Saturday, November 28, 2020

Early Winter 2020_High Knob Massif

         Wayne Browning Photograph © All Rights Reserved

                        Black Lives Matter Statement

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A real wintry pattern is poised to take hold of the mountain region as December 2020, and Meteorological Winter, begins.

Moderate-Heavy Snowfall Potential

From a climatology perspective, this is the month that typically experiences a large upward increase in snowfall.

Analogous to an incline plane, snowfall amounts typically display a sharp increase from the foothills of southeastern Kentucky across the stateline as air is lifted along the Appalachian front range, above Clintwood and Wise, toward the High Knob Massif and Tennessee Valley Divide on WNW-N air flow trajectories.

Although many different air flow trajectories can comprise the snowfall for any given month, season, or even event, a significant portion naturally comes via air flow possessing westerly and northerly components.

Seasonal snowfall more than doubles from the 
top of the Kentucky foothills upward into the high plateau surrounding Wise, then doubles once again with continued lifting to the summit level of the High Knob Massif.

Enhancement of snowfall over the High Knob Massif 
is also due in part to the effect of mountain width, as discussed in research above, which acts to partially compensate for vertical height (or the lack of).

A summary of above research that is applicable 
to the High Knob Massif includes the following:

*Precipitation Efficiency increases with mountain width, with greatest increases occurring relative to narrow mountains like are typical in the Appalachians

*Mountain width acts to increase orographic robbing of moisture and to decrease the spillover or amount that survives in cross-barrier flow into leeward valleys

(Precipitation Efficiency = the ratio of total precipitation rate to the total condensation rate over a unit area)

A large decrease in snowfall typically occurs with forced descent and subsidence as air sinks leeward of the High Knob Massif and Tennessee Valley Divide into the Great Valley.

Although a very simplistic graphic can be used to illustrate what collected data reveals, the actual situation is far, far more complex than merely "upsloping" and "downsloping" of air as would 
be dictated by a terrain profile.

This leeward decrease being partly due to increased robbing associated with mountain width (discussed above) and the linked seeder-feeder mechanism (highlighted below) that often operates during cold season precipitation events.

While rime may be celebrated for its beauty, even without snowfall, rime also plays an important role in orographic enhancement of snowfall.  This occurs as falling flakes become rimed as part of a cold-season, seeder-feeder cloud precipitation process where by windward facing mountain slopes act to concentrate low-level moisture into terrain engulfing or scraping clouds (otherwise, called fog by those on the ground).

Orographic Seeder-Feeder Mechanism

The lack of resolution of this cold season orographic mechanism, which also possesses a well studied warm season form, is one reason forecast models struggle to accurately predict snowfall amounts across complex terrain of the Mountain Empire.

It is this process that negates the impact of sheer elevation.  In other words, while elevation is certainly important and can become the main factor in conditions where elevation dictates where snow can fall and stick, during many storm events the concentration of low-level moisture by windward slopes can generate more snow than would be predicted by sheer elevation alone.

The production and subsequent extraction of this moisture from air can also require that the air flowing downstream (air that has already dropped snow) be lifted into higher elevations (than otherwise predicted) along the secondary front of the mountains in order to equal the amount of snow generated by lower elevations with initial lifting along the primary front range (where air is initially lifted).

If air has limited moisture, secondary lifting is not sufficient to equal the amount of snow produced by lifting along the primary front of the mountain range.

The Cumberland Front, or southwestern extension of 
the Allegheny Front, denotes the southeastern edge 
of this initial lifting with respect to W-NW-N air flow trajectories (in this case, it becomes the primary front 
of the mountain range and the Blue Ridge become 
the secondary front).

Geologically, this southeastern edge also correlates to the Appalachian Structural Front upon which the folded and faulted sedimentary massif of High Knob forms a notable bulge (above).

Change air flow trajectories to E-SE-S and the Blue Ridge become the primary front of the Appalachians (where initial lifting occurs) and the Cumberland-Allegheny Front becomes the secondary front of the mountain range.

This explains in part why locations such as Boone 
and Banner Elk, despite being at significantly higher elevations, receive less snowfall (especially on W-NW flow) than Clintwood, Norton-Wise given westerly component flow is the most common within middle latitudes of North America.  The significance of this 
fact is unfortunately not well studied and recognized (and resolved) by models nor forecasters.

