History Of Christmases Past
(1964-2019)

An ALERT For Severe Conditions Developing From Christmas Eve Into Christmas Day

Extremely hazardous conditions are expected to develop between 3:00 to 5:00 PM, worsening into this evening with high snowfall rates and plunging air temperatures.

Plan to be in place, where you will stay tonight, before this critical period of 3 to 9 PM on Christmas Eve.

Snowfall Forecast

Along And W-NW Cumberland Front

Christmas Eve PM to Christmas AM
General 4″ to 8″
(Locally higher amounts)

Christmas Sunrise-Sunset
Additional 1″ to 4″
(Locally higher amounts)

Clinch-Powell-Holston River Valleys

Christmas Eve to Christmas AM
General 2″ to 4″
(Locally higher amounts)

Christmas Sunrise-Sunset
Additional local accumulations
(in instability snow squalls)

Strong wind gusts combined with a temperature around 40 degrees to generate melting snow atop the High Knob Massif during afternoon hours of 23 December 2020, although, wind chills made it feel significantly colder (nothing like what is coming for the Christmas Holiday).

Heavy Snow & Bitter Cold Christmas

A period of severe winter weather is expected to develop during Christmas Eve afternoon into Christmas day.

The following are preliminary thoughts on the evolution of this event.

1). Rain will change to snow Christmas Eve afternoon with a temperature plunge into the evening. Hazardous travel conditions are expected.

2). The potential for enhanced snowfall along the front is being monitored, with the European and Canadian models being most aggressive in developing a wave of low pressure along the arctic front Christmas Eve.

Christmas Eve Day Model Update

I have included an array of model predictions, to include their Kuchera forecast where available.

The Kuchera Method takes into account how snow amounts change with air temperature versus the much more rigid 10:1 method that says 10″ of snow = 1.00″ of water.

In this case, due to strong cold air advection, snow density will rapidly drop with high snow to water ratios (that is, a lot of snow for limited water) developing into this evening and Christmas day.

3). Convective bursts of heavy snow are likely following the frontal snow band between midnight and sunset on Christmas day, beneath extremely cold air aloft.

It is because air will become outrageously cold aloft that snow amounts will generally be higher than most models predict for Christmas day, with potential for whiteout type snow squalls (bursts) in very low density (high snow to water ratio) snowfall.

While orographic forcing tends to make these squalls more productive and frequent over and upstream of windward mountain slopes and major barriers, strong vertical lapse rates during day time (especially) can cause them to form with instability within any location.

4). This will not be a classic NW flow setting across the southern Appalachians, with WSW-WNW trajectories that are not Great Lake connected.

Cold air aloft, and steep lapse rates, will help to make up for the lack of more perpendicular flow with cross-isobaric convergence favored along the main mountain barriers (especially the Cumberland-Allegheny Front and windward side of Blue Ridge).

Low snow density (high snow to water ratios) will work with lingering moisture to continue accumulating snow (especially in bursts) through Christmas day.

5). Dangerously cold conditions are expected through Christmas day, with air temperatures in the single digits and teens combining with gusty winds to generate single digit and below zero wind chill factors.

ALERT For Strong SSE-SSW Winds Developing By Late Wednesday Afternoon Through Christmas Eve Day Morning (23-24 December)

Strong winds will begin in upper elevations by late Wednesday afternoon, with downward mixing across the area into Thursday morning. Mountain wave winds in favored breaking zones (downward momentum transfer) will enhance gusts during Wednesday evening into the overnight.

A strong pressure gradient in advance of a major winter storm will generate strong winds across the mountain area Wednesday night into Thursday.

More Storms In The Pattern

Both the European (stronger) and GFS model groups show important features heading into the beginning of January 2021. High-latitude blocks and a classic negative phase of the North Atlantic Oscillation.

The Greenland Block is important for USA storms, like the present Christmas Holiday snowstorm and signals more storms in the pipeline ahead.

Storm potentials are currently showing up just prior to and following New Year (at least one of these could be a major snowstorm).

The Ural Block could be important for the longer-term via enhanced wave forcing that could work to weaken the Polar Vortex, increasing the threat of major arctic intrusions into middle latitudes.

Previous Discussion

Winter Storm For Christmas 2020

There is now a higher than average confidence level for a white Christmas 2020, with both the European (which I can not legally show) and GFS models (and ensemble means) being in rather amazing agreement at 500 MB across the Northern Hemisphere by Christmas.

Details remain to be worked out, of course, but this is part of a now long advertised pattern where features in the Atlantic Ocean and at high latitudes in the Northern Hemisphere are beginning to dominate over forcing from the Pacific Ocean Basin (which is now trending more favorable for wintry, eastern USA conditions).

In other words, the push-back from the Pacific Basin against very favorable conditions across the Atlantic Basin is not as strong now as it has been, favoring a colder and more wintry setting over the eastern USA into the New Year than previously suggested.

An aspect that will be different with this event from previous December systems to impact the southern Appalachians will be the advection of bitterly cold air, with very low snow density likely aiding amounts that fall across the mountains as temperature plunge into morning hours of Christmas day.

Although temperature means for this month (through 20 December) have been near to a little below average, air temperatures during snowfall events have been marginal across the southern Appalachians for snow with exception of the early December NW flow snow.

December 2020 Snow Events (Elevation Biased)

Much more snow has fallen at highest elevations than even within lower sections of the upper elevations (below 3500 feet), and adjoining middle elevations (2000-3000 feet), due to air that has only been marginally cold enough to support snow.