Remain Alert For Possible Black Ice As Well As Slick Stretches On Secondary Roads. Bitter Conditions Will Continue Into Friday Morning.
Although coldest air temperatures will be felt in mountain valleys, with single digits to local below zero readings in coldest valleys at upper elevations into Friday morning, breezy to gusty winds will continue to produce notable wind chills along mountain ridges.
The development of strong inversional conditions overnight are expected to weaken with vertical mixing Friday, but surface air temperatures will continue to run cooler than forecast models indicate until widespread snow cover melts.
Melting and sublimation are endothermic processes that absorb heat from the environment, with cooling being a result. Insolation, or incoming solar radiation, from the sun is partially applied to melting and sublimating the snow instead of heating up surfaces that then heat the overlying air.
In addition, of course, snow possesses a high albedo ( reflectivity ) such that up to 80-90% of insolation is reflected and not absorbed when snow is fresh and clean ( albedo tending to decrease as snow ages ). Time and time again we observe forecast temperatures being too high when snow cover is on the ground. In complex terrain, factors such as orographically forced downsloping can help offset this effect in part as was observed during the afternoon of January 18 when 31.6 degrees was the MAX temp observed in Clintwood. I had predicted max temps up to low 30s anticipating that the downslope influence on SW flow would partially offset the cooling effect of widespread snow cover. So Clintwood 1 W, at nearly the same elevation as Tri-City Airport ( TRI ) was 4 degrees warmer during the afternoon due to downsloping on SW winds versus TRI where a SW wind is an up-valley wind possessing a rising trajectory ( not sinking across mountains ).
ALERT For Bitterly Cold Conditions And Slick Roads, Especially Secondary Mountain Roads, Into Thursday
Many roads will remain snow covered and slick into this morning, with secondary roads likely remaining hazardous through today into tonight-Thursday AM due to bitterly cold temperatures.
Additional flurries, snow showers and instability supported squalls will be possible into Wednesday evening. New fluffy amounts of up to 0.5-1″ will be possible, with locally greater amounts within the High Knob Massif.
Wednesday Morning Through The Afternoon
Partly to mostly cloudy & bitterly cold. Flurries, snow showers and local snow squalls remaining possible. NW winds 5-15 mph with higher gusts. Temperatures nearly steady in the single digits at upper elevations and 10s at lower to middle elevations along-north of the High Knob Massif-Tennessee Valley Divide. Wind chill factors -5 to 10 degrees at lower-middle elevations below 2700 feet, and from -5 to -15 degrees in upper elevations.
Wednesday Night Into Thursday Morning
Partly to mostly cloudy. A chance of flurries and light snow. Riming at highest elevations. NW-W winds 5-10 mph, with higher gusts, on mountain ridges and exposed plateaus below 2700 feet. NW winds 5-15 mph, with higher gusts, on mountain ridges above 2700 feet. Temperatures varying from -15 to 5 degrees above, tending to rise on high mountain ridges overnight into morning. Below zero mins likely in many valleys if skies clear. Wind chills 5 above to -5 below zero at elevations under 2000 to 2500 feet, and -5 to -25 below zero over 2500 feet.
Partly to mostly sunny. SSW-WSW winds 5-10 mph, with higher gusts, below 2700 feet. WSW-WNW winds 10-20 mph, with higher gusts, on upper elevation mountain ridges. Temperatures in the 20s to lower 30s. Wind chill factors in the 10s to low 20s ( coldest highest elevations ).
Thursday Night Into Friday Morning
Mostly clear. Large vertical temperature spread developing between frigid mountain valleys & milder mountain ridges & exposed plateaus. Winds SSW-W at 5-15 mph with higher gusts, especially on upper elevation mountain ridges. Light valley winds. Temperatures varying from below 0 degrees in coldest mountain valleys to upper 10s & 20s on exposed middle to upper elevation mountain ridges. Wind chills in the 10s to lower 20s, with single digits possible in gusts on highest peaks.
Mostly sunny. Seasonally chilly. SSW-WSW winds 5 to 15 mph, with higher gusts. Max temperatures rising into the upper 20s to lower 30s at upper elevations to the upper 30s to lower 40s ( warmest in downslope locations ). Wind chill factors in the 10s and 20s to low 30s ( coldest at highest elevations in the High Knob Massif ).
Terrain models in combination with the European group are advecting low-level moisture into the area on gusty SW winds during the weekend, which would really put the lid on temps. Please stay tuned for updates and do not plan to break out the ”bermuda shorts” just yet! At least, not in the mountains.
Weather Discussion ( Bitter Air )
Recap & Analysis Of Event
A nice GOES-16 composite as the Circle of Illumination is poised to pass over the area as Earth rotates from daylight into darkness on January 18, 2018.
Northeast Canada and the northeastern USA are already going into night, with city lights noted around Lake Ontario. Observe snow cover across the piedmont of Virginia & North Carolina, as well as from the mountains west and northwest toward the open waters of Lake Michigan.
The coldest daylight period of the 2017-18 winter season to date was observed Wednesday as continental arctic air was able to invade the region across extensive snow cover that stretched from the Appalachians into Canada.
Note that flat surfaces and gently rolling terrain will appear brighter than complex terrain with snow cover; therefore, it is not possible to tell depth with this type of visible image. I always find it interesting that whenever Powell Valley has snow cover it will appear much brighter than the adjacent high country even though it always has much less snow depth. Part of that is due to the open nature of the Valley versus the thickly forested massif, but mostly this difference is due to the changes in albedo, or reflectivity, that are associated with highly complex, forested terrain.
A parting of the clouds was timed right for the daily MODIS pass of NASA’s Terra Satellite to capture the snow covered expanse of the High Knob Landform ( lower right above ) surrounded by lingering clouds ( close up below ).
Day time maximums barely broke above 0 degrees atop the High Knob Massif where the morning low dipped to around -8 degrees below zero ( with much colder wind chills ).
