Mountain Area Forecast ( Jan 3-5 )

ALERT For Bitter Cold During Thursday-Saturday With A Reinforcing Surge Of Continental Arctic Air – Expect The Most Severe Conditions Experienced So Far During This Cold Wave Of Late 2017-Early 2018

Bitter cold air and dangerously cold, life threatening wind chills will develop into Thursday-Friday as a large mass of cA ( continental Arctic ) air is pulled south across the region when the pressure gradient tightens around a deep cyclone in the western Atlantic Ocean.

ALERT For Whiteout Snow Squalls In NW Upslope Flow

*As of 5:30 AM up to 2″ of snow have already fallen in parts of the area, especially with persistent snowstreaks into upper elevations of the High Knob Massif from High Knob along Little Mountain.  Expect drifting at upper elevations due to strong NW winds.

Extremely bad conditions have developed along State Route 619 & Route 237, as well as other middle-lower elevation roads across Wise-Dickenson counties, with plunging temperatures.

Although big snowfall will remain near the Atlantic Coast, moderately strong NW upslope flow will carry enough low-level moisture south to produce accumulating snow along the upslope side of the Appalachians, with a general 1-4″+ currently expected in the favored orographic areas ( higher amounts may occur from the High Knob Massif northeast to Snowshoe-Canaan Mountain with squalls & snowstreaks ).  Roadways are expected to become slick.

Due to stronger winds during this period, versus the most recent cA blast, wind chills will be lower and conditions more dangerous over a prolonged period of time.

Overnight Into Wednesday Morning

Increasing high clouds.  Light winds becoming SW at 5-15 mph, with higher gusts, on mid-upper elevation mountain ridges.  Temperatures varying from 10s mountain ridges to single digits in valleys, except around or below zero degrees in colder mountain valleys.  Wind chills in the single digits above and below zero along higher mountain ridges.

Wednesday Afternoon

Partly-mostly cloudy ( high clouds ).  SW winds generally less than 10 mph.  Temperatures varying from upper 10s to lower 20s at highest elevations to the upper 20s to low 30s.

Wednesday Night Into Thursday Morning

Increasing clouds and lowering cloud bases overnight into morning with snow showers & flurries developing.  Winds shifting NW & increasing to 10-20 mph, with higher gusts, mid-upper mountain ridges and plateaus.  Turning colder with temperatures dropping into single digits to low 10s, except to around 0 degrees on highest peaks.  Riming at upper elevations, mainly above 3300 feet.  Wind chills from 5 above to -15 degrees below zero, except -15 to -25 below at elevations above 3300 feet.

Thursday Afternoon

Mostly cloudy.  Chance of snow showers, flurries and local snow squalls. Bitter. NW winds 10 to 20 mph with higher gusts.  Temperatures varying from 0 to 5 degrees along highest crest lines to the low-mid 10s ( milder south into river valleys of the Clinch, Powell, and Holston ).  Wind chills 5 above zero to -15 below zero at elevations below 3000 feet, -15 to -25 below zero chills above 3000 feet.

Thursday Night Into Friday Morning

Snow showers, flurries and local snow squalls.  Bitterly cold.  WNW-NW winds 10-20 mph, with higher gusts on mountain ridges.  Temperatures falling to 5 above zero to -5 degrees below zero, except colder on highest peaks.  Wind chills 0 to -20 degrees below zero, except -20 to -30+ degrees below zero highest elevations.  Rime formation in super-cooled orographic clouds at upper elevations.

Snowfall Forecast – Thursday AM Through Friday AM

General 1″ to 4″ along the upslope side of the mountains, with higher amounts possible in the High Knob Massif and within other most favored high mountain areas. 

The potential for extremely slick conditions are being monitored by early next week as a mix of precipitation falls upon surfaces frozen by prolonged sub-freezing temperatures.

 

Weather Discussion ( Harsh Period )

Early Thursday Evening Update

Light snow, flurries, and locally heavier snow showers are continuing across the area and will persist as the low-level moisture increases into overnight as the 850 MB thermal MIN ( coldest air ) pushes into the mountains.

A slick and snowy overnight-morning ruled the area with widespread snow accumulations in NW upslope flow from northern Lee and northern Scott counties across Wise and Dickenson counties into Buchanan County.

Snowfall accumulations varied from around 0.5″ on the low end to widespread 1-2″ ( Pound, Norton-Wise, Clintwood ) upward to between 3-4″ ( High Knob-Little Mountain area ).

It was a classic NW flow setting with simply gorgeous mountain waves forming from the Cumberland Overthrust Block ( where the first true mountains rise to perturb air flow ) southeast to where visible waves dissipated leeward of the Blue Ridge.

A larger-scale view revealed snow on the ground toward the Atlantic Coast beneath clearing skies with subsidence in the wake of intense western Atlantic cyclogenesis and sinking air lee of the Appalachians.

Bitter cold has made this a nasty event, with ”warmest” temperatures occurring at the very beginning of January 4.

Joe & Darlene Fields measured 2″ of snow in the High Chaparral community into Thursday morning, with heavier 3″+ depths toward the Little Mountain-Cox Place communities.

Afternoon MAX temperatures varied from 3 degrees on Eagle Knob to 16 degrees in Clintwood.  The problem, it never felt that warm!

A max of 4.7 degrees was recorded by the Black Mountain mesonet, with wind chills as low as -22 degrees below zero during the day.

This was especially true at middle-upper elevations where 20-30+ mph wind gusts were common.

 Please use extreme caution if needing to be outdoors.

 

Previous Discussion

Following a Tuesday morning MIN of -3 degrees below zero in Clintwood the afternoon MAX reached 23 degrees, then fell back to 2 degrees by 2:00 AM Wednesday.

Temperatures have been much colder in snow covered upper elevations of the High Knob Massif, with colder valleys quickly dropping back to below zero Tuesday night beneath clear skies before increasing high clouds-winds stopped the plunge. 

Increasing high altitude clouds overnight into Wednesday is a signal of the upcoming weather change that will again feature a huge surge of bitterly cold air.

Although bitter cold will be the big weather story again, the development of NW flow snow showers, flurries, and snow squalls should not be overlooked with slick road conditions developing within favored upslope locations along & north of the High Knob Massif-Tennessee Valley Divide, and in a narrow zone from Mount Rogers-Whitetop southwest along the TN-NC border toward the summit level of the Smokies.

A stronger and somewhat more cyclonic flow will help orographic forcing to be stronger than the previous blast, when up to 1″ or so of snow accumulated at the summit level of the High Knob Massif ( on meager moisture-lift ).

Reference my 123117 Forecast for details and photographs.

Bitter air = very low snow density and much greater amounts than 10:1 model ratios are predicting with orographic NW upslope flow given any moisture.

*If the western Atlantic low pressure tracks closer to the coast, and the pressure gradient is stronger than currently forecast, upslope snowfall amounts along the Appalachians will trend greater than I am currently forecasting.

Wind chills are going to be a major factor during the Thursday-Saturday period and models are too course to accurately resolve true chills amid complex terrain, so it is hoped that NWS Forecast Offices will post an advisory or warning for all of the mountain area.

With more snow expected along the upslope side of the mountains, an enhancement of bitter cold temperatures will help make this blast worst than the previous event.