Reference Early Spring 2021_High Knob Massif for an update on recent and Winter 2020-21 conditions.

Another Heavy Precipitation Event

Alert For The Potential of Heavy to Locally Excessive Rain Wednesday

If you think that it is taking less rainfall to cause rises and responses to local mountain streams, then that is correct. Why is this true?

The past 3-years have been very wet and water tables are elevated. Locations in watersheds that are above impermeable stratigraphy at depth have become most flashy with rapid responses to significant rainfall (as are karst terrain systems when conduits are full, see Recap).

Downward infiltration of surface water takes time. Once surface layers become saturated the processes of Through Flow, Interflow, and Overland Flow can contribute more rapidly to run-off into streams (a portion of this stream flow being base flow from groundwater input).

If I were responsible for the official excessive rainfall forecast below, I would make a distinction between the central (much drier) and southern (wet) Appalachians, where dramatically different rainfall regimes have been ongoing through March and previous months.

Take Elkins, in the northern mountains of West Virginia, for example, at the northern extent of the marginal excessive rainfall outlook below.

The 8.36″ total for 2021 is 0.12″ less than measured in the City of Norton during March and 1.90″ less than observed at Big Cherry Dam in March (Elkins had 8.82″ less rain in March than Big Cherry Dam).

Change To Snow Wednesday Evening

ALERT For Snow Showers And Squalls Developing From Midnight Into Morning Hours On Thursday (Reduced Visibility, Sharp Temperature Drops, And Low Wind Chills)

A transition to snow is expected Wednesday Night into the overnight and predawn hours of Thursday amid an influx of late season winter air.

A general 1″ to 3″ of accumulation will be possible, mainly within upslope locations along and northwest of the Cumberland-Allegheny Front and Blue Ridge.

More than 3″ will be possible at summit levels of the highest mountains, and in locations that may be impacted by training snow squalls known as snowstreaks (also called snow streamers or narrow bands).

Snow showers and flurries will continue into Thursday night and the morning hours of Friday amid unseasonably cold, early April air.

Due to a prime Great Lake connection, bursts of snow in squalls will be possible. With sharp temperature drops into early Thursday, this will create hazardous travel at upper elevations and within locations receiving heavier snow at lower-middle elevations in upslope corridors.

This setting is favorable for snowstreak formation, which features narrow bands of training snow squalls that become aligned along low-level streamlines.

There are many analogs, with one of the most extreme being 2 April 1992 when a snowstreak dropped more than 8″ of snow on Clintwood, causing hazardous travel.

Recap Of Flooding Rains

The corridor receiving greatest amounts of rainfall, within the High Knob Landform (Powell Valley Anticline) contains karst (calcareous stratigraphy).

An Example of Karst Flash Floods

Land Use In Karst Terrane_Temperate Karst Ecosystems

Interaction of highly heterogeneous karst + non-karstic terrain complicates this setting and is one reason that the modeling of streams in the Upper Tennessee River Basin, along the Clinch, Powell, and Holston is difficult and that forecast hydrographs for flooding often have to be adjusted more than in non-karst terrane.

A major reason for increased difficulty in forecasting, is that subterranean drainage systems in karst terrain can often not be directly correlated to surface basins.

Although the above corridor had the highest observed rain gauge totals, the entire area experienced heavy to locally excessive rain amounts.

Although there is no stream gage on the South Fork of Powell, draining Big Cherry Basin downstream of Big Cherry Dam, gages on the Powell River at Big Stone Gap and on Big Stony Creek (labeled Stoney Creek above) can often be used to judge levels when the lake is in overflow.

Forecast downward stream-level trends will be changed in the next couple days as more heavy rain impacts the watersheds.

Reference Early Spring 2021_High Knob Massif for an update on recent and Winter 2020-21 conditions.

ALERT For Heavy Rain And The Potential For Strong-Severe Thunderstorms During Thursday Into Early Hours Of Friday

Waves of rain, with embedded thunderstorms, will develop from Thursday mid-day into the afternoon. Additional showers and thunderstorms will occur Thursday Night into early Friday.

Update At 3:30 PM on Thursday (25 March 2021)

A general 0.75″ to 1.25″ of rain has fallen with this initial wave on headwater creeks from the High Knob Massif into Black Mountain. Those living and driving along streams will need to remain alert for strong water level rises as rain-thunderstorms redevelop.

Specific Point Measurements to 5 PM include:

Birchleaf 1 SW: 0.38″

Clintwood 1 W: 0.41″

Norton (Downtown): 0.70″

Big Cherry Dam: 0.84″

Black Mountain: 0.87″

Eagle Knob: 0.88″

High Chaparral-Robinson Knob: 1.20″

Breaks in the overcast behind this first wave of rain are beginning to create an unstable atmosphere that will become favorable for severe thunderstorms, with possible tornadoes, by late afternoon into this evening.

The SPC Supercell Potential at the Wise Gridpoint increases to 45% by 8 PM, to 60% between 11 PM and 2 AM Friday, then falls sharply to less than 20% after 4-5 AM Friday.

Rain cooled air north of a warm frontal boundary will help limit severe potential during the day. While rotating storms will be possible along the warm front in converging air streams, the main risk will be in the warm sector (between the warm front and cold front) where shear, lift, and instability will be greatest.

The greatest threat for rotating supercell thunderstorms will exist Thursday evening into the overnight hours of Friday (between sunset and 5 AM Friday).

