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

ALERT For Slick Surfaces Developing Overnight Into Friday Morning

Freezing Fog (low clouds) developing on upsloping northerly winds will combine with redevelopment of light precipitation to generate additional light icing on surfaces later tonight into Friday. In addition, water from melting snow-sleet-ice will be refreezing.

Due to a high concentration of low-level moisture this is expected to generate gorgeous conditions on trees in locations experiencing these frozen conditions.

ALERT For Low Visibility – Especially At Mid-Upper Elevations Along & North of the High Knob Massif-Tennessee Valley Divide

Snow accumulations of a dusting up to 1″ are expected, with locally higher amounts possible at highest elevations in the High Knob Massif and Black Mountain.

The coldest influx of air will arrive Friday Night into Saturday Morning with widespread 10s (teens) to around 20 degrees F (in “warmest” locations). Single digits at the highest elevations will combine with WNW-NW winds to generate below zero wind chills.

Storm Event Through Thursday PM

A period of heavy snow to sleet generated widespread 1″ to 4″ snow-sleet amounts across much of the area in the overnight (reference above map for partial reports) hours of Thursday (18 February 2021).

Heaviest snow amounts fell along and northwest of Pine Mountain and Cumberland Mountain, with totals on the above map being somewhat low in places due to melting and/or settlement (with sleet-rain) by morning when most people measured.

As an example, I measured 2.0″ of snow depth in Clintwood at 2:00 AM Thursday as the transition to sleet-freezing rain started. By sunrise, the mean depth had decreased to 1.0″ to 1.5″ even though the air temperature did not rise above freezing. Such differences, for research purposes and accurate documentation of storm history, must be noted as I am doing even though the above observations will not change.

The most significant icing occurred in lower elevation valleys north of the High Knob Massif-Tennessee Valley Divide, and at upper elevations in the High Knob Massif where cooling on upslope flow held temperatures in the 20s to around 30 degrees. Areas in between, had minor accumulations of ice.

The storm fit Miller B climatology with a surge of above freezing air streaming up through the Tennessee Valley and High Knob Landform aloft.

This layer of above freezing air was due to synoptic-scale warm air advection and was aided by downsloping leeward of the Great Smokies across the Great Valley into the Valley-and-Ridge and Appalachian Plateau physiographic provinces.

Former Alert and Forecast

ALERT For A High Impact Winter Storm Event During Thursday (18 Feb 2021)

A high impact winter storm is expected Thursday, but details with respect to precipitation type and amount of each type remain uncertain.

This forecast possesses higher-than-average uncertainty.

Precipitation Forecast By Frozen Type

Snowfall

1″ to 3″ below 2500 feet

3″ to 6″ above 2500 feet

(Locally Higher Amounts Possible)

While lower snowfall amounts will also be possible, I stress the fact that higher amounts are possible across the entire area given that would create a much higher impact with high density snow versus lower amounts and less impact.

Sleet

Trace up to 1″

Freezing Rain

Trace up to 0.33″

Models are in general agreement on a Miller B storm scenario, but a few like the new GFS maintain a colder vertical temperature profile with much more snow.

There will likely be a mesoscale band of heavy snow but, as seen above, models are divergent on where and how intense that band will become.

This system currently has intense, mesoscale banding of snow to the southwest and this is the potential possessed as it continues moving northeast into Thursday morning.

Previous Discussions

I will discuss some of these differences below. Note these are NOT my forecast, but the model predictions.

The Current Dilemma: Ice Storm or Snow Storm

Forecast models are currently divided (as of Tuesday PM) between a Miller A versus Miller B storm track, and that difference will make a world of difference for the local mountains in terms of precipitation type.

The latest NAM Model forecast is for a crippling ice storm, with 1″ of accumulated ice at the Wise gridpoint. That would be devastating and hopefully is an extreme solution that will not verify.

The old GFS and NAM model groups are predicting a Miller B type storm with warm air aloft quickly overspreading the area to change precipitation into sleet and freezing rain (above).

The new GFS (above) is one of the extreme solutions with 10.6″ of wet snow predicted at the Wise gridpoint (it is likely too widespread with heavy snow to the south).

The European Model group, new GFS Model, and the German Model are all predicting a fall of heavy, wet snow. The operational European predicts 7.5″ at the Wise gridpoint, while the 18z German predicts 6.6″ .

NOTE: The 18z European Model (operational) just came in with 8.3″ at the Wise gridpoint, with 6-10″ in counties along the Virginia-Kentucky border. This would be a crippling solution given the model tends to under-estimate snowfall for upper elevations (especially).

This solution features a Miller A storm track which holds the invasion of any above freezing air aloft to a minimum, especially in locations along and northwest of the Cumberland-Allegheny Front.

For those who follow snow density, it is critical to note that instead of increasing snow output being forecast using the Kuchera Method, models are increasing output at 10:1 to suggest that this snow will have potential to be 8:1 or perhaps even higher in density.

Which Solution Is Correct?

It is hoped that neither of the extreme solutions are correct, given one would generate a crippling ice storm and the other a crippling fall of wet snow.

While reality may “fall” in the middle, and no one can know for certain at this point, I am leaning more toward wet snow based upon Baroclinic and Quasi-geostrophic Theory which basically would state: with an expanded mass of arctic air the storm track will seek out the max thermal gradient (baroclinic zone) which will tend to be south and east of the previous winter storm track.

A baroclinic atmosphere depends on both pressure and temperature, with imbalances in mass, momentum, and energy being generated by geostropic advections in synoptic-sale weather systems. The atmosphere will seek to compensate for these imbalances by generating large-scale vertical circulations that change the vorticity structure of its mass field as well as temperature field (through adiabatic ascent-descent).

A critical point being that storms, neither small-scale thunderstorms nor large-scale storm systems, do not move from Point A to Point B, instead they move by continuously developing and redeveloping across space and TIME. So, time will now dictate which model solution is correct.

Stay tuned for updates.