Temperatures tonight ( Wednesday ) have again dropped below 0 F.
Morning snow depths generally varied from 4″ to 8″ across the high country, with 2″ to 4″ of new snow adding to old snow already on the ground.
Joe & Darlene Fields had 2.5″ of new snow in the High Chaparral community, on top of 2″ already on the ground ( a general 3″ to 4″ or 4″ MEAN depth at their southern exposed location ).
Note a single snow does not = a snowpack. Multiple snows that generate snow cover add to form a snowpack. The exception to this might be when a monster snow dump occurs such that it lingers for a long time and experiences a variety of changing weather conditions. High country in the High Knob Massif is certainly one of the very few places in Virginia where true snowpacks tend to develop in winter. The largest snowpack recently observed was during February 2015 when multiple snowstorms generated 3 to 4 feet of average depth.
The winter of 2009-10 featured a long-lived snowpack with a general 1-2+ feet of snow depth across the high country of the High Knob Massif from December into March.
Flurries and light snow has been falling atop the High Knob Massif since before sunset, and recently have started again in Wise and Clintwood. As typical, Lonesome Pine Airport reported the “hottest” afternoon temp at its elevation.
The official NWS maximum reached 14.1 degrees in Clintwood at 1560 feet above mean sea level. If the mean Environmental Lapse Rate of 6.5 C degrees per kilometer is then applied ( 3.5 degrees Fahrenheit per 1000 vertical feet of elevation increase ) the result would translate to a MAX temperature of 14.1 – 3.9 = 10.2 degrees for the elevation at Lonesome Pine Airport ( or just about what was recorded at the 2650 feet elevation of Nora 4 SSE on Long Ridge ).
So whatever is causing the Lonesome Pine Airport air temperature to run too warm for its elevation, whether it be exposure near the runway, the radiation shield, a thermistor calibration problem, or a combination of factors it has now been ongoing for years. How do I know, because in years past I could almost always look at my temperature and subtract 4 degrees F to get what LNP reported. Other ASOS sites have reportedly also developed this problem!
The above being applicable to typical daytime conditions, as large temp differences typically develop at night between the exposed plateau and mountain valleys, or local sinks on the plateau, where cold air drains.
A couple of factors made this a difficult system to forecast over the mountains. Number 1, as I recognized, orographic forcing was to be weak. This was good if you were a lover of snow in downslope locations since it translated to a much lower magnitude of inverse forcing ( sinking air ). It also generated less low-level lift on the upslope side.
I heard the WCYB crew drove the new StormTracker from the TRI along U.S. 23 to Norton, then along Alt. 58 back toward the Great Valley. While I do not know exactly what they found, orographics would suggest that worst conditions would have been found from upper portions of Powell Valley, where U.S. 23 begins approaching the Overlook, on through Norton to Coeburn then improving upon dropping downward toward the Clinch River in St. Paul. While elevation changes upon passing through the Cumbeland Front a couple of times would be a factor, the biggest impact typically would be that the above noted trek was on the windward or upslope side of the High Knob Massif. In this case; however, orographics were weak so perhaps it did not make such a difference like often observed with stronger forcing.
With NW-N flow the Jasper to East Stone Gap valley corridor is hindered by sinking air off Wallen Ridge and the Little Stone Mountain arm of the massif, with the significant lift typically being seen upon looking toward the great cliffs which line the flanks of the massif core ( right side of view when driving north along U.S. 23 from Duffield to Norton ). The cliff line is a huge weather change marker, with locations above it often being obscured by clouds or snow-rime during winter events.
Number 2, as these graphics highlight, was harder to resolve since the system was undergoing a transition in its dynamics from being supported by upper air divergence- ageostrophically driven Direct Transverse Circulation into the RRQuad ( right-rear entrance region ) of a high altitude jet streak at 250-300 MB ( above ), to a setting driven by an Indirect Transverse Circulation in the LFQuad ( left-front exit region ) of a high altitude jet streak ( below ). This feature proving most significant since its ageostrophic circulation was able to tap into Atlantic moisture!
Much of the weather we experience, as Lackmann notes, is the result of these ageostrophically driven secondary circulations which are formed by the atmosphere in order to re-establish thermal wind balance ( Gary Lackmann: Midlatitude Synoptic Meteorology ). That is a very nice and important way to view instantaneous states of the fluid atmosphere ( weather ).
The isentropic lift which I outlined in the 011418 Forecast generated widespread snow, so a hit or miss of the forecast was not in question, just the amounts and where would any enhanced bands develop. Enhanced snowbands developed both west of the mountains and east of the mountains, but due to the transitioning phase were generally lacking over the high terrain of the Appalachians.
Had orographic forcing been stronger it could have helped to make up for a lack of upper air dynamics, with the lift generated by the High Knob Massif still doing well given its highly exposed nature as evidenced by the miles and miles of rime coated terrain alone.
The transitioning in the upper dynamics, as noted above, placed the main mountain chain of the Appalachians in its weakening phase. Ideally, of course, as past climatology well records, the greatest systems to impact the mountains occur with phasing of strong orographics with strong upper dynamics.
The closest enhanced band fell along the Daniel Boone National Forest in eastern Kentucky, with significantly higher amounts in local places in far western Kentucky.
Totals of 4″ on Beech Mountain and 6″ on Mount Mitchell were among the highest reported in the NC mountains, with Mitchell closest to the developing left front exit region.
Mount LeConte, at the top of the Great Smokies on the western side of the Appalachians, reported only 1″ of snowfall and depth.
Another notable aspect, of course, being much milder temps along the Blue Ridge and farther south across high peaks in the Smokies versus locations amid bitter surface-850 MB air along and west of the Cumberland-Allegheny Front ( which has been a trend during most of this 2017-18 winter ).
Just prior to sunset, and thickening clouds, the webcam was able to capture a line of Kelvin-Helmholtz waves with other orographic wave clouds. Note they appear like ocean waves and are always great to observe.
