Key message 3: A High Wind Watch will be issued for the east side of the NC mountains and the NC Foothills from 00Z Saturday through 00Z Sunday. Wind gusts of 45 to 55 mph will be possible, especially near the Blue Ridge Escarpment on Saturday morning, which may knock down tree limbs and cause power outages. Some locations east of the Blue Ridge could get near Red Flag Warning conditions due to frequent strong gusts and afternoon RH dropping down to 25% or less, leading to dangerous fire behavior.

In the wake of the short wave that moves across the region late in the day on Friday, model guidance shows the 850 mb flow really picking up from the NW during the overnight hours. Winds at 850 mb improve to greater than 50kt east of the spine of the Appalachians after 06Z Saturday with 60-65kt winds indicated near the Escarpment around 12Z-15Z Saturday. Meanwhile, the pressure difference across the mountains reaches 10 mb Saturday morning, which is usually good enough to result in howling winds on the east side of the mtns during strong NW flow cold advection regimes. Fcst soundings are split on how deeply the boundary layer will be mixed during this time, with the NAM suggesting that at least Advisory-level gusts will be frequent across the NC foothills. The prob of wind gusts greater than 45kt climbs into the 50% range Saturday morning in that area as well. The wind gust potential supports at least an Advisory in these areas, but there are indications of a greater impact and some warning-criteria potential. So...a High Wind Watch will be issued for the zones on the east side of the mtns and then out across the NC foothills and we will let the next shift decide if any of that area would warrant a Warning. Expect a Wind Advisory to cover a larger footprint than the High Wind Watch if the model guidance continues to trend in the windier direction.

By Saturday afternoon, model guidance continues to trend even drier than previous runs, with large sections east of the Blue Ridge dropping down below 30% RH in the afternoon. Our forecast often underestimates the amount of downslope drying and mixing in the wake of a cold front, so if anything, the RH may trend farther downward. Meanwhile, there is good potential that wind gusts will remain frequent and at least 25 mph. If the forecast doesn't change, a Fire Weather Watch will have to be considered for parts of the area with coordination from land management agencies.

Key message 4: Cold again Saturday night across the mountains, possibly cold enough for a Cold Weather Advisory, but a warming trend is still expected next week.

Looks like one more cold night on Saturday night as high pressure moves across the central Appalachians, with low temps dropping down into the 10-15 degree range below normal. If not for the wind also dying down, we would be looking at a Cold Weather Advisory for parts of the mountains once again, but as it is the situation looks borderline right now.

That might do it for the cold for awhile. The upper pattern will finally become unblocked and progressive beginning on Sunday. Temps start to rebound then, but will still be below normal. The flow pattern flattens considerably by Monday and it should start to feel like Spring with high temps finally above normal. Guidance continues to indicate highs in the 60s by mid-week, with a southern stream system moving through Wednesday that would bring only rain even to the high elevations.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

At KCLT and elsewhere: Drier air working into the region has allowed for the band of precip to gradually dissipate and should fully end over the next couple of hours. Kept a TEMPO for -SN mention and associated restrictions for KGSP/KGMU/KCLT through 19Z, but could end before that. Otherwise, dry conditions will be in store through the end of the forecast period. Northeast winds continue for most of the terminals through today before going light and variable overnight (KAVL maintains a north-northwesterly component). Lingering MVFR/Lower VFR cigs are expected to last through 20-21Z before lifting and scattering across the area. Some fast moving VFR mid/upper-level clouds will move in overnight tonight with a cold front approaching the region, but should scatter out after daybreak Friday and the lower cloud cover and precip remaining confined to the mountains along the NC/TN border. Can't rule out a few pockets of vsby restrictions, especially where snow fell today, so included an MVFR mention at KHKY before daybreak Friday. Winds turn southwesterly on Friday and should pick up at 5-10 kts by the afternoon, with low-end gusts developing and should increase by the evening hours.

Outlook: Dry, VFR conditions are expected through the weekend. Gusty winds develop later Friday in association with a cold front and linger across the mountains through Saturday, but will gradually taper off east of the mountains early Saturday.

GSP WATCHES/WARNINGS/ADVISORIES

GA...None. NC...Winter Weather Advisory from 1 PM Friday to 7 AM EST Saturday for NCZ033-048>050. High Wind Watch from Friday evening through Saturday evening for NCZ033-035-049-050-053-063>065-501>510. SC...High Wind Watch from Friday evening through Saturday evening for SCZ103.