textproduct: Greenville-Spartanburg

WHAT HAS CHANGED

Snow totals have increased again in the highest elevations of the NC mountains, but remain very low elsewhere in the mountains.

The aviation discussion was updated this evening to concur with the latest 18Z TAF issuance.

KEY MESSAGES

1. Accumulating snowfall is expected to begin Wednesday afternoon at high mountain elevations along the TN border. Snow level falls over the mountains as temps cool through the evening. Appreciable accumulations and the most significant travel impacts still look to be confined to elevations above 3500 feet. Flurries cannot be ruled out Wed night or early Thu morning in the Piedmont, though accumulation is not currently expected. 2. Wind chills late Wednesday night and early Thursday morning could prove dangerous in some higher elevations, requiring adequate protection from winter clothing. 3. Significantly colder temperatures continue Thursday through the weekend, albeit with a slight warmup on Saturday. Dangerously cold wind chills will be possible above 3500 ft Saturday night and Sunday morning. 4. A clipper type low pressure/cold front may bring some light snow to the mountains Friday night possibly into Sunday.

DISCUSSION

Key message 1: Accumulating snowfall is expected to begin Wednesday afternoon at high mountain elevations along the TN border. Snow level falls over the mountains as temps cool through the evening. Appreciable accumulations and the most significant travel impacts still look to be confined to elevations above 3500 feet. Flurries cannot be ruled out Wed night or early Thu morning in the Piedmont, though accumulation is not currently expected.

As noted in previous forecast cycles, a compact southern-stream shortwave over the western Gulf Coast will phase with and be absorbed by a substantially deeper shortwave associated with an Arctic cold front tonight into early Wednesday. Low- to mid-level flow will back and amplify over the CWA during this time, ahead of the trough axis. Moisture return is limited at or below 850 mb, though by Wed morning deep saturation is seen above 800-700 mb; left-exit region of southern stream jet streak somewhat overlaps with right-entrance region of the northern one to concurrently bring some dynamic lift across GA and the Carolinas, which results in light QPF response near our southeastern CWA border in some models. This would be of little consequence given temps well above freezing in those areas. Saturation occurs closer to the surface during the afternoon just ahead of the surface front such that mentionable PoPs develop over the mountains; RH will be high at ice nucleation temperatures well aloft, so snow is possible wherever temps are cold enough in the PBL. Only the highest peaks appear likely to be cold enough for snow at onset, though rates greater than an inch per hour appear possible there. CAA from around 21z onward should begin to bring the snow level lower, particularly considering the wet-bulb temp profile, though temps still don't cool fast enough to allow accumulation to begin in the valleys until after 8 PM. Lift will be enhanced for a time as the base of the shortwave shifts overhead, though drying quickly occurs aloft behind that. However, with temps continuing to fall and still decent low-level moisture, the PBL cools to support ice nucleation near the surface, with the needed lift then mainly resulting from NW upslope flow. Snow rates decline where NWF is not a factor. With strong (near-advisory) winds/gusts continuing, the upslope factor is respectable, and near 1"/hr rates look to continue on the peaks until the low levels too dry out after midnight Thu morning; additional accums after that time appear on the order of an inch or less, even in the higher elevations.

Altogether it still appears this will be a highly elevation dependent event. Event totals of 4 to 8 inches now are expected in the highest elevations of the Smokies, with 3 to 6 inches on some of the higher peaks in the northern mountains and near Mt Mitchell. The larger cities in the mountain zones may not even see an inch per the current forecast, and averages for the lower-elevation portions of the zones are well below the 2 inch threshold for Winter Wx Advisory. Hence no Advisory is being hoisted at the current time, per local policy of considering the above- and below-3500 ft averages. Avery is considered as one zone as usual and does not meet the 2 inch criterion either. All this said, even the light accums may lead to some travel issues given overnight temps falling mostly into the teens, so will continue HWO mention.

For the Piedmont, 15-30% PoPs are warranted Wed evening ahead of the base of the shortwave, though temps will be too warm for anything but rain at that time. Most model depictions show the cold air just barely chasing the precip, and particularly with saturation aloft and wet-bulb profiles near freezing down to the surface, it is reasonable to expect a few flurries will be seen. With the late overnight onset of subfreezing sfc temps, however, and the very light QPF depicted from those models producing it at all, any accumulation looks unlikely. Did see fit to add a flurry mention for the eastern I-85 and I-77 corridors where the overlap looks most likely to occur.