Flurries and light snow continues to fall from Clintwood to the summit of High Knob as of 4:30 AM Tuesday; although, this is generally not being picked up by Doppler.
A total of 2.6″ of new snow was measured in Clintwood at 3:00 AM, as steady, fluffy flakes continued to fall ( around 20:1 density and getting lower as the temperature continues to drop ).
Air temperatures have dropped below zero at the top of the mountains, even with low clouds and light snow continuing. Riming on the anemometer is likely causing wind speeds to be reported too low ( as orographic cloud vapor can be seen blowing across the summit of Eagle Knob, where riming is ongoing ).
A dramatic temperature drop was observed during Tuesday afternoon and evening as arctic air surged into the western slopes of the mountains.
Up to 30 degrees of temperature difference was observed between upper elevations in the High Knob Massif and the Tri-Cities ( lower 40s ) prior to cold air spilling across the mountains into the Great Valley of northeastern Tennessee and southwest Virginia.
This was part of a huge contrast in weather conditions across the Appalachians during Tuesday afternoon.
Mount Mitchell missed out on the January 12-14 snow event and had no snow prior to Tuesday night into Wednesday ( when snow should have finally covered the highest mountain in the East ).
A vigorous upper air wave will cross the mountains through Wednesday afternoon, with a pocket of very cold air aloft, so despite limited moisture in the vertical column this will continue to offer the chance for snow showers, flurries, and even some local squalls…especially on the upslope side of the mountains with respect to NW-N air flow.
Regardless of additional snowfall a bitter Wednesday is upcoming, even with some arctic sunshine this afternoon.
ALERT For Accumulating Snowfall From Mid-day Tuesday Into Predawn Hours Of Thursday, With Bitterly Cold Air Developing Tuesday Night Into Wednesday
A storm system with two phases, defined by the processes or mechanisms by which snow will be generated, will impact the mountain area from Tuesday afternoon into Wednesday night.
Phase 1 develops Tuesday with widespread snowfall generated by isentropic lifting and an influx of some moisture from the Gulf of Mexico. General 1″ to 3″ amounts are expected to accumulate by sunset Tuesday. Due to interaction between the two processes I am highlighting, I now have posted an alert for heavy snowfall late Tuesday afternoon into Tuesday evening.
A Heavy Snow Alert For Late Tuesday Afternoon And Tuesday Evening For Locations Along & Adjacent To The Cumberland Mountains
My latest analysis suggests that the arctic cold front will bring these two Phases together late Tuesday into Tuesday Night so I have now put out an Alert for Heavy Snow as the arctic front and upslope flow interact with deeper moisture for a period of time. I now think heavy snow will become widespread along the upslope side of the mountains, especially, with squalls remaining possible into Wednesday ( as already indicated ).
Phase 2 now develops by Tuesday sunset and continues through Wednesday evening ( possibly predawn of Thursday as flurries ), with the focus shifting to NW upslope snow, squalls & flurries.
A general 2″ to 3″ of snow is now expected from sunset Tuesday into Wednesday, especially within the favored upslope belt along and northwest of the Cumberland-Allegheny Front and possibly in a more narrow zone from Mount Rogers-Whitetop southwest along the Tennessee-North Carolina stateline.
Due to increased low-level lapse rates, local accumulations may also occur into the Great Valley on Wednesday.
Snowfall Forecast Tuesday AM – Thursday AM ( January 16-18 )
Phase I ( 10 AM to 5 PM Tuesday )
A widespread 1″ to 3″
Phase II ( 5 PM Tuesday to 7 AM Thursday )
2″ to 3″ along and west-northwest of the Cumberland Front and in narrow zones near Clinch Mountain and from Mount Rogers to the summit level of the Smokies
Locally up to 1″ within downslope locations of the Clinch, Powell, Holston-Great Valley corridor in NE Tennessee and SW Virginia
Storm Total Snowfall ( 7 AM Tuesday-7 AM Thursday )
3″ to 6″ with locally higher amounts possible in the High Knob Massif
Locations along and west to northwest of the Cumberland-Allegheny Front will be most favored for the higher snow totals during the entire event ( Phases I + II )
*Reference my updated forecast discussion for more details.
Sunday Night Into Monday Morning
Partly-mostly cloudy. A chance of morning flurries. Light valley winds. Winds becoming SSE-SSW at 5-10 mph, with higher gusts, on mountain ridges-exposed plateaus below 2700 feet. SSW-WSW winds 5-15 mph, with higher gusts, along mountain ridges above 2700 feet. Temperatures in the single digits & 10s, coldest in the upper elevations and mountain valleys with snow cover at low to mid elevations. A tendency for steady to slowly rising temps on mid-upper elevation ridges overnight. Wind chills in single digits & 10s on exposed mountain ridges, except below zero along highest ridges by late evening-overnight.
Partly to mostly cloudy ( mid-high clouds ). SSW-SW winds 5-15 mph, with higher gusts, below 2700 feet. SW winds 10-20 mph, with higher gusts, above 2700 feet. Temperatures varying from the mid 20s to the mid-upper 30s. Wind chills in the 20s on middle elevation ridges & plateaus and in the 10s at upper elevations ( single digits in gusts high peaks ).
Monday Night Into Tuesday Morning
Mostly cloudy. SSW-SW winds 5-15 mph with some higher gusts on mountain ridges-plateaus below 2700 feet. SW winds 10-20 mph, with higher gusts, on mountain ridges above 2700 feet. Temperatures varying from the lower 20s to around 30 degrees. Wind chills in the 10s to lower 20s, with single digits in gusts at highest elevations.
Snow developing. Heavy at times. SW winds shifting NW-NNW at 5 to 10 mph, with some higher gusts. Temperatures near steady to falling in the 20s during mid-late afternoon, falling into 10s at upper elevations toward sunset. Riming at upper elevations.
