Caution for the redevelopment of slick roads tonight into early Saturday. Otherwise, snow covered secondary roads continue to be an issue for travel at highest elevations and in some local middle-lower elevation sites.

Light snow, flurries, and heavier snow showers will continue into tonight and Saturday along the upslope side of the mountains with respect to northerly (NW-NE) air flow trajectories amid lingering low-level moisture.

Cold air advection will be strongest below 850 MB, helping to establish a low-level temperature inversion into tonight and Saturday. Rime forming low clouds at upper elevations may drop into the middle elevation zone (below 3000 feet) in windward locations upstream of the High Knob Massif-Tennessee Valley Divide (Cumberland Front).

Please be cautious of enhanced slickness if fog forms in your location tonight (with air temperatures well below freezing).

Raw, cold conditions will continue through much of Saturday with hopes for eventual weakening of the inversion to allow for some sunshine. This will be a struggle, with temperatures remaining below freezing throughout most (all if no sunshine develops) of Saturday in Norton-Wise and the 10s to around 20 degrees at highest elevations in the High Knob Massif.

Additional accumulations will vary from less than 1″ to locally 2″ in favored upslope locations of the High Knob Massif, where current snow depths vary from 3″ to 6″ at elevations below 3300 feet to 6″ to 12″+ in the highest elevations (above 3300 feet). Lingering “old” snow of recent days added to depth at highest elevations.

A general 3″ to 5″ of snow fell across central-southern Wise County into portions of Lee and Scott counties by mid-day Friday, with lesser 1″ to 2″ amounts across northern Wise and Dickenson-Buchanan counties.

Huge snowflakes were a characteristic of this event, especially overnight into early Friday, but even into the afternoon hours in some locations (above).

Previous Forecast & Discussion

ALERT For Snow And The Development of Hazardous Conditions Into Friday

There is higher than average uncertainty associated with this event. Although a general 1″ to 8″+ is likely, there is potential for large variability for any given location and therefore a larger potential range.

The map below shows the favored zones based upon climatology and the current mean of forecast models. Appalachian topography is, of course, the base map.

In reality, these predicted accumulation zones may very well shift on the map below. Time will soon tell. An idea of model variability can be obtained by watching the slide show of all model runs from today (7 January).

I included all model runs but those from the European Model group (50 ensembles + the operational run + the control run), given I am not allowed to show them without a license.

I think one or more mesoscale snow bands will be possible, but where they form can not really be determined with confidence in advance. This map is, therefore, generalized with a heavy terrain focus.

This map implies, as an example, that those traveling north along U.S. 23 (between Big Stone Gap and Norton) will encounter more snow as they pass from Powell Valley in Wise County through Little Stone Gap (at Powell Valley Overlook) to the northern side of the massif.

The (+) allows for both model and terrain variability that exists depending upon which model solution (or blend) verifies. Typically, no single model is ever exact.

Greatest uncertainty exists within the Great Valley where downsloping air flow trajectories will be opposed to favorable upper air dynamics, such that ultimate depths will depend upon boundary layer temperatures, precipitation type, and rates of fall.

Although rare, the Tri-Cities can occasionally receive more snow than Norton-Wise and even the summit of the High Knob Massif (very rare, but documented). When that has rarely occurred, it has been due to a southern tracking system whose precipitation shield did not fully reach into the northern mountains of the Mountain Empire.

Western North Carolina and the Tennessee-North Carolina border area (northeast to Mount Rogers) has the highest probability of reaching higher end totals based upon typical snow forecasting schemes related to the relationships between snowfall amounts and surface, 850, 700, 500 MB low tracks.

Higher end totals will also be possible in the High Knob Massif-Black Mountain corridor of far southwest Virginia given favorable air flow trajectories. Higher uncertainty here exists due to storm track, with northern portions of the map area above expected to be at the northern extent of the main precipitation shield.

Any south or north jog in storm track could change totals over northern portions of southwestern Virginia more than in other sectors of this storm system (with air temperatures cold enough for all snow, the amount of available moisture will be a critical factor here).

It should be kept in mind, as well documented during past similar events, that the evaporative cooling potential will be somewhat greater over locations along and northwest to north of the Cumberland Front (High Knob Massif-Tennessee Valley Divide), given advection of lower dewpoints on NE-N trajectories.

Snow density will tend to be lower in this sector of the storm than farther south. Somewhat lower snow density can help compensate for less moisture (greater fluff factor), and if moisture is greater than expected will generate a snowfall maximum in this sector of the storm due to the combination of lower density and favorable orographics with significant rise on NE-N trajectories felt by communities such as Clintwood, Norton, and Wise (to name a few).

Additionally, any change in intensity (strength) of this system will be important given it is not expected to intensify until entering the western Atlantic.

Heavy Snowfall Potential for Friday

Snow will develop overnight into Friday morning with hazardous conditions expected from around Breaks Interstate Park south and southeast.

Updated Model Predictions from 7 January 2021 Runs:

Previous Discussion

Caution for slick conditions on roads at highest elevations within the High Knob Massif into Thursday (7 January 2021)

A general 2″ to 5″ of snow accumulated at highest elevations in the High Knob Massif. Extreme caution should be used for snow covered roads.

The solution to this dilemma may not be known until the system develops over the Mountain Empire late Thursday into early Friday.

The difference between a “big” snow and a “little” snow for Wise County, on the latest European Model run at 18z Wednesday, is less than 50 air miles (around 35-40 miles). This is literally nothing for a global-scale model.

The operational GEM (Canadian) Model has been most consistent with this system, while the GFS has been the weakest and farthest south. Wednesday trends feature increasing heavy snow potential being predicted by the Short Range Ensembles, NAM group, and the German Model (ICON). The GEM has predicted heavy snow across southwestern Virginia for days now.

While the operational European Model has been wobbling, and recently edged just a bit south, at least 14 ensembles of its group continue to predict heavy snow over southwestern Virginia. The European Ensemble Control has been consistent in keeping heavy snow over far southwestern Virginia.

To illustrate how models are struggling, the 00z NAM group coming in tonight is to the south and appears to be suffering from some feedback issues.

A problem being that this system is being taken across the highest mountain chain in the eastern USA, such that models are all trying to figure out how to handle this process across both space and time.

Much like an individual thunderstorm, a storm system in the atmosphere does not move from Point A to Point B, but instead it redevelopments at each point across space and time. In this case, a major problem is the redevelopment to the lee of the mountain chain.

Bitter Cold By Mid-Late January

There are signs that a major (big-time) eastern USA winter storm could develop toward the middle of January (one of numerous upper air waves within the flow field).

Getting a major winter storm requires specific conditions, such that only time will tell if these can align in such a way to support a big-time winter storm.

This would occur as cross-polar flow develops across the Northern Hemisphere.

Ensemble Model Means are beginning to reveal impacts of the major Sudden Stratospheric Warming (SSW) that is ongoing, with development of cross-polar flow expected by mid-late January.

This cross-polar flow pattern will open the door for bitterly cold, arctic air to invade the USA.

The full potential of this would be best realized, of course, above snow cover.

Reference My 27 December 2020 Discussion for more information about the ongoing major warming in the stratosphere that will continue to help change North American weather during January 2021.