Parameters for rotating storms and tornadoes look to extend farther north and northeast than last week. The Storm Prediction Center may upgrade the region by tonight or early Thursday.

An Enhanced Risk of Severe Thunderstorms with possible Tornadoes has officially been extended northeast into southwestern Lee County, Va., and Bell County, Ky., however, the entire area will be at risk tonight.

The potential for heavy to locally excessive rainfall amounts also exists, with ponding and flooding of low-lying and poor drainage areas, as well as strong rises on streams.

This marks the beginning of a stormy 5-7 day period that could generate widespread 3.00″ to 5.00″+ rainfall totals across the southern Appalachians.

Please monitor NOAA Weather Radio and your favorite media sources for possible watches and warnings that may be needed.

MRX NOAA Weather Radio

Previous Discussion

Nice early spring weather conditions are expected in the short-term, and while some showers will be possible Wednesday the next chance for significant rain and thunderstorms will hold off until late week.

The main focus for the second half of this weekend into early next week will be on more significant rainfall, with heavy additional rain amounts.

Reference Early Spring 2021_High Knob Massif for an update on recent and Winter 2020-21 conditions.

ALERT For Elevated Stream Levels And Ponding Of Water In Flood-Prone Low-Lying Areas Into Early Friday

While locally strong-severe thunderstorms remain possible this afternoon, residents and travelers will need to closely monitor stream levels due to a general 2.00″ to 2.50″ of rain that has fallen on headwater creeks from the High Knob Massif into Black Mountain.

Any additional downpours, if prolonged or training, could quickly trigger flash flooding.

An influx of cold air tonight into early Friday will change rain to snow at upper elevations, with 1″ to 3″ possible (mainly above 3300-3500 feet).

A pocket of cold air associated with an upper-level low currently over western portions of Kentucky-Tennessee will support a transition to snow at high elevations into early Friday. Travel along routes 619, 238, 237, 704, 160 and others that run above 3300-3500 feet could become slushy overnight into early Friday morning.

Previous Discussion

Heavy rain with the potential for strong to severe thunderstorms is being monitored for Thursday (18 March 2021).

Conditions favorable for a major outbreak of severe thunderstorms are coming together over the Deep South and lower Mississippi River Valley. Strong-severe thunderstorms will move northeast into or toward the Appalachians Wednesday night into the early hours of Thursday.

An outbreak of severe thunderstorms, with tornadoes, across the Deep South on Wednesday will spread into the Appalachians on Thursday with a continued chance for strong to locally severe storms.

Strong to severe thunderstorms will be ongoing early Thursday from the main outbreak to the southwest, and will need to be closely tracked.

Additional development will be possible along the mountains into the afternoon.

Potential for heavy to locally excessive rainfall will occur and depend upon where bands of thunderstorms and downpours develop.

Stay tuned to NOAA Weather Radio and your favorite media sources for the possible issuance of watches and warnings.

Thunderstorms To Snow?

A rapid influx of cold air aloft and toward the surface will create a potential for rain to end as snow during the predawn-morning hours of Friday, especially at upper elevations where accumulation can not be ruled out above 3000 to 4000+ feet.

Reference Late-Winter 2021_High Knob Massif as well as Mid-Winter 2021_High Knob Massif for a recap of recent conditions, and a 2020-2021 precipitation update.

Update At 9:30 PM on 1 March 2021

Caution For High Water Levels Continues Into Tuesday For Creeks And Mainstem Rivers

Main stem rivers in Dickenson and Buchanan counties crested below flood stage, but many tributaries creeks reached near to or exceeded flood stage, as did creeks draining the High Knob Massif into Wise, Scott, and Lee counties of the Upper Tennessee Basin.

Orographically enhanced rainfall totals in upper elevations of the High Knob Massif pushed the Clinch River into minor flood downstream of its tributaries (*).

*Orographically enhanced rains via upslope flow and the seeder-feeder mechanism are often not well detected by Doppler radar given much of this enhancement occurs in supersaturated air beneath and just above the summit levels which Doppler beams overshoot (therefore, totals are locally well above regional totals observed within adjacent lowlands).

This can led to river stage predictions which are below what is actually observed, as in this event where initial hydrologic forecasts were for the Clinch River to remain below Action Stage.

Update At 5:30 AM_Monday_1 March 2021

Numerous roadways have become impassible across Wise, Dickenson, and Buchanan counties due to flooding.

Storm rainfall totals of 3.00″ to 5.00″+ have become widespread. All streams are running high or flooding.

Widespread ponding of water in low-lying areas will otherwise generate hazardous travel conditions through this morning. If you do not need to travel, please remain at home.

Flood Potential Increasing

Heavy To Excessive Rainfall Will Continue High Water Levels And Increase The Flash Flood And Flood Risk From Sunday Into Monday Morning

Widespread 1.50″ to 2.50″ of rain have been observed across the mountain area, with strong rises on streams and ponding of water in low-lying, flood prone locations as of early Saturday afternoon (27 February).

*A total of 2.83″ of rain had been observed on Eagle Knob of the High Knob Massif as of 6:30 PM Saturday (27 February), with local totals also near 3.00″ along portions of Pine Mountain that drain into northern Wise and Dickenson counties (Letcher-Pike counties in SE Kentucky).

Embedded convective rains (downpours) are a major concern along with backbuilding and training of heavy rain repeatedly along the same corridors.

