Reference Mid-Winter 2021_High Knob Massif for a recap of recent conditions, as well as a 2020 precipitation update.

ALERT For A High Impact Snow Event During Wednesday Night Into Thursday Morning (27-28 January 2021)

A High Impact event is expected due to rapid temperature drops and moderate to high snowfall rates following sunset.

NOTE this has to be considered a HIGH impact event. Travel along I-81, the busiest road, through the Great Valley will feature no snow accumulation in the Knoxville area to 6″ or more centered upon the Tennessee Valley Divide (around Mountain Empire Airport) where travel may become impossible for a period of time.

Snowfall Forecast

2″ to 4″ below 2500 feet

4″ to 8″ above 2500 feet

(+/-) 1″ error potential to imply a target snowfall of 3″ to 5″ at 2500 feet elevation (above mean sea level)

Snow accumulations will mainly occur between 6:00 PM Wednesday and 10 AM Thursday, with the bulk happening between 7 PM and 7 AM.

At the latitude of Wise, the solar angle at Noon is approximately 34.7 degrees for 27 January. This means that 1 unit of solar radiation striking the surface at a 34.7 degree solar angle above the horizon will spread out to cover 1.75 area units. This will diminish the intensity of insolation to just 57.1% of the value of a vertical beam.

In complex, three-dimensional terrain many slopes and valleys receive limited insolation (locally little to none) with such low sun angles due to shading by adjacent mountain ridges.

The Bottom Line, recent above average temperatures will not be a major factor given such a strong influx of cold air and limited insolation at this time of year in complex terrain.

While low clouds and a morning inversion mixed out, the mixing downward of drier air aloft is a key feature that will support a rapid deterioration of conditions by around and after 7 PM on Wednesday evening.

The 0 Celsius (32 F) wet-bulb temperature goes below freezing prior to the onset of precipitation, indicating that initial mix or rain will rapidly change into snow.

This will support a rapid development of widespread hazardous conditions, with thin icing beneath, during the evening in locations along and north to east-northeastward of the Cumberland Front (along the Tennessee Valley Divide).

Heavy snow is expected to develop and stretch from the High Knob Massif and Black Mountain corridor eastward along the Tennessee Valley Divide, with models centering the core of heaviest snow on the Mountain Empire Airport region of the Tennessee Valley Divide.

My caution for significant run-off will continue into Wednesday, with streams still above Red Alert levels.

Caution For High Water Levels On Streams Draining The Wise-Scott-Lee Border Area Into Wednesday

Snowstorm Potential Next Week

A Miller B Winter Storm will develop to impact the mountain region beginning late this weekend into early-middle portions of next week.

This typically means initial frozen precipitation (snow, sleet, freezing rain) that changes into a cold rain at lower-middle elevations (below 3000 feet), with cold rain or freezing rain in upper elevations.

Initial snow-sleet tends to change to sleet or freezing rain along the eastern slopes of the Appalachians, with only rain occasionally falling on high summits (standing in above freezing air aloft) following initial frozen. Farther northeast, into the central Appalachians, deeper cold air can support mostly snow or snow-sleet.

A transition back to snow occurs as the secondary low becomes dominant. The track-intensity of this low then dictates how much snow falls on NW-N upslope flow.

In the BIG Picture…

Signals for a major winter storm began showing up a couple weeks ago, and I initially highlighted this potential in my 011321 Forecast Discussion.

Models have struggled with this pattern, but seem to have finally found the wave that may generate a major winter storm whose impact will likely be greatest from northern Virginia into New England. This is not the only major storm potential showing up in the extended.