Tuesday Night Into Wednesday Morning
Snow giving way to snow showers & snow squalls. Whiteout snow bursts possible, especially along and northwest of the Cumberland Front ( High Knob Massif and Tennessee Valley Divide ). Turning bitterly cold. NW-N winds 5-10 mph with higher gusts. Temperatures dropping into the single digits above and below zero. Wind chills 5 above to 10 below zero, except as cold as -20 below zero or lower on highest peaks. Rime formation at upper elevations.
Wednesday Morning Through The Afternoon
Mostly cloudy & bitterly cold. Flurries, snow showers and snow squalls ( bursts of snow ). NW winds 5-15 mph with higher gusts. Temperatures nearly steady in the single digits at upper elevations and the 10s at lower to middle elevations along and north of the High Knob Massif and Tennessee Valley Divide. Wind chill factors 0 to 10 degrees at lower-middle elevations, and 0 to -10+ degrees below zero at upper elevations. Blowing & drifting at high elevations in the High Knob high country.
Weather Discussion ( Another Blast )
An array of beautiful lenticular ( mountain wave ) clouds at sunset signals that some interesting weather is on the way.
As I have been highlighting now for days, a system with two distinct phases will impact the mountain region during the next couple of days.
Phase 1, as I highlight above using the 850 MB moisture field, is essentially associated with the warm conveyor belt. Streamlines show some connection to the Gulf of Mexico to denote that higher energy air, with latent heat, will advect northward and will be rising and undergoing condensation.
Although Phase 1 will not have a huge connection to the Gulf of Mexico, there is still a notable warm conveyor belt that is showing up on the modeling ( as noted previously ) and to be a warm conveyor belt it does not need to have a textbook type of Gulf connection ( indigenous to the USA ).
This process can be envisioned by looking at the Monday 294K surface at 7 AM, with air at the 800 MB level near the Gulf of Mexico that will begin to turn northeast toward 700- 600-500 MB air ( air will begin blowing across isobars from higher to lower pressure = rising air along the isentropic surface ). The process noted at 7 AM ( above ) is now being observed on the 7 PM Monday chart ( below ), with moisture transport denoted by mixing ratio contours in green.
Due to diabatic processes, like latent heat that is released during condensation and freezing, the continuity of the isentropic surface will tend to be disrupted and the level to see processes best may need to be changed or looked at in vertical cross-sections.
For more information reference: Isentropic Analysis from which much of this information is courtesy of the late Dr. James ( Jim ) Moore who helped me learn about this important way to analyze the atmosphere.
Most everyone seems to agree on Phase 1, with widespread Winter Weather Advisories now in effect to 7 PM ( Wise ) or Midnight ( Dickenson-Buchanan counties ) on Tuesday. The second phase of this system should not be neglected and is likely to generate Winter Storm Warning criteria as it will add to initial isentropic lift snow ( warning criteria was also reached in the High Knob high country during the previous January 12-14 weather event ).
Air begins turning bitterly cold Tuesday evening with NW-NNW upslope flow and a low-level moisture connection to the Great Lakes, such that snow showers and squalls will be developing in the upslope flow behind the widespread snow associated with Phase 1 that will be moving eastward.
Although atmospheric moisture in a vertical column will be dropping to between 0.10″ to 0.20″, air temps will be in the single digits above and below zero ( at top of mountains ) by Wednesday morning.
Cyclonic vorticity advection + a pocket of -30C air aloft will be moving over the mountains from Wednesday morning into the afternoon ( above ). This will help increase low-level lapse rates to take advantage of what moisture is present amid the bitterly cold air ( below ).
As observed during the January 12-14 AM period, flakes of snow will be likely to fall along the upslope side of the mountains until the last low-level cloud dissipates!
The Bottom Line…Widespread snow develops Tuesday to impact the mountain region during Phase 1. Expect hazardous travel as snowfall rates increase and temps begin dropping through the afternoon, with the heavy snow potential currently appearing to be increasing for late Tuesday afternoon into Tuesday evening.
The event does not end at 7 PM when current Winter Weather Advisories are set to expire, but instead will continue Tuesday Night into Wednesday with flurries, snow showers, and squalls ( some featuring whiteout bursts of snow in local places ).
While NW-N upslope flow will tend to favor the typical lifting zones, dynamics in the upper air and increasing low-level lapse rates means activity may not be restricted completely to the NW upslope flow corridors on Wednesday. Bitterly cold temperatures and wind chills will make conditions worse, especially at middle-upper elevations. Please take care.
A couple of Sunday morning views from the high country of the High Knob Massif where low temperatures dropped well below zero. There is more of that coming!
Storm Total Snowfall January 12-14
Joe & Darlene Fields High Chaparral Community 4.5″
Steve & Cody Blankenbecler Eagle Knob of High Knob Massif *6.0″
*( 3-8″+ ground depths )
Morning view looking across lofty High Knob Lake Basin toward an inversion layer along the Tennessee Valley.
Snowflakes literally fell until the very last low-level cloud dissipated, with light snow-flurries all night into Sunday morning from Clintwood to Norton-Wise and into the high country of the High Knob Massif.
While the High Knob Massif may not be the tallest mountain in the Appalachians, it is one of the most distinct from high resolution satellites in space. See if you can pick it out?
An aspect which helps make it such a prolific snow mountain in winter, precipitation enhancer + microclimate generator through all seasons is that its sprawling ( wide ) mass rises isolated within the atmosphere ( as clearly seen above from 22,300 miles up! ).
An array of mid-high clouds decorated the sky as Sunday afternoon temperatures held in the 10s to lower 20s within locations along and north of the High Knob Massif and the Tennessee Valley Divide ( forecasters observe the chilling, albedo effect, of snow cover ).
As most people who live in Wise know, the Airport tends to report the “warmer” air temperatures on the Wise Plateau, with Nora 4 SSE along the adjacent Sandy Ridge Plateau often being more in line with UVA-Wise and what many shaded thermometers read in Wise.