An additional wave of rain into Sunday morning will be followed by the heaviest wave expected to develop Sunday night into Monday morning on an already super-saturated to flooded landscape.

Life-threatening Flash Flooding Can Quickly Develop Under Saturated Conditions. Please Remain Alert.

Additional model rainfall totals into Monday morning all show notable mesoscale banding, but are variable in its positioning. Locations within bands will tend to have the highest flood risk over ground already saturated.

Previous Discussion

The first of multiple waves of rain will begin impacting the mountain region Friday, with a heavy to excessive rainfall potential this weekend into early next week.

A strengthening 850 MB low-level jet will be the initial forcing for heavy rainfall this weekend, with orographic enhancement expected from headwaters of the Upper Tennessee, Cumberland, Kentucky, and Big Sandy river basins toward the west-southwest.

Axes of heaviest rainfall are variable in modeling, but climatology of past similar events favors the greatest rain totals to be along and west to southwest of the western Appalachians.

A WSW-SW upper-level flow is climatologically favored for heaviest rainfall amounts to occur (as documented in the past) along and west of the western side of the Appalachians.

Those living and driving along streams, rivers, and within low-lying, typically flood prone locations will need to remain alert for water level rises.

Previous Discussion

From a hydrology perspective, snow cover and high mountain snowpack has now retreated north of the southern Appalachians.

Although some snow or frozen could fall with the first wave in this upcoming pattern by later Friday at high elevations, run-off from snow is not expected to be a factor in the flood potential ahead.

Despite what GRACE may indicate, soil moisture is high across the area and a limited amount of rainfall will be needed to reach saturation, such that additional rainfall contributes to run-off.

Stay tuned for updates.

Reference Late-Winter 2021_High Knob Massif as well as Mid-Winter 2021_High Knob Massif for a recap of recent conditions, and a 2020-2021 precipitation update.

ALERT For Slick Surfaces Developing Overnight Into Friday Morning

Freezing Fog (low clouds) developing on upsloping northerly winds will combine with redevelopment of light precipitation to generate additional light icing on surfaces later tonight into Friday. In addition, water from melting snow-sleet-ice will be refreezing.

Due to a high concentration of low-level moisture this is expected to generate gorgeous conditions on trees in locations experiencing these frozen conditions.

ALERT For Low Visibility – Especially At Mid-Upper Elevations Along & North of the High Knob Massif-Tennessee Valley Divide

Snow accumulations of a dusting up to 1″ are expected, with locally higher amounts possible at highest elevations in the High Knob Massif and Black Mountain.

The coldest influx of air will arrive Friday Night into Saturday Morning with widespread 10s (teens) to around 20 degrees F (in “warmest” locations). Single digits at the highest elevations will combine with WNW-NW winds to generate below zero wind chills.

Storm Event Through Thursday PM

A period of heavy snow to sleet generated widespread 1″ to 4″ snow-sleet amounts across much of the area in the overnight (reference above map for partial reports) hours of Thursday (18 February 2021).

Heaviest snow amounts fell along and northwest of Pine Mountain and Cumberland Mountain, with totals on the above map being somewhat low in places due to melting and/or settlement (with sleet-rain) by morning when most people measured.

As an example, I measured 2.0″ of snow depth in Clintwood at 2:00 AM Thursday as the transition to sleet-freezing rain started. By sunrise, the mean depth had decreased to 1.0″ to 1.5″ even though the air temperature did not rise above freezing. Such differences, for research purposes and accurate documentation of storm history, must be noted as I am doing even though the above observations will not change.

The most significant icing occurred in lower elevation valleys north of the High Knob Massif-Tennessee Valley Divide, and at upper elevations in the High Knob Massif where cooling on upslope flow held temperatures in the 20s to around 30 degrees. Areas in between, had minor accumulations of ice.

The storm fit Miller B climatology with a surge of above freezing air streaming up through the Tennessee Valley and High Knob Landform aloft.

This layer of above freezing air was due to synoptic-scale warm air advection and was aided by downsloping leeward of the Great Smokies across the Great Valley into the Valley-and-Ridge and Appalachian Plateau physiographic provinces.

Former Alert and Forecast

ALERT For A High Impact Winter Storm Event During Thursday (18 Feb 2021)

A high impact winter storm is expected Thursday, but details with respect to precipitation type and amount of each type remain uncertain.

This forecast possesses higher-than-average uncertainty.

Precipitation Forecast By Frozen Type

Snowfall

1″ to 3″ below 2500 feet

3″ to 6″ above 2500 feet

(Locally Higher Amounts Possible)

While lower snowfall amounts will also be possible, I stress the fact that higher amounts are possible across the entire area given that would create a much higher impact with high density snow versus lower amounts and less impact.

Sleet

Trace up to 1″

Freezing Rain

Trace up to 0.33″

Models are in general agreement on a Miller B storm scenario, but a few like the new GFS maintain a colder vertical temperature profile with much more snow.

There will likely be a mesoscale band of heavy snow but, as seen above, models are divergent on where and how intense that band will become.

This system currently has intense, mesoscale banding of snow to the southwest and this is the potential possessed as it continues moving northeast into Thursday morning.

Previous Discussions

I will discuss some of these differences below. Note these are NOT my forecast, but the model predictions.

The Current Dilemma: Ice Storm or Snow Storm

Forecast models are currently divided (as of Tuesday PM) between a Miller A versus Miller B storm track, and that difference will make a world of difference for the local mountains in terms of precipitation type.

