ALERT For Accumulating Snow And Hazardous Travel At Upper Elevations Between Midnight And Noon Monday

Snow is expected to develop later tonight into Monday at upper elevations, with a general 2″ to 4″ expected above 3000 feet in the High Knob Massif-Black Mountain area. Locally higher amounts will be possible, especially in the 4000 to 6000+ foot elevation zone from Mount Rogers and Whitetop southwest to Roan and the Great Smokies.

What appeared to be snow falling, during a period of Sunday afternoon sunshine, was rime crystals dropping off trees and towers on Eagle Knob where around 1″ of snow initially stuck and covered roads into early Saturday.

This forecast temperature time series off the high-resolution NAM 3 KM Model is predicting an elevation snow event, with the summit-level of High Knob in freezing air versus above freezing air in Wise. What the model can not typically do is properly resolve the local terrain and orographics, such that orographically force rising will cool the air and allow snow to fall at a lower elevation than suggested. The forecast dilemma being how low will the sticking snow level get? Ideally, if rising was coupled vertically it would reach all the way to the floor of Powell Valley and the Powell River Valley, however, as I discuss below, models are not currently showing a coupled unidirectional flow and instead predict too much cross-terrain flow above the surface SW flow (with sinking over mountain ridges working against upslope and up-valley cooling on the surface).

For those who got excited at the possibility of a SW Upslope Flow snow impacting Powell Valley and the Powell River Valley, I can not officially pull the trigger due to predicted model air flow trajectories which currently do not match my composite of SW Flow snowfall events observed in the past.

Although low-level winds will be SW, there is too much cross-terrain flow above the surface to match past events. This does not mean it could not happen, it only means that unless the flow above the surface is more SSW-WSW than currently predicted by models there looks to be too much terrain induced cross-isobaric ageostrophic flow for a coupled SW Flow snowfall event. In other words, too much sinking across mountain ridges into the valley to counter up-valley and up-slope flow adiabatic cooling, and air temperatures will not cool enough to allow sticking to the valley floor. On the other hand, if deeper SSW-WSW flow actually forms then snow falling above will be able to work down to the surface.

A coating of snow covered high elevations throughout the weekend, as visible across northern slopes of the High Knob Massif along the horizon (above). A trend that more often than not has dominated the 2019-20 winter season up to this point in time.

Weather Headlines

*Colder air will dominate the weekend with intervals of light snow, flurries and snow showers into early Sunday. Low cloud bases will generate riming (freezing fog) at the upper elevations. Any sticking of snow will be light and mainly at upper elevations, with generally a dusting up to 1″ expected (locally more possible on highest peaks).

A break in activity is expected Sunday afternoon in between the current system and the next upper air disturbance arriving into early Monday.

*A upper air disturbance and Great Lake moisture will offer a better chance for more widespread accumulations Monday into Tuesday. Locations typically experiencing upsloping on SW to NW flow will be most favored. The potential for 1″ to 4″ of snow is a preliminary estimate for these upslope locations (heaviest upper elevations).

School schedule changes will be likely for Monday and Tuesday (January 27-28).

While upslope cooling on SSW-WSW flow and snow in the corridor from Wise southwest through Powell Valley and Big Stone Gap into Lee County can not be ruled out, the current pre-event air flow predictions do not look as favorable for middle-lower elevations.

The level of sticking snow remains to be seen, with upper elevations around and above 3000 feet currently being most favored for accumulations. A vertical shift of 500 vertical feet or more could easily occur in the eventual level of sticking snow depending upon actual air flow trajectories and cooling-warming with rising-sinking along the High Knob Landform corridor.

Short-Term Forecast

Friday Night Into Saturday Morning

Lowering clouds with rain showers redeveloping and becoming mixed with or changing to snow at middle-lower elevations. Changing to snow upper elevations. Turning colder. Winds SW to W at 5-15 mph with higher gusts. Temperatures dropping into the mid-upper 20s at upper elevations and the 30s at middle-lower elevations. Wind chills falling into the 10s and 20s, except single digits in gusts toward morning on highest ridges. Accumulations up to 1″ above 3000 feet. Riming developing at elevations above 3000-3300 feet.

Saturday Morning Through The Afternoon

Cloudy and cold. Intervals of light snow, flurries and heavier local snow showers. Winds SW to W at 5-15 mph, with higher gusts. Temperatures holding nearly steady, in the 20s at upper elevations and 30s to near 40 degrees at middle-lower elevations. Low cloud bases with riming at highest elevations. Wind chills varying from 10s at the upper elevations to the 20s to lower 30s.

The best sticking potential for snow, outside of upper elevations, will exist Saturday evening into Sunday morning for locations below 2500 feet. That is, temperatures will tend to be above freezing at elevations below 2500 feet prior to Saturday night. Moisture will be limited and accumulations, if any at lower elevations, are expected to be light.

Saturday Night Into Mid-Morning Sunday

Cloudy and seasonally cold. Intervals of light snow and flurries. Heavier local snow showers possible. Winds SW to W at 5-15 mph, with higher gusts. Temps varying from upper 10s to upper 20s to around 30 degrees. Low cloud bases with riming at upper elevations. Wind chills from single digits at highest elevations to the 10s & 20s.

Sunday Afternoon

Low clouds breaking to increasing mid-level clouds. Winds SW-W at 5-15 mph, with some higher gusts. Temps varying from upper 20s to lower 30s to the upper 30s to lower 40s. Wind chills varying from 10s and 20s to 30s, coldest at highest elevations.

The potential for a light SW upslope flow snow is being monitored for Sunday Night into Monday on SSW-WSW flow. Locations along and southwest of the Tennessee Valley Divide and High Knob Massif will be most favored for sticking snow at lower-middle elevations (through the defined High Knob Landform corridor of Lee County into Powell Valley) in comparison to lower-middle elevations under downslope flow across northern Wise and much of Dickenson-Buchanan counties.

Sunday Night Through Mid-Morning Monday

Lowering cloud bases with snow developing at upper elevations. Mixed snow-rain at lower elevations. Winds SW-WSW 5-15 mph, with higher gusts, below 2700 feet. WSW-WNW winds 10-20 mph, with higher gusts, along mountain ridges above 2700 feet. Temperatures varying from 20s at highest elevations to the 30s (warmest in the downslope locations of northern Wise and Dickenson-Buchanan counties). Riming at high elevations. Wind chills varying from 10s to low 20s at upper elevations to the 20s & 30s at middle-lower elevations.

Following a likely break Monday afternoon, a shift to NW air flow will change the locations receiving the best potential for more snow to stick during Monday night into Tuesday to typical NW flow locations.

*The potential for a major winter storm system is still being monitored for the end of January-early February period, however, the setting is looking less favorable at the current time (26 January) for a major event across the southern Appalachians. There remains plenty of time for changes (which will occur in the modeling) so stay tuned for later updates.

Will the snow drought starting in early 2019 ever end? Reference my 012120 Extended Discussion.