My Forecast For Today ( January 22 )
A Crippling Fall of Very Heavy Snow, With Extreme Snowfall Rates, Is Likely For Friday ( Predawn Into Mid-Day )
*Even the downslope wake zone of the High Knob Massif may have very heavy snowfall during this time period
Please Do Not Travel If Possible
Many Roads May Become Impassible during the Friday Morning to Early Afternoon Period. A Break In Activity Will Be Followed By NE-NW Upslope Flow Snow, With Blowing & Drifting, Dropping Temperatures & Low Wind Chills From Friday Night Into Saturday To Generate Additional Severe Conditions. Travel Is Not Recommended During This Event.
Very strong LIFT ( Upward Vertical Motion ) will be moving across the region Friday. This has an excellent chance of overwhelming downslope sinking-warming of air in wakes to generate very heavy snowfall during the Sunrise to Early Afternoon Period of Friday.
Wake = the zone of sinking air on the leeward side of major mountain barriers like the High Knob Massif. In this case air will be sinking in locations to its northwest-west-southwest as air flow streams across the massif from the SE-E-NE during this major event before shifting N-NW ( changing the lee wake zone to the south & southeast of the massif and Tennessee Valley Divide ).
Locations in the downstream wake of the High Knob Massif and the Tennessee-North Carolina border, on easterly air flow, will likely have a period of very heavy snow but do remain zones within NE TN and SW VA with the highest uncertainty.
Overnight Into The Predawn
Increasing clouds with snow or a mix developing at onset, changing to snow ( any mix lasting longest in the Powell Valley-Big Stone Gap corridor and adjacent locations in the downstream wake of the High Knob Massif ). Winds becoming E-SE at 5-15 mph, with higher gusts, on ridges-plateaus below 2700 feet. Winds ESE-SE 10-20 mph, with higher gusts, along mountain ridges above 2700 feet. Temps varying from 10s in colder mountain valleys to lower-middle 30s on exposed upper elevation mountain ridges ( temps rising overnight in colder valleys and dropping on ridges following onset of precipitation ).
*The Predawn to Early Afternoon period remains the most uncertain for locations in the downslope wake of the High Knob Massif, with respect to precipitation type. It is very difficult to get snow in Powell Valley on easterly air flow, but if dynamic lift aloft is strong enough and evaporative cooling sufficient it may be able to overcome warming with sinking air into the Valley corridor to generate heavy snow.
If a valley mix of precipitation types continues a TIM Circulation across the High Knob Massif will develop to generate even more extreme snowfall rates in the high country above 3000 feet.
Predawn To Early Afternoon
Snow becoming very heavy. Extreme snowfall rates of 1-3″+ per hour possible during peak of intensity. ESE winds 10-20 mph, with gusts 30-40+ mph at highest elevations. Blowing & drifting snow upon exposed mid to upper elevation mountain ridges & plateaus. Cold with temperatures from upper 10s to mid-upper 20s. Wind chills in single digits & 10s, except below zero on high crestlines of the High Knob Massif ( in gusts ).
Heavy snow diminishing. A break in activity possible before new snow & snow showers redevelop late. ESE to SE winds becoming ENE at 5-15 mph, with higher gusts. Temperatures varying from upper 10s to upper 20s ( coldest at highest elevations ).
Tonight Into Saturday Morning
Snow. Heavy at times. Turning colder. NE winds shifting NNW-NNE at 10-20 mph, with higher gusts, along mid-upper elevation mountain ridges-plateaus. Cloud bases dropping ( possibly into mid elevations ). Temperatures dropping into the 10s along upslope side of mountains, with upper single digits to around 10 degrees at summit level of High Knob Massif. Wind chills dropping into the single digits to low 10s, except 0 to -10 below along upper elevation ridges. Blowing and drifting snow along mid-upper elevation ridges.
*Additional fluffy snow accumulations are likely thru Saturday into Saturday Night and the overnight hours of Sunday along the upslope side of the mountains with respect to northerly air flow. Cloud bases will be low through this period, with continuation of rime formation across upper elevations where heavy amounts are expected to bow small trees.
Weather Discussion ( January 21-22 )
It has been quite a day across the mountains. As of mid-afternoon a general 5″ to 10″ of snow had accumulated from the High Knob Massif north & east, with MUCH LESS amid the downslope wake ( shadow zone ) of the massif where 1″ or so has been reported amid the Powell Valley to Big Stone Gap corridor ( with much mixed precipitation ).
Although not as great as past events ( with a TIM Circulation this time not associated with a pure rain-snow boundary but a snow-mixed boundary ), differences in snow depths and new snow were still very significant through this AM between Big Stone Gap and the High Knob Massif.
Reference TIM Circulation for more information.
The effect has been nicely illustrated this afternoon by some gorgeous views captured via the High Knob Massif Webcam at UVA-Wise, where a break in activity allowed downslope breaks leeward of the great massif in this easterly flow regime to become visible.
While only a tiny portion of the massif is actually visible, upon looking to the left of these views it is possible to see subsidence breaks ( caused by orographically forced sinking air on E winds ).
Following very heavy morning snow, with 1-2″+ or so per hour accumulation rates in parts of the area, the afternoon has featured bands of snow with on-and-off activity.
Afternoon bands have been manageable in comparison to the very heavy morning snow which made some roads in the area impassible or nearly so.
Snow began falling atop the High Knob Massif around 2 AM and quickly became heavy, with around 1″ falling in just 30 minutes or so. A little mix may have occurred initially, but the bulk was snow above 3000 feet. The heavy snow spread into Norton-Wise, with more mix-sleet in Norton vs more snow in Wise ( the difference…more sinking air off the mass into Norton on ESE air flow ).
Southwest of Norton, into the Powell Valley to Big Stone Gap corridor, sinking was significant enough to even beat out the very strong upward vertical motion that was developing aloft in the atmosphere.
Once again this illustrates how difficult it is to get just snow, and significant snow, on easterly air flow streaming across the High Knob Massif as I had indicated in my earlier discussions. I should have left my preliminary snow forecast for this zone ( 6″ or Lee ) alone ( of course, this event is not yet over ).
My friend Chris Allgyer reported freezing rain, sleet, and snow all in Big Stone Gap that formed a slushy mess, as the new accumulation above can be seen on his sidewalk ( with lingering snow from the previous event beside the walk ).
My friend Sharon Daniels reported about the same amid the Head of Powell Valley ( around 1″ through the morning ).
This effect of sinking air adjacent to the High Knob Massif also impacted snow amounts in Norton, with more mix than in nearby Wise. My friend Sandy Earls measured 4″ of depth at 12:43 PM.
A classic TIM Circulation reflectivity pattern, as viewed from the JKL Doppler, found highest echoes on the Wise County side of the massif that formed, and reformed, over time.
My friend Caleb Ramsey of the City of Norton Water Plant reported 3″ of new snow, with 6″ on the ground, at 9:00 AM after MUCH overnight mixed precip ( NWS 8″-diameter rain gauge total will be given later to illustrate how much mix occurred to contrast with mostly snow high atop the main crest zone of the High Knob Massif ).
As winds shift NE-NW this evening into Saturday snow will increase again across the mountain area with snow in all locations. Snow will become very heavy once again in upslope locations northwest to northeast of the High Knob Massif and Tennessee Valley Divide ( widespread 6″ to 10″+ of additional snow in upslope locations verses 4″ or less in the downslope sites ( e.g., toward the Tri-Cities ).