The latest NAM Model forecast is for a crippling ice storm, with 1″ of accumulated ice at the Wise gridpoint. That would be devastating and hopefully is an extreme solution that will not verify.

The old GFS and NAM model groups are predicting a Miller B type storm with warm air aloft quickly overspreading the area to change precipitation into sleet and freezing rain (above).

The new GFS (above) is one of the extreme solutions with 10.6″ of wet snow predicted at the Wise gridpoint (it is likely too widespread with heavy snow to the south).

The European Model group, new GFS Model, and the German Model are all predicting a fall of heavy, wet snow. The operational European predicts 7.5″ at the Wise gridpoint, while the 18z German predicts 6.6″ .

NOTE: The 18z European Model (operational) just came in with 8.3″ at the Wise gridpoint, with 6-10″ in counties along the Virginia-Kentucky border. This would be a crippling solution given the model tends to under-estimate snowfall for upper elevations (especially).

This solution features a Miller A storm track which holds the invasion of any above freezing air aloft to a minimum, especially in locations along and northwest of the Cumberland-Allegheny Front.

For those who follow snow density, it is critical to note that instead of increasing snow output being forecast using the Kuchera Method, models are increasing output at 10:1 to suggest that this snow will have potential to be 8:1 or perhaps even higher in density.

Which Solution Is Correct?

It is hoped that neither of the extreme solutions are correct, given one would generate a crippling ice storm and the other a crippling fall of wet snow.

While reality may “fall” in the middle, and no one can know for certain at this point, I am leaning more toward wet snow based upon Baroclinic and Quasi-geostrophic Theory which basically would state: with an expanded mass of arctic air the storm track will seek out the max thermal gradient (baroclinic zone) which will tend to be south and east of the previous winter storm track.

A baroclinic atmosphere depends on both pressure and temperature, with imbalances in mass, momentum, and energy being generated by geostropic advections in synoptic-sale weather systems. The atmosphere will seek to compensate for these imbalances by generating large-scale vertical circulations that change the vorticity structure of its mass field as well as temperature field (through adiabatic ascent-descent).

A critical point being that storms, neither small-scale thunderstorms nor large-scale storm systems, do not move from Point A to Point B, instead they move by continuously developing and redeveloping across space and TIME. So, time will now dictate which model solution is correct.

Stay tuned for updates.

Reference Late-Winter 2021_High Knob Massif as well as Mid-Winter 2021_High Knob Massif for a recap of recent conditions, and a 2020-2021 precipitation update.

For additional information on ice, please reference my 020921 Forecast page.

Previous ALERT-Discussions

Caution For Localized Freezing Conditions In Valleys Into Monday Night, With Rain And Gusty Winds Across Higher Elevations (Strong Wind Gusts At Upper Elevations).

Sheltered mountain hollows north of the High Knob Massif and Tennessee Valley Divide will be most at risk of having air temperatures near freezing (across northern Wise, Dickenson, and Buchanan counties on the Virginia side of the stateline) into Monday evening.

The official NWS temp at 5:00 PM is 34 degrees at Clintwood 1 W.

While localized temperatures around freezing will remain possible into Monday evening, the vertical depth of below freezing air has become very shallow.

Two things in the above image from UVA-Wise indicate important changes. First, cloud bases have finally lifted off middle elevations and, second, mountain waves on SSE-SSW winds have developed above the High Knob Massif. Both indicate enhanced vertical mixing that is finally eroding a low-level temperature inversion firmly in place for the past five days (5 long days of raw, cold, nasty conditions with icing).

Much colder air will begin to invade the mountain area toward Tuesday morning. Caution should be used for possible freezing of moisture remaining on roadways.

Miller A versus Miller B

The track of this next winter storm is trending farther east, with a current problem to be solved revolving around if it will be of a Miller A or Miller B type?

The current possibilities:

Miller A = Heavy snowfall event

Miller A (inland variant) = Crippling snowfall event

Miller B = Snow to rain back to snow

The basin meteorology behind a farther east trend is that a more expansive mass of cold air will cause the baroclinic zone to be pushed to the south and east to result in a much greater snowfall threat.

Numerous European and GFS model ensembles are showing the trend, while models like the NAM have not yet come into agreement and maintain more of a freezing rain-sleet-heavy rain threat.

Stay tuned for updates.

Weather Headlines

Mainly rain is expected by later Monday into Monday night when heavier rain arrives on strong SE-SW flow.

Freezing rain, with the potential of a major ice storm, remains likely for locations northwest of Pine Mountain across the foothills of eastern Kentucky into southwestern-western portions of West Virginia (especially in locations removed from downsloping SE-SW air flow).

Much colder air will flood the mountain area Tuesday with a chance of frozen precipitation early before drier air arrives during the day.

Temperatures during the daylight hours of Tuesday will vary from 10s at highest elevations to the 20s to near 30 degrees at lower elevations.

Because cold air advection (CAA) will occur across extensive, new snow cover west to northwest of the mountains, it will likely be stronger than most forecasts and forecast models predict (I have allowed for this with mainly 10s and 20s in locations along and NW of the Cumberland Front).

High Water-Flood Potential

While rises will occur on already elevated creeks, the flooding potential into Tuesday currently looks to be limited outside of typical poor drainage and low-lying locations where ponding of water is common.