Sunday MAX temperatures varied from 22.1 degrees in Clintwood ( official NWS temp ) to low teens at the summit level of the High Knob Massif.
**The mean snow depth reached 2″ in Clintwood with a total snowfall of 3.2″ ( mostly a low density snow after the initial transition occurred in milder temps ). Interestingly to some, perhaps, 1.7″ fell after 9 AM Saturday into Sunday morning.
The upcoming system should not be taken for granted given its nice upper air dynamics, with the only aspect holding me back from calling for a major dump of snow being that the orographic forcing will be weak-moderate.
Starting at the top, synoptic-scale lift into the right-rear quadrant of a 100-130+ knot jetstreak at 300 MB will be in a climatologically favored position to support widespread snow as the system develops Tuesday.
Air turns bitterly cold Tuesday night into Wednesday AM such that the bottom drops out of snow densities on NW-N upslope flow into the Appalachians. Synoptic lift will also continue to be a factor aloft as divergence and cyclonic vorticity advection occurs into Wednesday morning.
While model QPF can be used as general guidance, one can not forecast a system like this using 10:1 density, especially as air turns bitterly cold into the NW upslope zones. As just seen, snowfall with the most recent system was higher than model projections and lasted much longer than they said.
The Bottom Line…This system will impact a wide region due to general isentropic lift of higher energy air above lower energy air as part of Phase 1 during the day Tuesday. If the shift to NW upslope is faster and a little stronger it will enhance amounts along and west of the Cumberland Front versus the Great Valley; however, this type of setting can be favorable for the TRI area if up-valley flow occurs and/or the sinking on NW flow is weak given that the upglide process develops snow independently of the terrain on a larger-scale.
Dr. Charles Doswell would say, upper level air is always over-running lower level air so we do NOT want to call it overrunning unless a physical mechanism is stated to explain what makes this over-running any different from other cases ( in my own words to highlight why I include a physical mechanism above ). In this case a Direct Transverse Circulation will develop as “milder” air rises upward into the right-rear entrance of the upper-level jet and colder air sinks with development of an ageostrophic flow to covert potential energy into kinetic energy of the system.
Phase 2 develops Tuesday Night into Wednesday as advection ( transport ) of bitterly cold air begins with a focus shifting to NW upslope snow. Although the low-level orographic forcing will tend to be weaker than this most recent system, upper air dynamics + weak-moderate forcing will combine with a plunge in snow density to support bursts of snow as nearly all moisture within the vertical column is forced out.
Alert For Significant Frozen Precipitation And The Development Of Extremely Bad Road Conditions By Friday Evening With A Sharp Temperature Plunge Expected Initially Along And North To West Of The High Knob Massif & Tennessee Valley Divide During The 6:00 PM To Midnight Period Friday ( Including Far Western Lee County ) – Followed By Continued Drops And Very Low Wind Chills Into Saturday Morning
A wind shift to a northerly upslope direction, with influx of cold air, is currently timed for late Friday into Friday evening. Due to initially shallow cold air advection a transition from rainfall to freezing rain will become possible first at mid-upper elevations within the lifting zone along and north of the High Knob Massif and Tennessee Valley Divide and Pine Mountain. As the cold air deepens vertically a transition into sleet and snow will occur across the area. Bitterly cold air and dangerous wind chills are expected to develop as temperatures drop into the single digits and teens at mid-upper elevations into Saturday morning.
Bitterly cold conditions will continue through Saturday with daytime temperatures expected to hold in the 10s to around 20 degrees at lower-middle elevations, and in the single digits to around 10 degrees at upper elevations, in locations along and north of the High Knob Massif & Tennessee Valley Divide.
ALERT For Ponding Of Water Along Roads And Strong Rises On Creeks With Upslope Rains Within The High Knob Massif-Black Mountain Corridor Into Friday In Advance Of The Temperature Plunge
As of 1:00 AM Friday more than 2.00″ of rain had fallen within upslope locations of the High Knob Massif and Black Mountain corridor of far southwestern Virginia. Due to partially frozen ground, especially across northern slopes, run-off on creeks will need to be closely monitored in watersheds of the Cumberland, Clinch, and Powell basins.
Lingering ice along some creeks could also be problematic. Folks living and driving along streams should remain on alert as periods of heavy rain continue to fall through Friday afternoon.
**A break in rainfall will be possible Friday morning before new rains, heavy at times, redevelop in advance of a strong cold front during Friday afternoon. Remain alert for strong rises on streams.
Overnight Into Thursday Morning
Mostly cloudy ( mid to high clouds ). Large vertical temp spread between sheltered valleys & gusty mountain ridges and exposed plateaus. SE-S winds 5-15 mph, with higher gusts, along mountain ridges and plateaus below 2700 feet. SSE-S winds 15-30 mph, with higher gusts, along mountain ridges above 2700 feet. Temperatures varying from around 30 degrees in colder, sheltered valleys to the mid 40s to low 50s on windy mountain ridges. Rising valley temperatures overnight into morning in most locations.
Mid-Morning Thursday Through The Afternoon
Mostly cloudy with rain & showers developing into the afternoon. Windy. SE-S winds 10-30 mph, with higher gusts. Temperatures varying from the lower 50s to lower 60s ( coolest at highest elevations ).
Thursday Night Into Friday Morning
Rain likely with a chance of thunder. Downpours possible, especially in upslope locations within the High Knob Massif-Black Mountain corridor. SSE-S winds 5-15 mph with higher gusts along mountain ridges-plateaus below 2700 feet. SSE to SSW winds 15-25 mph, with higher gusts, along mountain ridges above 2700 feet. Low cloud bases with widespread dense fog at upper elevations. Areas of fog at middle to lower elevations.
