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 ).

Mid-Late Afternoon

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 )

Afternoon Update

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 ).

 

My Overnight Discussion

An inversion over fresh snow cover in combination with upsloping air flow worked to keep temperatures along and north of the High Knob Massif much cooler during Jan 21 than locations to the south and southeast.

A PM MAX temperature of 28 degrees at the official NWS observation station in Clintwood fell to 15 degrees by 11 PM on January 21, beneath increasing mid-high clouds.

*The MAX temperature in Clintwood being the same as on High Knob during the afternoon, before evening temperature rose on High Knob in warm air advection ( creating a large vertical temp spread across the area between valley and peak ).

These colder temperatures look to play an important role in this storm event, with potential now high for a impressive period of heavy snowfall development into the predawn to mid-day period of this TGIF.

While deep convection over the Deep South can interfere with northward moisture transport, in this case it is not likely to disrupt what is already developing a structure featured in many great winter storms of the past.

My forecast suggests a general 1 to 2 feet of snow will fall across the area, and now has support of nearly all models, with the Friday Night to Saturday period having snow to water ratios ( density ) much lower than the 10:1 used by forecast models.  It will be surprising if snow totals do not top 2 feet in the High Knob Massif, with large to massive drifting possible in places along the main crest zone.

What you have on the ground by mid-day to late Friday afternoon will be added to Friday Night through Saturday Night ( with 6″ or MORE of additional snowfall likely for upslope locations on NE-NW air flow ( northerly ).

This snowfall will be low density, fluffy snow and be hard to keep track of as it will blow around over the harder, snow mass on the ground from earlier Friday.

 *Upslope locations including Norton-Wise, Pound, Clintwood, all communities along Sandy Ridge and others in Wise & Dickenson and, of course, the High Knob Massif of Wise-Scott-Lee.

While more snow now looks to fall amid wake zones of major mountain barriers like the High Knob Massif and TN-NC border ( in NE Tennessee ), these lee wakes will likely also have the highest probability of power outages due to higher density ( wetter ) snowfall.

Time will tell.  That was the trends being shown Thursday.  As already noted, it is very hard to get significant snow to accumulate in the Powell Valley to Big Stone Gap corridor when air flow has to cross the wide expanse of the High Knob Massif on easterly air flow trajectories.  It will only occur if atmospheric dynamics and thermodynamics can overcome the subsidence warming.  Timing and model lift forecast Thursday suggested it would be possible.

As of 2:00 AM snow had started to fall atop the High Knob Massif, so time will soon tell when winds shift easterly and increase if snow will continue to be able to overspread Powell Valley ( amid inverse orographic forcing for the Valley corridor…forced sinking ).

As of 2:30 AM around 1″ or a little more of snow had already accumulated on Eagle Knob of the High Knob Massif amid gusty easterly winds.

The NAM 4 KM Model predicts 11″ of snow in Wise by 7 PM Friday, most of that accumulating between Sunrise and early afternoon ( 9″ ).

The NAM 12 KM has a storm total of 18″ in Wise with a bulls-eye of highest totals centered on the High Knob Massif area with respect to southwestern Virginia.

The ECMWF ( European Model ) continued to replicate previous forcast’s with a snowfall total of 14″ for Wise.

As already noted, all models are under-estimating snowfall amounts amid the cold ( and dendritic ) NE-NW upslope flow during the Friday Night-Saturday Night period.

If the lights stay on, I will update later today.

Have A Safe TGIF.