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.