Key message 2: Wind chills late Wednesday night and early Thursday morning could prove dangerous in some higher elevations, requiring adequate protection from winter clothing.

As noted above, winds will be near Advisory criteria in higher mountain elevations, where temps will fall into the lower teens (upper single digits above zero above 5000 ft). Resultant apparent temperatures fall into the -5 to -15 range which would support Cold Weather Advisory. Confidence is moderately high for such conditions above 3500 ft, but not yet confident if lower elevations may meet criteria. There remains time for re-evaluation on tonight's forecast cycle, so will let the next shift take a look.

Key message 3: Significantly colder temperatures continue Thursday through the weekend, albeit with a slight warmup on Saturday. Dangerously cold wind chills will be possible above 3500 ft Saturday night and Sunday morning.

Any lingering light snow showers/flurries should end Thursday morning as moisture dissipates. The bigger story will be a cold air mass that moves in behind a departing cold front. Highs Thursday will be much colder than Wednesday, as much as 15 to 20 degrees below normal. Lows Thursday night will also be as much as 15 to 20 degrees below normal With winds diminishing overnight, locations with 5 below wind chill values should be limited to mainly locations above 4000 feet.

Temperatures moderate Friday and Saturday high pressure moves east and a southwesterly flow develops. Lows Friday night and highs Saturday will range from near normal to up to 5 degrees below normal. Highs Saturday will be near normal with steady for slightly lower temps across the mountains.

Another cold front crosses the area late Saturday or Saturday night bringing cold temperatures back into the region. Lows Saturday night through Monday nights will be nearly steady 10 to 15 degrees below normal. For now, winds appear to be low enough to keep wind chill values out of advisory territory for all but the highest peaks. Highs Sunday will be 15 to 20 degrees below normal then 5 to 10 degrees below normal Monday and Tuesday, with Monday the warmer of the two days.

Key message 4: A clipper type low pressure/cold front may bring some light snow to the mountains Friday night possibly into Sunday.

The cold air mass mentioned in the earlier key message will be proceeded by a clipper type cold front. The guidance varies greatly regarding this feature. The GFS favors a double barreled system with light snow showers for the mountains Friday night and early Saturday, then the second system on Sunday. The Canadian has a stronger system Friday night and Saturday with some precip spreading out of the mountains, then a series of weaker systems through Tuesday for mainly the NW flow precip areas. The ECMWF is similar to the GFS for Friday night and Saturday then a weaker system on Monday similar to the Canadian. And just to make things even more uncertain, the Canadian and GFS have a different flavor of a weaker font/clipper for Tuesday while the ECMWF is dry. The LREF mean shows light precip Fri nite/Sat then again Sat nite/Sun. The GEFS mean shows a light mountain event Fri nite/early Sat then a stronger event Sun/Sun nite. The Canadian ensemble mean shows a stronger event Fri nite/Sat with a weaker event Sat nite/Sun. Obviously, this makes for a confusing and low confidence forecast. As usual in these situations, have followed the model blend which shows light snow potential for Fri nite through Sat night with the possibility of some precip escaping the mountains across the I-40 corridor Saturday. Stay tuned as this will likely change.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

At KCLT and elsewhere: Dry and VFR through most of the 00Z TAF period outside of -RA ahead of a cold front leading to MVFR cigs and vsbys at KAVL and KCLT towards the end of the period. Cirrus will push east over the next hour, allowing SKC skies to return across the western North Carolina terminals. SKC should linger through the late evening hours before cloud cover gradually increase from SW to NE late tonight ahead of a cold front, becoming OVC by daybreak Wednesday. Skies may briefly scatter out somewhat Wednesday afternoon, but confidence is low. Thus, maintained OVC skies throughout Wednesday. Wind direction east of the mountains will remain SW ahead of the approaching front through the period. The exception is KCLT (as the TAF goes out 30 hours) where winds will turn NW behind the front ~05-06Z Thursday. Winds at KAVL will be light and VRB this evening into tonight, picking up out of the WNW/NW around daybreak Wednesday. Winds at KAVL may toggle more WSW Wednesday afternoon ahead of the front but confidence is low. Wind speeds will increase throughout Wednesday, with low-end gusts of 18- 22 kts developing. Confidence on gusts developing at KHKY and KAVL is lower so opted not to mention gusts at these terminals for now.

Outlook: Dry and VFR conditions return by late Wednesday night and linger through at least Friday. Precipitation chances as well as associated restrictions may return this weekend but confidence is very low.

GSP WATCHES/WARNINGS/ADVISORIES

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