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.

ALERT For Possible Hazardous Travel Early Wednesday then For Accumulating Snow Wednesday Night Into Thursday Morning

Frozen precipitation types will be possible at the onset of precipitation early Wednesday prior to a change into rain. Precipitation will then change into snow, from highest elevations down, beginning Wednesday afternoon into Wednesday night.

More limited wrap-around moisture, and a small change in air flow trajectories, has resulted in a reduction in forecast amounts through 17 December at all elevations.

Snowfall Forecast
(16-17 December 2020)

Generally 1″ or less below 3000 feet

1″ to 2″ at elevations above 3000 feet
(with locally higher amounts)

Target Snowfall: 1″ [(+/-) 1″ Error Potential] for Norton-Wise. This implies a dusting to 2″ of snowfall will be possible into 17 December 2020 across Wise and Dickenson counties. The greatest snowfall amounts, of 2″ or more, are expected at highest elevations in the High Knob Massif where rime ice is also expected.

Reference Early Winter 2020 for a brief recap of the recent event.

Miller B Type Winter Storm

An initial low that moves west of the Appalachians weakens as a secondary low begins forming along the Atlantic Coast. This second low may then impact the Northeastern USA, especially if the track is inside the 40 degrees N / 70 degrees W benchmark point, with heavy snow and wind (generally referred to as a Nor’easter).

While Miller B type Nor’easters can occasionally have a large impact locally with icing and heavy snow, (especially at upper elevations in the High Knob Massif), it is the Miller A type Nor’easters that tend to be crippling winter events for the local mountain region.

There are important aspects to be monitored, with these being climatological characteristics (all of which apply, at least partially, to each event of this type).

1). Evaporative cooling with dry air initially in place could support snow or a mixed period of frozen types at the beginning and front of the surface-reaching precipitation shield (early Wednesday).

2). The most significant snow and/or icing in far southwestern Virginia typically occurs at upper elevations in the High Knob Massif.

3). To the east, cold air wedging and upslope on easterly component low-level flow typically results in more prolonged frozen types across western North Carolina and the eastern portion of southwestern Va.

4). To the northeast, deeper cold air in the vertical typically supports more snow from the Greenbrier Valley of eastern West Virginia north and east along the West Virginia-Virginia border area.

5). Wrap-around and orographic upslope snow on the backside of the storm is typical, and sometimes can be very significant in favored upslope locations as temperatures drop with cold air advection.

For this event, wrap-around moisture is expected to be limited versus more productive events already documented in history.

Potential Christmas Winter Storm

The big news in the weather world, at least for those with hope for a white Christmas, is a signal for the potential (emphasis this far away on potential) development of a Christmas Holiday winter storm.

Although negative phases of the Arctic and North Atlantic oscillations are expected to continue (as they are currently negative), it is the formation of a positive Pacific North American oscillation phase that is a new trend that could aid deepening of a eastern USA upper wave (i.e., aid storm formation).

The persistence of blocking near Greenland and in the Ural Mountain region is an important aspect that is also working to generate atmospheric waves capable of impacting the Polar Vortex, which in the long-term (January-February) may influence future conditions across eastern North America.

Stay tuned for later updates.

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.