011418 Forecast

Mountain Area Forecast ( Jan 14-17 )

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 )

Location Of The Cumberland Front – Courtesy of Google Maps
*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.

Monday Afternoon

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.

Tuesday Afternoon

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.

High Knob Massif Webcam – University Of Virginia’s College At Wise

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.

NAM 12 KM Model 850 MB RH Forecast At 7:00 AM Tuesday – January 16, 2018

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.

Conveyor Belts – Courtesy of The COMET Project
The UCAR-COMET Project

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 ).

Isentropic Surface At 294K as of 7:00 AM Monday – January 15, 2018

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.

Isentropic Surface At 294K At 7 PM Monday – January 15, 2018

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 ).

NAM 12 KM Model 925 MB Relative Humidity Forecast At 10 PM Tuesday – Jan 16

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.

NAM 12 KM Model 500 MB Vorticity Forecast At 7 AM Wednesday – January 17

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.

NAM 12 KM Model 500 MB Temp-Wind Field Forecast At 1 PM Wednesday – Jan 17

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 ).

NAM 12 KM Model Surface-850 MB Lapse Rate Forecast At 4 PM Wednesday – Jan 17

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.


Previous Discussion

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!

Sunday Morning Sunrise on Eagle Knob of High Knob Massif

Storm Total Snowfall
January 12-14

Joe & Darlene Fields
High Chaparral Community

Steve & Cody Blankenbecler
Eagle Knob of High Knob Massif

*( 3-8″+ ground depths )

View From Eagle Knob of High Knob Massif – January 14, 2018

Morning view looking across lofty High Knob Lake Basin toward an inversion layer along the Tennessee Valley.

High Knob Massif Webcam – University Of Virginia’s College At Wise

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.

GOES 16 Visible Image At 10:47 AM on January 14, 2018

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! ).
High Knob Massif Webcam – University Of Virginia’s College At Wise

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 ).

Lonesome Pine Airport In Wise

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.

Nora 4 SSE – Elevation 2650 feet on Long Ridge of Sandy Ridge

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.

NAM 12 KM Model 300 MB Jet Stream Forecast At 7 PM Tuesday – Jan 16

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.

NAM 12 KM Model 500 MB Vorticity Forecast At 7 AM Wednesday – Jan 17

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.

NAM 12 KM Model Total Snowfall Forecast To 7 AM Wednesday – Jan 17

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.

Stay tuned for later updates.