Periods of rain. Chance of thunder. Downpours likely. Winds shifting SW to NW at 10-20 mph, with higher gusts. Sharply dropping temperatures toward sunset into early evening. Temps in the upper 40s ( highest elevations ) to upper 50s prior to falling into the 30s shortly after sunset. Widespread fog at upper elevations with orographic clouds, then dropping cloud bases into middle elevations behind the frontal passage; otherwise, local areas of fog.
Friday Night Into Saturday Morning
Rain changing to freezing rain, sleet, and snow. Snow heavy at times after midnight. Turning bitterly cold. Winds NW-N 10-25 mph, with higher gusts. Temperatures plunging into the 10s to around 20 degrees by morning, with single digits to 10 degrees at highest elevations, along and north of the High Knob Massif-Tennessee Valley Divide. Milder amid downslope areas of the Clinch, Powell, Holston valleys ( 20s to around 30 degrees ). Wind chills falling through the 20s into the 10s and single digits, except to below zero at upper elevations. Riming upper elevations.
Saturday Morning Through The Afternoon
Mostly cloudy. Bitter. Snow showers & flurries. NW-N winds 10-20 mph, with higher gusts. Temperatures near steady in the 10s to around 20 degrees across lower-middle elevations, along-north of the High Knob Massif-Tennessee Valley Divide, and in the single digits to around 10 degrees across upper elevations. Riming at upper elevations. Wind chills in the single digits above and below zero, except -10 degrees below zero or colder at highest elevations.
A prolonged period of winter cold, with additional snowfall chances, is being monitored through next week ( Jan 14-20 ).
Weather Discussion ( Huge Change )
Thursday Evening Update
My previous forecast holds with this update, with new additions being to highlight persistent rains that have dropped locally more than 1.50″ in the past few hours within portions of Wise & Harlan counties.
*More than 2.00″ of rainfall as of 1:00 AM Friday.
The latest run of the NAM terrain model has done a nice job of picking up this band, being forced by a combination of low-level lift and upper-level vertical motion into the right entrance region of a high altitude jet streak, as well as the enhancement with rise along the Blue Ridge in the extreme southern portion of Virginia and western North Carolina.
When a model initializes current conditions well then this helps to improve the odds that its short-term outlook will be on target, thus the thinking does not change that very bad conditions will become likely Friday evening into the morning hours of Saturday as a major temperature drop occurs with influx of bitter air into locations along and westward of the Cumberland-Allegheny Front.
Temperatures will begin to initially plunge first in the far western portion of Lee County, and along the Cumberland Plateau in Tennessee, as winds have more of a westerly flow component during Friday afternoon.
During Friday evening, as winds develop more of a NW-N flow component, the strongest temperature drops are then expected to become concentrated along and north to west of the High Knob Massif-Tennessee Valley Divide with a period of low-level terrain blocking likely acting to increase temp differences between Norton-Wise and the Tri-Cities more than the high resolution NAM 3 KM Model is suggesting.
Temperature drops will locally be stronger Friday evening in communities such as Norton-Wise, Clintwood and Pound than in Big Stone Gap, Gate City, Fort Blackmore and Dungannon as the flow works to accentuate terrain influences.
The Bottom Line…Conditions Are Expected To Become Bad Friday Evening Into Saturday Morning As A Major Temperature Plunge Interacts With Moisture. Places Along And West Of The Cumberland-Allegheny Front Are Likely To Experience The Worst Conditions.
Towering cliffs along the Cumberland Front, the White Rocks of present day Cumberland Gap National Historical Park, were first described by early frontiersmen such as Daniel Boone. The bulge of high country along this front associated with the High Knob Massif was a refuge for Robert ( Bob ) Benge, who terrorized the frontier of eastern Tennessee and southwestern Virginia during the long and bloody period of 1774-1794.
Bob Benge was a Chickamauga Cherokee war chief, the mixed-blood son of a Cherokee woman and a Scots-Irish trader named John Benge ( reference Wikipedia ), with places such as Benges Basin and the Chief Benge Scout Trail in the High Knob Massif named after him today.
Hazardous Roads Could Develop Across The Region Due To Such A Sharp Temperature Drop, Even In Locales Which Receive Little Frozen Precipitation.
A huge weather change back into winter is approaching the mountains as unseasonably mild conditions into Friday give way to a sharp temperature plunge Friday Night into early Saturday. Precipitation types during the Friday and Friday Night period could run the gamut from wind driven rain, spiced with thunder, to freezing rain, sleet, and snow.
Temperatures currently are forecast to drop below freezing beneath the 850 MB level between 7:00 PM and 10:00 PM Friday evening, as an above freezing zone lingers aloft. This vertical temperature profile would result in freezing rain and sleet.
All eyes in the forecast community should be on soundings, with the next few model runs being critical as to helping better determine how this period will unfold.
The positions of developing surface and 850 MB low pressure centers will be important, with a somewhat farther east solution than currently shown by the NAM Model allowing cold air to advect into the western front range of the Appalachians a little quicker. A farther west solution would tend to delay the cold air. Climatology in this type of digging upper trough setting suggests that a leeward low track ( east of the Eastern Continent Divide ) tends to be favored.
While the current NAM Model is in basic alignment with the latest European Model and its mean Ensembles & Control in showing a pronounced orographic signature, with heaviest snowfall along and upstream of the western front range of the Cumberland-Allegheny Front, there will likely be some changes in the placement of this during the next 24-48 hours. Stay tuned for important updates.
Bitterly cold air flooding into the mountains will be a most notable part of this event.
Although changes may occur in this forecast, the drop in 850 MB temperatures on this type of air flow agrees with past climatology of observed events.
The development of the coldest 850 MB temperatures in the time between 7:00 PM Friday ( above ) and 1:00 AM Saturday ( below ) aligns with the strongest low-level orographically forced lifting along and north of the High Knob Massif and Tennessee Valley Divide on northerly air flow. So while the complete three-dimensional nature of this complex terrain tends to not be fully resolved, one has to think that this run of the model is picking up on part of the orographics. This aspect, along with general alignment with the ECMWF, is the main reason I chose this model to highlight how the Friday evening period will unfold ( remembering this is a setting that may, and will likely, change in some ways ).