Depending upon the amount of frozen versus rain, the flooding potential will be much higher with the second winter storm during 18-19 February 2021.

Second Winter Storm

There is an increasing snow potential with a second winter storm expected to impact the mountain area Thursday into Friday (other precipitation types will also occur, but snowfall totals are beginning to look higher for counties along the Virginia-Kentucky stateline).

Stay tuned for updates.

ALERT Continues For Freezing Fog Into Monday Morning – Especially In Locations Along & North Of The High Knob Massif-Tennessee Valley Divide (15 February 2021)

Freezing fog and areas of freezing drizzle will be widespread overnight into early Monday across lower-middle elevation mountain ridges and plateaus, as well as some windward upper elevations below 3600 feet.

The good news, deepening SE-SW flow will allow temperatures to rise and limit freezing rain potential when the main wave of precipitation arrives by later Monday into Monday night.

Reference Late-Winter 2021_High Knob Massif as well as Mid-Winter 2021_High Knob Massif for a recap of recent conditions, and a 2020-2021 precipitation update.

ALERT For Freezing Fog On Ridges-Elevated Plateaus In Locations Along And N-NE Of The High Knob Massif-Tennessee Valley Divide

Update at 5:20 PM on Friday (12 February 2021)

Note that the main concern now is not freezing rain, but freezing fog that will add to icing in locations along and north of the High Knob Massif-Tennessee Valley Divide.

This type of fog is always dangerous from a low visibility perspective, given that it becomes widespread within the upslope zone and is NOT localized & patchy. I have talked about this more times than could be counted over the years, and will not now.

Power outages have increased, with 1034 homes-places out of electricity as of the time of this map below. This is only on the AEP System in Virginia, it does not include the Old Dominion Power network or any outages on the other side of the stateline in southeast Kentucky.

Ridges and elevated plateaus at lower-middle elevations have generally seen greatest icing, with icicle formation being problematic for adding weight in some of these areas (reference my notes on icicles below).

Freezing fog will be widespread were air flow rises on northern component air flow trajectories and have the greatest impact on ridges-elevated plateaus of Dickenson and Wise counties into Saturday morning.

Previous Discussions

ALERT For Significant-Major Icing With Freezing Rain Thursday Into Friday AM (11-12 February 2021)

The potential for icing with freezing rain is expected to develop Thursday into early Friday, placing the area along and north-northwest of the Cumberland Front at highest risk for icing on trees, power lines, and roads.

If your area has been in dense overnight fog, or just beneath this low cloud deck, then your area will be at risk for icing Thursday into Friday morning on northerly upslope flow and cooling.

The highest probability of most significant icing will be across lower to middle elevations of northern Wise and Dickenson counties.

However, if the new high-resolution NAM terrain model is accurate then significant to major icing may also extend upward to the crest of the High Knob Massif where it will be enhanced by freezing fog (orographic clouds).

The reality of what happens will depend upon air flow trajectories and depth of low-level sub-freezing air, but chances for significant-major icing is increasing for all locations from the High Knob Massif crest zone north across Wise-Dickenson and Buchanan counties.

These events can generate interesting and unexpected conditions that forecasters must take into consideration. Typically, the freezing level drops vertically from upper elevations into lower elevations. In these events, however, that may not happen with the freezing level being monitored for upward movement over time from lower into middle-upper elevations.

The above develops when cold air advection is restricted to only the lower portion of the boundary layer, with neutral to warm advection above this in air flow that typically possesses a southern component. This increases the chance for terrain blocking and can greatly complicate the weather setting within three-dimensional, complex topography as an inversion develops and/or strengthens.

Upward movement into mid-upper elevations can develop over time if the low-level cold advection flow direction deepens to allow adiabatic cooling with air being lifted upslope (an ageostrophic flow component forms). Sub-freezing air may not be able to cross the main mountain barriers, and if it does will warm on subsidence flow leeward of them to prevent further sub-freezing advection.

Reference my Case Study section below for more information regarding icing along the western slopes of the Appalachians.

The icing risk will diminish, by contrast, when surface air flow (winds) develops a southern component, such that wind direction will be a key factor.

The European Model (not shown) has trended south with the boundary and freezing rain potential during Thursday into Friday.

Given the dense nature of low-level arctic air, this fits climatology of past events and must be taken seriously.

If sub-freezing air becomes very shallow it could become dammed against Pine Mountain and blocked, at least at times, from crossing into Virginia southwest of Breaks Interstate Park (Pine Mountain ends at Breaks Park). This complicates the local setting. If sub-freezing air is deeper, then adiabatic upslope cooling could assist its development and expansion-advection into windwards slopes of the High Knob Massif-Tennessee Valley Divide.

Complexity Of Ice Accretion

The accretion (accumulation) of ice is complex. Much like snow, it is largely dictated by ice-to-liquid (ILR) ratios that vary significantly with atmospheric & surface conditions in analogous ways to snow-to-water ratios.

A few basic facts learned by research include:

Ice accretion tends to be inversely proportional to precipitation rate. Light precipitation results in more accumulation than heavy precipitation.

That makes common sense, since rain can fall faster than it can freeze such that there tends to be more run-off with high rainfall rates versus lower rainfall rates (a slow falling rain has more time to freeze).

Ice accretion tends to be proportional to wind speed, with more ice accumulating in higher winds versus light winds (efficiency of icing increases with speed).

This explains, at least in part, why icing often tends to be greater along exposed mountain ridges and slopes than within wind sheltered locations.

