Mountain Area Forecast ( Dec 26-29 )

The Coldest Air Mass Of The Season Is Expected To Arrive During New Year’s Weekend

A large mass of bitterly cold air is expected to pour south into the mountains during New Year’s Weekend.  While some snow will be likely with northerly upslope flow, amounts continue to look light.

This will be the beginning of a prolonged period of bitterly cold conditions that may eventually allow for a more important winter storm to impact the Appalachians in the first week of January.

Former ALERTS

Air Will Turn Increasingly Bitter & Much Drier ( Arctic Dewpoints ) Later Wednesday Into Thursday Morning With Northerly Flow Behind A Passing Disturbance

The coldest temperatures of this winter season to date are expected Wednesday Night Into Thursday Morning as bone dry arctic air combines with light winds and mainly clear skies to allow temperatures to tank.

Morning lows into December 28 varied from below zero in colder valleys of the High Knob Massif to 5 degrees on Long Ridge of Sandy Ridge and 6 degrees in Clintwood.

 

Detailed Forecast

Tuesday Night Into Wednesday Morning

Mostly cloudy.  Cold.  Small chance of snow flurries.  Light northerly winds during the evening increasing to 5-10 mph, with higher gusts, along mid-upper elevation mountain ridges overnight into morning.  Temperatures dropping into the low-mid 10s to the low-mid 20s ( coldest within upper elevations ).  Wind chills dropping into the single digits and 10s, locally below zero on peaks by morning.

Wednesday Afternoon

Becoming mostly clear into evening.  Colder.  Light snow or flurries possible prior to clearing.  NNW to NNE winds 5-10 mph with higher gusts.  Temps near steady or falling in the 10s upper elevations and 20s middle to lower elevations.  Wind chills in single digits and 10s, except below zero in gusts higher peaks.  Riming highest ridges.

Wednesday Night Into Thursday Morning

 Mostly clear & bitterly cold.  Light winds.  Temperatures dropping into the single digits to 10 degrees, except around 0 degrees or below zero in upper elevation valleys.

Thursday Afternoon

Increasing mid-high clouds.  Light winds becoming S-SW at 5 to 10 mph, with higher gusts on upper elevation mountain ridges.  Temperatures rising into the low-mid 20s to lower 30s ( coldest upper elevations ).  Wind chill factors in the 10s and 20s ( single digits to around 10 in gusts along high mountain crest lines ).

Thursday Night Into Friday Morning

Partly to mostly cloudy.  Light valley winds.  SW-W winds increasing to 10-20 mph, with higher gusts, along middle-upper elevation mountain ridges ( especially at elevations above 2700-3000 feet ).  Temperatures in the 10s to middle 20s.  Wind chills in single digits & 10s on higher ridges.

Friday Afternoon

Partly to mostly cloudy.  Cold.  Variable winds generally less than 10 mph.  Temperatures varying from 20s to lower 30s, coldest upper elevations.  Wind chills in the 10s & 20s.

Reference my 122217 Forecast page for a review of the Christmas Holiday period and The High Knob Landorm for more information.

 

Weather Discussion ( Arctic Air )

Thursday Afternoon ( Dec 28 ) Update

For a 4th consecutive day temps have remained below freezing in locations along-north of the High Knob Massif and Tennessee Valley Divide ( MAX 28 degrees Clintwood ).

For the 2nd consecutive winter a snow drought is ongoing, with totals so far this season ( October-December 2017 ) of around 12″ atop the High Knob Massif and 16″ on Snowshoe Mountain in West Virginia.  How often can it be said, at this point in a winter season, that places in Georgia have had more snow than these 4,000+ foot peaks which are among the most favored for heavy winter snowfall totals?

Although snow continues to cover the ground in the high country of the High Knob Massif, it is nothing like it often would be during this time of year with season-to-date snowfall being much less than it typically is just during December.

If it were not for the freak southern snow band of early December there would not have been hardly any snow this season atop the highest southern Appalachian peaks, where the summit of Mount Mitchell is currently snow-rime free with no white Christmas in 2017.

While Snowshoe Mountain will pick up some snow from clippers tracking through the northern stream flow into New Year’s weekend, the southern Appalachians will be mostly in between northern and southern jet waves.

A significant north-to-south temp gradient has and will continue to bias places at higher latitudes along the Appalachian chain for colder temperatures.  For example, morning MINS on December 28 reached 12 degrees on Mount Mitchell versus 1 degree Fahrenheit on High Knob and -7 below on Snowshoe Mountain.  Despite being significantly higher in elevation, the 6684 foot summit of Mitchell was standing amid much milder ( relative speaking ) 850-800 MB air compared to these lower summits at higher latitudes.  Far to the north, the 6288 foot peak of Mount Washington, NH recorded -34 degrees below zero ( -89 F below wind chill ) on December 28.