I had highlighted this system for a long time as one to watch, and as it gets closer the reasons are becoming more and more distinct with a huge temperature gradient acting to drive imbalances within the thermal energy, mass and momentum of the atmosphere. Quasi-geostrophic theory says that the atmosphere will thus respond by changing the vorticity structure of its mass field ( see map below ) and by changing the temperature of air parcels by adiabatic rising and sinking motions as demanded by thermal wind balance.
By 10 PM Friday Night the 500 MB trough ( above ) has taken on a negative tilt, with cyclonic vorticity advection as well as mass divergence ( not shown ) contributing to upward motion aloft as orographics lift the lower atmosphere.
A 30 to 40 degree temperature plunge is expected to occur between 4:00 PM Friday and 1:00 AM Saturday, with more declines through sunrise on Saturday.
These are surface temperature forecasts and do not reflect colder conditions at higher elevations nor the wind chills that will really make this a notable event.
This will mark the beginning of another prolonged period of cold conditions, and if significant snow accumulates it may be able to generate readings significantly lower than are being currently predicted into next week.
Meanwhile, lets not forget about the strong winds blowing across the high country which will tend to mix downward more into middle-lower elevations during Thursday.
ALERT For Areas Of Dense Fog Overnight Into Tuesday Along With Localized Icy Patches On Some Roadways
Lingering low-level moisture trapped beneath an inversion will continue to produce abundant fog overnight into Tuesday, with areas of drizzle. While most temperatures will remain near to above freezing, a few valleys may remain near to below 32 F.
Radiational cooling by Tuesday Night into Wednesday AM will keep near saturated to saturated conditions at lower to middle elevations while drier air overspreads upper elevations, above 3000 feet.
An Alert For Mountain Wave Wind Gusts, And Strong General Winds At Upper Elevations, May Be Needed By Late Wednesday Into Thursday
Overnight Into Tuesday Morning
Low clouds with areas of dense fog. SW-W winds 5-15 mph with higher gusts on mid-upper elevation mountain ridges and plateaus. Temperatures widespread in the 30s. Wind chills in the 20s to lower 30s ( coolest highest elevations ).
Low clouds with areas of dense fog, especially at higher elevations. Light winds, except W-SSW at 5-10 mph along upper elevation mountain ridges. Chilly and damp with temperatures mainly in the lower-middle 40s.
Tuesday Night Into Wednesday Morning
Low clouds with areas of dense fog at lower-middle elevations. Becoming partly-mostly cloudy at upper elevations. Winds SSE-SSW 5-15 mph, with higher gusts, along mid-upper elevation mountain ridges. Temps in the 30s, except near steady or rising in the 40s along higher mountain ridges.
Wednesday Morning Through The Afternoon
Becoming partly-mostly sunny. Milder and gusty. SSE-S winds increasing to 10-20 mph with higher gusts. Temps varying from 40s to low 50s in upper elevations to the upper 50s to around 60 degrees.
Wednesday Night Into Thursday Morning
Partly-mostly cloudy. Windy. Unseasonably mild. SE-SSE winds 10-25 mph, with higher gusts, at elevations below 2700 feet. SE-SSE winds 20-30 mph, with higher gusts, along mountain ridges above 2700 feet. Temperatures widespread in the 40s to around 50 degrees.
The potential for major storm development, with rain to snow and an influx of bitterly cold air, is being monitored for the late week and weekend period ( January 12-14 ).
Weather Discussion ( Brief Thaw )
Abundant low-level moisture is trapped beneath an inversion layer left over in wake of the Monday system which dropped everything but the kitchen sink on the mountain landscape ( snow, sleet, freezing rain, rain ).
If this low-level inversion does not mix out Tuesday, abundant moisture will continue as some terrain models suggest with drier air advecting into upper elevations by Tuesday Night into early Wednesday to set the stage for a very nice but increasingly gusty Wednesday.
Wednesday is looking to be the best day of the week, in general, as strong winds develop by Wednesday night into Thursday on SE-S flow to increase moisture into the Blue Ridge.
All eyes will then focus upon a upper wave digging into the eastern USA by later this week, with potential for phasing between sub-tropical and polar jet streaks suggesting the potential of major cyclogenesis ( storm development ).
Although details remain to be worked out, a setting like this could generate rain-thunderstorms in its warm sector and a synotpic-scale snowband with wrap-around and/or NW Flow snow developing on its cold side. Stay tuned.
While frozen precipitation types will be possible in all locations, a main concern today is frozen surfaces from more than a week of sub-freezing temps. Even if air temperatures are above freezing surfaces subjected to prolonged bitter cold will not have had time to warm prior to the development of precipitation.
There will be an enhanced potential for frozen precipitation forms, including snow, within the SW flow upslope zone within the High Knob Landform corridor where SW air flow rises, such that temperatures will remain colder than forecast models predict from the Tennessee Valley Divide ( Norton-Wise-Sandy Ridge ) into portions of Lee County. If snow develops within this zone then a general 1-3″+ will be possible; otherwise, other frozen types are expected from mid-morning into the afternoon.
Strong SW Winds Will Develop Sunday Night Into Monday At Middle-Upper Elevations Along The Cumberland Mountains
Although temperatures are expected to display near steady to rising trends across middle-upper elevation mountain ridges and exposed plateaus during Sunday Night into Monday, development of strong to ROARING SSW-WSW winds will continue to generate low wind chill values. Wind gusts of 25-50+ mph will become likely as a low-level jet interacts with higher elevations.