The most efficient icing often takes place when wet-bulb temperatures are between 27 to 30 degrees F.

Freezing is an exothermic process, which releases heat into the surrounding air, such that a somewhat lower wet-bulb will help to compensate for the latent heat of fusion associated with the phase transition from liquid to solid.

Smaller drop sizes generate more efficient ice accretion.

It takes less time for a small drop to freeze versus a large drop, and there is more chance for some water to run-off the accretion surface before it can freeze with large drops.

Accretion efficiency tends to be inversely proportional to droplet temperature, with drops falling through a deep, mild layer being less productive at generation of ice accumulation than colder drops.

An above freezing layer aloft that is in the 30s, for example, is more conducive to icing than an above freezing layer aloft that is in the 50s. If drop temperatures warm with WAA aloft, they may become able to melt ice already accreted in sub-freezing surface air.

A cold ground and surfaces promote icing versus a warm ground and surfaces. Following a very cold winter period, icing may occur even with air temperatures well above freezing.

Events observed during the 1970s winters come to mind, with icing in above freezing air following prolonged arctic cold. In addition, this also helps to explain (in part) why icing is often more efficient on northern-eastern exposed mountain slopes-valleys versus those with southern-western exposures.

The rate of evaporation of water not immediately freezing can impact ice accretion efficiency.

Evaporation is an endothermic process that consumes (takes or extracts) energy from the surrounding air in order to break hydrogen bonds holding it in liquid state. This is a cooling process that can aid icing.

Conditions favorable for icicle formation feature rain that falls faster than it can freeze, with abundant icicles becoming problematic in terms of total weight.

While ice is typically measured with respect to a flat surface, or radially with respect to a tree branch (in which radial accretion is recorded), the development of abundant icicles makes ice measurement more problematic as they also act to add more total weight than radial measurement would suggest.

Freezing Fog is an important form of icing, especially in mountainous terrain, with all the aspects I have talked about in the past with respect to rime being applicable.

Freezing and deposition are exothermic processes that release heat into the surrounding air. While this may slow accumulation in marginal temperature conditions during daylight hours when insolation is present, it typically is easily overcome after sunset by cooling associated with adiabatic processes on upward flowing air forced by orographic lift.

In this case, it will add to pre-existing ice with fine droplets freezing on contact to coat ice-icicles. There is preference for greatest rime-ice accumulation on windward facing sides of objects.

For more information, please reference:

Analysis of Ice-to-Liquid Ratios In Freezing Rain

Strong Water Level Rises

Another concern, as observed during late January, will be strong water level rises on creeks with a combination of rain and snow melt through Thursday-Friday.

It appears likely that freezing rain will slow run-off from highest elevations, but rain may fall faster than it can freeze such that water level rises will still occur.

The more significant threat for high water and flooding now looks to be a greater concern into next week, which will be particularly true if the main storm track continues to keep the mountain area within the warm sector of cyclonic storm systems.

Western Slope Cold Air Damming

Ice Storm – Case Study
(3-7 February 1989)

Cold air damming (CAD) along eastern slopes of the Appalachians is well documented, studied, and taught in meteorology classes. Western slope CAD, by contrast, is not and the recognition and usage of CAD with respect to the western front of the mountain chain is not even talked about by meteorologists (in general).

This is rather ironic. A mountain chain does have two sides. What can happen on one side can also happen on the other, albeit, less common in nature for various reasons (not the least of which is the mean westerly flow field in middle latitudes).

A classic example of western slope CAD was documented during the first week of February 1989, as a cold front stalled along the Appalachians and Arctic High pressure funneled bitter air down along the eastern slopes of the Rockies and across the central USA.

A key feature of this western slope CAD event was advection of shallow, arctic air on mean NE flow near the surface…

beneath mean SW flow at 850 MB, to form a vertical temperature inversion with increasing temperatures from the surface upward in elevation (*).

*This is common at night in complex terrain, but atypical during daylight hours when temperatures typically cool with increasing elevation.

The result of this event was 0.25-0.50″ of ice accretion across lower elevations of Dickenson-Wise counties, with 0.50″+ at middle-upper elevations along the Tennessee Valley Divide into the High Knob Massif.

Only rain, with no icing, was observed within the Great Valley of northeastern Tennessee as CAD kept cold air banked against the western side of the Cumberland Front throughout this event.

Air temperatures at 1300 hours (1:00 PM) on 6 February varied from 18 degrees in Lexington, Ky., to 51 degrees in Asheville, Nc., for a 33 degree cross-barrier temperature contrast at lower elevations.

Although the summit level of the High Knob Massif is close to the 850 MB level, icing occurred as low-level N-NE flow was strong enough to undercut the milder air with local cooling being reinforced by adiabatic upslope cooling to generate temperatures just cold enough for ice accretion (with freezing fog in orographic clouds adding to ice accumulation above 3000 feet).

In stronger static stability-850 MB flow fields this may not occur, but my observations find that if cold advection can reach the level of the Wise-Sandy Ridge plateaus that it typically will be forced upslope across the windward slopes-crests of the High Knob Massif. Dramatic changes then occur, with sinking air and warming, across northern Scott County.

In-situ (developing in place) CAD can also develop along the western slopes, and that was documented with even greater ice accumulations a couple years later (I do not have time to illustrate that now). Additional western slopes CAD examples could be cited.

