Weather Headlines

ALERT For Accumulating Snow From Friday Afternoon Into Saturday Morning

Moisture transport from the Great Lakes will combine with WNW-NW-N upslope flow to generate snow showers, snow flurries, and locally heavy snow squalls Friday into Saturday morning. Accumulations are expected to begin at the highest elevations early Friday, with sticking snow levels dropping through middle into lower elevations during Friday afternoon and evening.

The best sticking and most widespread impacts to travel are expected along the upslope side of the Appalachians during Friday night into Saturday morning.

Update At 5:00 PM on Friday_6 March 2020

Wind blown snow showers covered roadways at highest elevations in the High Knob Massif Friday afternoon, with deteriorating conditions expected Friday evening into middle and some lower elevation locations along and north to northwest of the Tennessee Valley Divide and High Knob Massif.

Upslope snow showers will continue developing in Great Lake moisture transport as air temperatures drop.

ALERT For Much Colder Air Temperatures And Low Wind Chill Factors

*Expect air temperatures to fall into the 20s at middle elevations, and into the upper 10s at highest elevations, during Friday afternoon (with much colder wind chills).

Snowfall Forecast

A general 1″ to 2″ below 3000 feet

**A general 2″ to 4″ above 3000 feet

**Locally higher amounts possible on highest peaks, with deeper depths due to blowing and drifting snow, being in contrast to only a dusting within some lower elevation sites at elevations below 1200 feet.

Any accumulations are expected to be limited leeward of the mountains into the Clinch, Powell, and Holston river valleys on downsloping air flow trajectories.

The worst travel conditions are expected to develop Friday night into Saturday morning, with falling temperatures and wind driven snow showers.

This snowfall forecast is primarily for WNW-NW-N upslope flow locations along and northwest of the Cumberland-Allegheny Front and highest elevations of Clinch Mountain and the Blue Ridge.

*Clearing and seasonally cold conditions are expected Saturday afternoon into Sunday morning, with coldest conditions developing within mountain valleys.

High pressure and sinking air, along with light winds and mostly clear skies are expected to set the stage for good outgoing longwave radiation (OLR) and compensative turbulent heat flux divergence from overlying air heading into late Saturday afternoon-evening. This will support strong temperature falls along slopes and initial drainage into valleys and basins where radiative flux divergence will aid cooling. Any lingering snow cover will also help to support cooling, and this is primarily expected to be the case for upper elevation cold air collecting basins and draining valleys-hollows of the southern Appalachians (middle to upper elevations in the central Appalachians).

Cold air collecting basins in the central Appalachians will have higher wind speeds into the evening that may initially make decoupling more difficult into the overnight hours [e.g., the classic northern Canaan Valley, WV frost pocket site appears to have a reflection point centered near 14 mph (6.3 m/s or 12 knots), with decoupling when speeds are under this point and partial to no-decoupling of basin winds when mean speeds are above 14 mph on adjacent Cabin Mountain at Dolly Sods]. The reflection point varies for different locations depending upon topographic factors.

Persistence In Longer Term

As I recently noted, persistence is the main guide for the medium range forecast through next week with warming and wet conditions (surprising…NOT!) developing once again in wake of early weekend cold and snow.

Although not exactly the same pattern as observed during February, the trend of the ensemble means of different model groups (GFS, GEM, and ECMWF) is becoming increasingly more like the Feb pattern.

This would increase wetness through the second-third weeks of March, with unseasonable warmth raising a concern for more strong-severe thunderstorms and convective rains.

The European Model is looking especially anomalous in terms of warmth during the upcoming 5-10 day period.

This anomalously warm pattern continues through mid into late March on the latest ensemble mean of the GFS Model group (below).

Observed SSW mean 925-850 MB flow trajectories during February (near the long-term climatological mean flow trajectories) provided plenty of orographic forcing and convergence for enhanced precipitation amounts along the Cumberland Mountains, Cumberland Plateau and Tennessee Valley during the month.

Precipitation Totals – February 2020

Clintwood 1 W
9.20″

City of Norton WP
10.47″

Jonesville 3.1 WSW
10.49″

Hunter Valley
10.70″

Black Mountain Mesonet
12.39″

Big Cherry Lake Dam
13.01″

High Chaparral-Robinson Knob
13.32″

Eagle Knob of High Knob Massif
13.47″