Bitter Cold Temperatures Will Continue Into Sunday Morning As A Large Temperature Spread Develops In The Vertical Between Frigid Valleys & Milder Ridges
A rapid Saturday evening-night temperature drop is expected in mountain valleys as ideal cooling conditions develop. Meanwhile, higher mountain ridges will begin to experience a slowing of falls and even rising temperatures overnight into Sunday morning as winds shift S-SW. This will generate a large vertical temperature spread between colder valleys that drop below 0 degrees and milder ridges which rise upward through the 10s.
Saturday Night Into Sunday Morning
Mostly clear. Bitter. Large vertical temperature difference developing between frigid valleys and ”milder” mountain ridges. Light winds becoming S-SW along high mountain ridges overnight-into morning at 5-10 mph. Temperatures varying from below zero in colder mountain valleys and on highest peaks ( early ) to the single digits, then rising thru the single digits into the 10s along higher mountain ridges overnight into morning. Wind chills from 5 above zero to -15 degrees below zero on mountain ridges.
Sunday Morning Through The Afternoon
Increasing mid-high altitude clouds. S-SW winds increasing to 5-15 mph, with higher gusts. Temperatures varying from the mid 20s to low-mid 30s. Wind chills in the 10s and 20s.
Sunday Night Into Monday Morning
Mostly cloudy. Becoming windy. Evening temp drop in sheltered valleys, then rising temps possible with mixing overnight into morning. SSW-SW winds 10-20 mph, with higher gusts, on mountain ridges-exposed plateaus below 2700 feet. SW winds 15-25 mph, with higher gusts, along mountain ridges above 2700 feet. Temperatures mainly in the 20s, except dropping into the 10s in sheltered valleys prior to rising by morning. Wind chill factors in the single digits and 10s.
Monday Morning Through The Afternoon
Cloudy with snow, sleet, freezing rain-rain developing. Icy conditions likely on many surfaces. Windy. SW winds 15 to 25 mph, with higher gusts ( decreasing beneath the top of any low-level inversion formation ). Temperatures in the 30s, then dropping to around or below freezing within the SW upslope flow corridor along and southwest of the High Knob Massif-Tennessee Valley Divide as well as beneath any low-level inversion. Wind chill factors in the 10s and 20s. Dense fog formation possible.
A January thaw period next week is expected to give way to bitter cold once again by late next week-weekend ( January 12-14 period ) as the winter pattern locks back into the eastern USA.
Weather Discussion ( Nasty )
Early Overnight Monday Update
Strong-roaring SSW-SW winds are blowing across the high terrain in advance of developing precipitation downstream of the mountains.
Strong and gusty winds will continue to mix downward to below the 2700 foot level into middle elevations overnight into Monday morning.
Dry air in lower levels of the atmosphere may keep precip from reaching the ground until the mid-morning period, with evaporative cooling helping create a setting where various types of frozen precipitation will be likely.
The best chance for snow, with possible accumulations, will be within the SW air flow upslope zone from the Tennessee Valley Divide-High Knob Massif southwest into portions of Lee County where air flow becomes orographically forced.
Unless a strong inversion develops, limiting mixing, downslope flow could help reduce frozen amounts in the Pound-Clintwood to Haysi-Grundy zone; however, this is not a typical setting with all ground surfaces being frozen to increase the hazardous aspect with only light precipitation amounts.
Another bitterly cold night, especially in mountain valleys, will be observed into Sunday morning before conditions get nasty in another way.
The best radiational cooling conditions of this great cold wave to date will be observed tonight into Sunday morning, especially within mountain valleys where temps will again drop below zero ( far below in high elevation valleys where temperatures were already dropping to 0 degrees at 6 PM ). Lower elevation valleys, like in Clintwood, with less snow cover were at 10 degrees at 6:00 PM Saturday.
Saturday ( January 6 ) MIN Temperatures
Nora 4 SSE -1 degree
Clintwood 1 W -5 degrees
City of Norton -6 degrees
Black Mountain -7 degrees
*The coldest temperatures occurred within the snow covered expanse of the High Knob Massif, which I will highlight later.
Low temperatures into Saturday morning were frigid from top to bottom, with most locations across Wise & Dickenson counties falling to 0 degree and below.
Saturday afternoon MAXS varied from 6 degrees on Eagle Knob of the High Knob Massif to 18 degrees in Clintwood. No wonder high valleys have had such a quick drop toward zero degrees with cold air drainage from the summits, with the temp on Eagle Knob falling back to 0 degrees at 6:15 PM.
The main difference tonight versus last night being that temps will not drop a lot more on the summits, but will begin to rise into the overnight and morning hours of Sunday as high valley temps continue to drop. A key factor to cooling being dewpoints within the 875-825 MB layer of the atmosphere for high country valleys surrounded by snow covered terrain.
After more than 40 years of recording mountain weather it has been learned well that coming out of a great cold wave is typically never easy, with strong winds causing low chill values being a typical ingredient having to be endured as baroclinicity increases ( horizontal temp gradient packs ) and generates a pressure gradient that cranks up strong winds to push against the mountains and drive the bitter air away ( an especially nasty process for everyone living within mid-upper elevations ).
Models will continue to struggle as they try to resolve what is going to happen with the vertical temperature profile in the atmosphere, but the fact remains they they simply will not be able resolve what 10-14 days of below freezing temps have done to surfaces ( freezing the ground deep ) which will not even begin to thaw prior to any precipitation.
An ice up on surfaces, a period of snow to mix, formation of fog over cold ground are all on the table, perhaps even more significant SW upslope flow snow, as the battle between warm air advection and arctic air develops Monday.
Roads treated with new cold weather solutions will certainly do better than many back roads and other surfaces which are not treated in such ways. No surface; however, should be taken for granted and everyone will need to be on high alert Monday with respect to decks, walkways, roads, etc…until the icing threat passes.
One aspect appears certain, given this great cold wave has not been just concentrated in the mountains, the typically milder domain of the Great Valley will also be subjected to potential chaos as this transition occurs.
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.
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.
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.
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 ).
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.