The Bottom Line…meteorologists should consider these cases and not be shy about using CAD with respect to this side of the ancient Appalachians.

Example From Present Event

Air temperature differences on 12 February 2021 illustrate terrain blocking from this present event, with two sites along the observed mean air flow trajectory being used for illustration.

A mean temperature difference of 16 degrees (F) was observed during this observation period, with maximum differences of more than 20.0 degrees.

Reference Late-Winter 2021_High Knob Massif as well as Mid-Winter 2021_High Knob Massif for a recap of recent conditions, and a 2020-2021 precipitation update.

Update at 1:30 AM_7 February 2021

While I will not change my snow forecast numbers, having gone up to 6″ for elevations below 3000 feet when factoring in my error potential, with winds having gone quickly N-NE I do think the upper end of snow numbers could be reached or exceeded with upslope being maximized across Dickenson-Wise counties toward the High Knob Massif-Tennessee Valley Divide. I also think that the upper end may be topped in the Powell Valley of Wise County given this rapid shift to more northerly flow (as suggested below).

Certainly, Winter Storm Warning criteria (as I already had in my forecast) will be easily met and the potential for Power Outages is concerning at lower-middle elevations where snow density will be higher than at upper elevations (where air temperatures are colder and snow density lower and more wind driven).

Blowing-drifting (as often is the case) will be a factor at upper elevations.

A Miller A Type Winter Storm is expected to generate widespread, hazardous conditions from late Saturday night into Sunday.

ALERT For Heavy Snow Developing Saturday Night Into Sunday Morning (6-7 February 2021)

The majority of snow accumulation is expected to occur between 10:00 PM Saturday and 10:00 AM Sunday, with precipitation falling as all snow along and northwest to north of the Cumberland Front.

While Powell Valley in Wise County is south of (outside) this red line denoting the Cumberland Front, I want to stress that the highest probability of mixed precipitation exists here due to an enhancement of sinking air off of the High Knob Massif on easterly component air flow until the early overnight of Sunday (when winds shift to reduce sinking-warming, allowing all snow to fall).

Models are generally not predicting a decrease in snowfall southwest of Norton toward Big Stone Gap, but if the easterly component flow verifies then it will occur. If the easterly flow does not develop then a TIM Circulation will not form and variations will not be as large.

Another factor that could prevent a TIM circulation from forming would be if evaporative cooling within initially dry air is strong enough to overcome subsidence warming forced by the terrain. Evaporation is an endothermic process that takes energy out of the air in order to break the hydrogen bonds necessary to transform liquid water into vapor. This process is typically more limited (there is just so much energy available to be extracted from air), while subsidence warming will continue for as long as easterly flow streams across the high country into Powell Valley.

Due to this, lower snow amounts are expected southwest of the City of Norton toward Big Stone Gap. At the same time, by contrast, an enhancement of snow is expected above 3000 feet in the High Knob Massif.

Note that some models do not begin accumulating snow until Midnight or after, such that the bulk will fall through the overnight-morning hours of Sunday (7 February 2021).

Snowfall Forecast

3″ to 5″ below 3000 feet

6″ to 8″ above 3000 feet (*)

A (+/-) 1″ error potential implies
a 2″ to 6″ potential at elevations
below 3000 feet

1″ to 3″ in downslope locations
of the Clinch-Powell-Holston valleys

(*) – Locally higher amounts will be possible as orographic upslope combines with a topographically anchored rain-snow melting zone to generate a Thermally Indirect Mesoscale couplet of enhanced rising and sinking air (what I have identified in field research that I call a TIM Circulation) for a period into the early overnight of Sunday.

Example TIM events on the High Knob Landform are much more significant than what this is currently expected to be, with only a short period featuring a potential circulation. It is important not to neglect, however, in anticipation of bigger and much more impactful winter storms to come.

A sharp cut-off in accumulating snow is expected toward the northwest, approximately along the edge of foothill terrain where highest ridges begin dropping under 1300-1500 feet (above mean sea level) within the Appalachian Plateau province.

Heaviest snowfall is expected to develop along and eastward of the Cumberland Overthrust Block and southern Appalachian fold-thrust belt.

The above follows MEAN climatology of Miller A Type Winter Storms (it is not currently expected to be an inland variant-Miller A or totals would be MUCH greater).

Reference Late-Winter 2021_High Knob Massif as well as Mid-Winter 2021_High Knob Massif for a recap of recent conditions, and a 2020-2021 precipitation update.

Update at 11:30 PM on 4 February 2021

Extreme caution should be taken on roadways above 2500-3000 feet in the High Knob Massif as snow-sleet has now changed to freezing rain with a drop in surface temperature to 27-28 degrees.

A few impacted roadways include:

State Route 619, State Route 706, State Route 699, Route 237, Route 238, Route 822 with the freezing level generally being a little lower on the Scott County side versus Wise County side due to strong upslope cooling on S-SSW winds which has dropped air temperatures below freezing.

A change back to snow may occur during the overnight, especially at the highest elevations.

Some tree limbs have also been broken due to high winds during the evening.

A mixture of sleet & rain has mainly changed to a cold rain at lower-middle elevations, but a quick temperature drop into the predawn should be respected for the possibility of black ice formation at lower elevations.

ALERT For Strong S-SW Winds Thursday Night Into Friday Morning – Especially At Elevations Above 2500 Feet And Locally Within Mountain Waves

Wind gusts to 50+ mph will be likely at highest elevations and locally with mountain waves to the north and northeast of the High Knob Massif-Black Mountain area across Pine Mountain and higher ridges in Dickenson-Wise counties (where many people live).

