ALERT For Heavy, Wet Snow Developing Along And N-NW Of The Cumberland Front During The Predawn-Morning Hours Of Monday (14 Dec 2020)

NOTE: The main departure from this will also include the area along and northwest of Clinch Mountain, northeast from Brumley Mountain, for the heavy snow potential

I am concerned about a crippling fall of snow. The potential is there based upon the setting and well documented, past events. Crippling in terms of the snowfall rates and also for the possibility of power outages. To be honest in every way, the potential exists for either a non-event or a crippling event. Reality may be in between these extremes, however, I must make everyone aware of this crippling threat.

Snowfall Forecast

2″ to 6″ below 3000 feet

6″ to 10″ above 3000 feet
with locally higher depths

Target Snowfall: 4.0″ in Norton-Wise (+/- 2.0″ error potential) implying the potential for 2″ to 6″ of snowfall through early afternoon Monday (14 December).

The deepest snow depths of more than 6″ are expected at upper elevations in the High Knob Massif and highest portions of Black Mountain. The majority of precipitation is expected to be snow at the summit level of the High Knob Massif, where 10″ or more could accumulate in a furious fall of heavy snow.

The above forecast is for locations along and northwest-north of the Cumberland Front.

This first wave presents a relatively easy forecast for locations toward the south into the Clinch, Powell, and Holston river valleys where it will be mainly a rain-mix event with limited to no snow accumulation, and to the northeast along the Allegheny Front of the highlands of eastern West Virginia where upper air temperatures will be colder to support mainly a snow event.

In the middle along the Cumberland Front, northeast from Cumberland Gap, it will be a war and the forecast bust potential will be much higher than average.

Forecast models are literally divided between a non-event and a heavy, pounding event that could include potential for power outages and downed trees.

Rain will overspread the mountain region initially, then processes that work to cool the air will become the main focus for locations along and just north and northwest of the Cumberland-Allegheny Front.

This forecast is leaning toward more wet snow based upon already observed, documented past climatology of events possessing an analogous surface low track and surface to 850 MB air flow trajectories.

Although no two forecast settings are identical, the reasoning for my lean toward wet snow is sound based upon past observed events and meteorology.

Column cooling, supported by dynamics, will develop downward via entrainment of cold air at higher levels as low-level northerly upslope flow develops to begin the process of low-level cooling via orographic lifting.

If these processes phase earlier in the event, a crippling snow could occur versus if they occur later during the event (that is the big unknown that gives this forecast setting a higher than average bust potential).

Anyone who is a student of past climatology should recognize this crippling potential immediately, given many have occurred and a few are even documented on the High Knob Landform website (even though it only came online during Summer 2009).

Storm Report At 1:50 AM on 14 December 2020

Snow began falling at the summit level of the High Knob Massif by around 1:00 AM, with sticking to roads by 1:30 AM (a gravel lot above at 1:50 AM).

It is likely that roads will become near impassible at these upper elevations by morning as winds shift and become stronger out of the north.

Snowfall Potential Increasing

High-latitude blocking over Greenland is expected to impact upper air waves moving across the continental USA next week.

This is part of a pattern that currently features favorable Atlantic Ocean Basin teleconnections for wintry conditions across the eastern USA, with negative phases of both the Arctic and North Atlantic oscillations.

The European ensemble mean is more negative than the GFS ensemble mean, but there are signs of warming during Christmas week. That remains to be seen as time gets closer to the Holiday period.

An initial (weaker) wave will support the first potential for accumulating snowfall into Monday (14 December), with a stronger secondary wave being monitored for the middle of next week (16-17 December 2020).

While the Pacific Ocean basin is supporting teleconnections in opposition to wintry conditions across the eastern USA, in general, there has been a trend toward +PNA by the middle to end of this coming week (centered on 17-18 December 2020).

Warm forcing from the Pacific Basin will attempt to win out in the longer term (Christmas Week into January 2021).

Potential Impact of First Upper-Wave

Advisory to local warning level amounts of snow are being highlighted by different model groups for an upper wave predicted to cross the Appalachians into Monday, with accumulating snow developing locally between midnight-sunrise on Monday (14 Dec 2020).

The European Model group (not shown) and NAM Model group are in general agreement with placement of an axis of moderate to locally heavy snow along and northwest of the Allegheny-Cumberland Front. The new ICON (German) Model and GFS run are also in basic agreement.

The Canadian Model (GEM Operational) remains the single model of the big groups to predict very little to no snowfall through Monday.

Extended Outlook – Toward Christmas

Although the extended outlook to Christmas, released today, from NOAA looks like a blow-torch, all hope is NOT yet lost for a White Christmas.

Above average precipitation is a notable indication of storminess, and there are some other signs of a variable up-down pattern as forcing factors in the Atlantic (cold) and Pacific (warmer) continue to battle for control.

Stay tuned for later adjustments.