Mountain Area Forecast ( Jan 17-19 )

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 ).

Previous Alert

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.

Thursday Afternoon

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.

Friday Afternoon

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.

 

Previous Discussion

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.