ALERT For Wintry Precipitation (Snow-Sleet-Freezing Rain) Developing Thursday Night Into The Overnight Hours Of Friday (Especially At Elevations Above 3000 Feet)

Snow-sleet accumulations varying from a dusting up to 1-3″ will be possible above 3000 feet. This spread is due to the unknown amount of specific precipitation type.

Wintry precipitation is expected to develop Thursday Night into the overnight hours of Friday at mid-upper elevations across southern Wise County, northern Scott, and highest portions of Lee (mainly centered upon the High Knob Massif and highest ridges of Black Mountain) due to evaporative and strong adiabatic upslope cooling on SSW-WSW winds.

While this alert is centered upon an 80-square mile zone with elevations above 3000-3300 feet, where 100+ families live in the High Knob Massif, there will likely be a mix of precipitation types in surrounding locations.

If only a single family lived above 3000 feet it would matter. Everyone counts. All must be treated equally. So, 1, 1000, or 10,000+ it does not matter. They all deserve to be treated the same with forecasts.

To not include any area within weather advisories (of any type) for impacts because it possesses a more limited population is a form of Environmental Injustice.

It is Human Rights Injustice.

High-resolution models are seeing the upslope cooling where they are programmed to recognize it, such as in the Snowshoe area of eastern West Virginia.

There is no frontal passage between Snowshoe and High Knob, and wind directions remain S-SW, but model bias and errors (which I have documented over, and over, and over again in the past and observed most recently during 30-31 January) show warming instead of cooling over the High Knob Massif.

I have no doubt cooling will occur up to 850 MB, but if warming above that level is strong enough to overcome rising air with orographic ascent (often along a standing mountain wave) then freezing rain and not snow will be observed at highest elevations.

The massif tends to generate standing wave clouds, with a upward ascent region that can lift air well above its physical height. That is the key to producing snow when forecast models predict only rain, or mostly rain, in these settings.

Even if all snow falls, it will generally only result in 1-3″ of accumulation, so what is the big deal?

It for recognition of bigger events, and in respect of those (every single person) who live, work, hike, travel across, etc. the High Knob Massif area in all seasons.

Bigger events have been observed many times in the past on SW flow, most recently during 30-31 January 2021 and well documented during February 2015 when ROARing SW winds accompanied a blizzard across the high country (above 3000 feet).

Extreme Winter Potential

Discussion of an extreme winter potential is not HYPE. It is real, very real. The problem, ongoing changes between the stratosphere-troposphere has models in chaos until the two fully couple together.

Observed changes this winter in the Polar Vortex appear to be unique in the observational record (since 1969), and while I will not get into the details here, an excellent discussion is given by Dr. Judah Cohen in his Arctic Oscillation and Polar Vortex Blog.

I expect the true potential of this upcoming winter pattern to become much more clear in a few more days.

A Miller A type winter storm can be a very potent snow maker for the Mountain Empire, so this setting will need to be closely followed as it evolves into this weekend.

While models are trending back toward a southern Appalachian fall of snow this weekend, I think they will also begin to better resolve an influx of bitterly cold, arctic air that will dramatically change the landscape across the central-eastern USA into mid-February.

As I noted way back at the beginning of this winter, a fight has been ongoing between Pacific Ocean Basin factors (unfavorable for eastern USA winter) and Atlantic Ocean Basin factors (favorable for eastern USA winter conditions).

It is not that the NAO is going negative, it has been negative since late December.

The Arctic Oscillation (AO) has been negative all winter as it is more intimately linked to the Stratospheric Polar Vortex. Reference Sudden Stratospheric Warming.

The AO is likely to remain negative through the final warming and Spring 2021.

The headline news, factors across the Pacific Ocean Basin are becoming more favorable for eastern USA winter (and not fighting against the -AO and -NAO as strongly as recently observed).

The EPO remains mixed, and not as favorable.

The Bottom Line…An extreme winter potential exists heading into mid-late February. Whether or not it is fully realized in the Mountain Empire remains to be seen.

Previous Discussion

Weather Headlines

Upslope flurries and light snow have redeveloped during early hours of Wednesday as a plume of Great Lake moisture is allowing clouds to form.

A dusting up to 1″ will be possible through this morning (Wednesday, 3 February 2021).

An ALERT for High Winds will be likely for Thursday Night into early Friday as ROARING and potentially damaging SSW to WSW winds develop.

A low-level jet with core speeds of more than 70 knots will generate Hurricane Force wind gusts at highest elevations, and locally within the descent region of mountain waves that develop across the High Knob Massif-Black Mountain corridor Thursday Night into the early hours of Friday.

A strong pressure gradient will generate strong winds centered on the sunset Thursday to sunrise Friday period. This will also cause blowing snow along high, exposed mountain ridges. At least localized power outages will be possible.

Wind driven snow, supported by strong orographic lift, will also be likely at upper elevations in the High Knob Massif during this time. How low will snow levels drop? That is what I will be looking at on new model runs.

Due to strong descent, only rain is expected in locations north-northeast of the High Knob Massif and Tennessee Valley Divide (models generally predict rain, but like the heavy snow that fell into 31 January, above 3000 feet, models will be wrong unless adjusted for orographics).

Stay tuned for updates.