If a big east coat storm bombs next week then a blob of this truly bitter air could get pulled farther south to impact the southern Appalachians.  Stay tuned for updates.

As bitterly cold air settles south on northerly flow there will be the chance for some snow accumulation, especially along and north of the High Knob Massif-Tennessee Valley Divide where air will be orographically forced to rise.  It takes very little to generate snow in such cold air with vertical lifting, so this will bear watching – the only ingredient needed will be a little moisture!

Although timing has varied and run-to-run differences have been common, models continue to suggest a major storm will develop in this pattern with current timing  being after New Year’s by the middle of next week.

Regardless of snow, the next week to ten days are going to be bitterly cold with a MEAN 850 MB temp of -11 to -12 C being predicted by the ensemble mean of the European Model group above the High Knob Massif area.

This is likely to generate temperatures well below zero as time passes, with -20 below being a target number for the continued extirpation of Hemlock Wooly Adelgids.

*Hemlock Wooly Adelgids ( Adelges tsugae ) were killed out of the high country of the High Knob Massif during the 2014-15 winter season when air temperatures plunged into the -20 to -30 degrees below zero ( Fahrenheit ) range; however, because conditions did not get that cold to the south and southeast of the massif area the adelgids survived and subsequently migrated back into the massif area during the past couple of years.

While many Canadian Hemlock ( Tsugae canadensis ) trees have been killed since adelgids were able to breech the mountain range since the late 1990s, many are also looking good currently thanks in part to the bitter cold of February 2015.  Adelgids were found in eastern Virginia as long ago as the 1950s, but were never able to cross the Appalachians into this area due to periodic bitter cold spells during winter ( -20 F being their kill threshold ). 

 

Wednesday Afternoon Update

My update this afternoon includes slowing down the clearing a little, until late afternoon-early evening, and introduces light, fine snow-flurries in upslope locations.

Low clouds are banked up against western slopes of the mountains & adjacent foothills of the Appalachians with lingering moisture in advance of bone dry continental Polar air.  A little light snow and flurries are falling out in upslope locations.

Temperatures at 1:00 PM varied from 15 degrees ( wind chill 3 F ) on Eagle Knob of the High Knob Massif to 36 degrees down in the Tri-Cities of the Great Valley and 41 degrees in Wilkesboro, NC.

Temperatures at 3:00 PM varied from 13 degrees ( wind chill 0 F ) on Eagle Knob of the High Knob Massif to 38 degrees down in the Tri-Cities of the Great Valley and 43 degrees in Wilkesboro, NC.

 

Previous Discussion

Arctic air is the main weather feature to rule the mountain landscape through coming days, with a reinforcing short-turn push featuring a dewpoint front that will introduce continental Polar Air during Wednesday into Thursday.

Although temperatures remained below freezing throughout December 26, a good amount of snow “disappeared.”  How can snow vanish like this?  While some insolation penetrates clouds, especially thinner sections, the main factor with a dry air mass is sublimination, a process by which a solid changes into a vapor without going through the melting phase.  Even at the highest elevations this process can cause snow cover to decrease.

Sublimation is an endothermic process, which means it absorbs heat and thereby aids cooling of the air.  This is one reason that afternoon temperatures in locations that had Christmas snow only managed to reach the lower 20s ( high elevations in the High Knob Massif ) to lower 30s during Tuesday ( December 26 ).

The vertical air column will be drying significantly behind a passing disturbance-dewpoint front into Wednesday, which sets the stage for bitterly cold conditions to develop into the evening hours of Wednesday and Thursday morning.

Dewpoints to around -40 Celsius ( -40 F below zero ) are being predicted within the 900-850 MB level by Wednesday Night, such that the only aspect to hinder temperature drops will be any areas of middle-high altitude cloudiness.  This will help prevent the full cooling potential from being reached, but still produce bitterly cold temperatures.  High valleys, especially with lingering snow within the High Knob Massif, could easily drop to 0 degrees or below in a setting like this ( as even over bare ground many mountain valleys will drop into single digits unless extensive cloudiness develops into Thursday morning ).

The last time 0 degrees was recorded in high valleys of the High Knob Massif featured a similarly bone dry lower atmosphere into morning hours of March 16, 2017 ( above ).

Although a huge blast of bitterly cold air is looking likely by New Year’s weekend, details with respect to how that plays out are yet to be determined with models currently in a state of disarray handling shortwaves traveling through the longwave flow field.  So stay tuned.