If inspecting the data in more detail, it is found that Boone and Banner Elk receive more snowfall than Clintwood and Norton-Wise on E-SE air flow trajectories.  This fact is well taught in meteorology classes and also well recognized by both forecast
models and forecasters.

This also explains in part how the High Knob Massif, despite being much lower in elevation above sea level than Mount LeConte and Mount Mitchell, can rival these highest summits with respect to longer-term, annual average snowfall.

Snowshoe Mountain, Canaan Mountain and Cabin Mountain in eastern-northern West Virginia (on the Allegheny Front) typically receive much more annual snowfall than the much higher summits of both Mount Mitchell and Mount LeConte thanks to added low-level moisture transport from the Great Lakes on W-NW flow.

Important Disclaimer: It should be noted that down-wind drift of snow on strong air flow is a major factor and hinderance to accurate measurement of snowfall within higher mountain terrain.  While this effect applies to all the high mountains listed above, it should in theory be greater at the highest elevations of Mount Mitchell and Mount LeConte (the Mount LeConte station likely being the best for more accurate representation of snowfall above 6000 feet in the Appalachians).

First Widespread Fall Of Snow

The first widespread snow of the 2020-21 season dropped a general 2" to 6" across the mountain area, with local variations from just over 1" to more than 6", especially along and north to northwest (N-NW) of the High Knob Massif and Tennessee Valley Divide (which includes Black Mountain on the Virginia-Kentucky stateline).

Eagle Knob of High Knob Massif

State Route 160 - Wise County (Black Mountain)

Snow Depth on Black Mountain (Wise County)

Long Ridge of Sandy Ridge

Looking Toward High Knob Massif
(2 December 2020)

Looking east from Black Mountain, VDOT snow plow driver John Varner captured a beautiful view of lingering clouds along the main high country of the High Knob Massif.

Little Stone Mountain joins the main high country mass at Little Stone Gap, the location of scenic Powell Valley Overlook, along U.S. 23 .

Breaks Interstate Park

2 December 2020
Dickenson County, Virginia
Russell Fork River of Big Sandy
Breaks Gorge In Breaks Interstate Park
   Wayne Browning Photograph © All Rights Reserved

An average of 2" was on the ground on north slopes of Breaks Interstate Park when I took these photographs of Breaks Gorge during afternoon hours of 2 December 2020.  A total of 3.5" was measured at Clintwood 1 W.

The Towers Formation of Breaks Gorge

Snowfall Event

(7 December 2020)

This snowfall event was elevation biased, with up to 2" of new snow at mid-upper elevations in the High Knob Massif and Tennessee Valley Divide (this fell on top of old snow at highest elevations).

A nice rime formation event also accompanied accumulating snow at upper elevations.

                                      Courtesy of WCYB-TV Chime In

Sunset (below) from the High Knob Peak.

Wayne & Genevie Riner measured 1.7" of snow at Nora 4 SSE, on Long Ridge along the Tennessee Valley Divide, in southern Dickenson County.

Snowfall (Non) Event

(14 December 2020)

Although up to 5" of snow fell at the summit level of 
the High Knob Massif, sticking was reduced (especially upon pavement and gravels) by mixing with rain.

This mixture occurred even though surface air temperatures remained below freezing.
How can that happen?

A thin layer of above freezing air just above the 
summit of the massif allowed falling snow to melt prior to reaching the ground, resulting in a rain-snow mixture during much of this event.

If the wave responsible for this had not been deamplifying (weakening) then the thermal advection would have been more significant and the transition more pronounced and impactful to better follow documented results of mean climatology of past systems featuring analogous surface low tracks and surface to 850 mb streamlines.

With surface temperature above freezing, lower-middle elevations (below 3000 feet) had only a minor impact from this system in terms of snowfall.

Foxy The Fox Squirrel
Wayne Riner Photograph © All Rights Reserved

14 December 2020
Long Ridge of Tennessee Valley Divide
Wayne Riner Photograph © All Rights Reserved

14 December 2020
Cow Paths Highlighted By Falling Snow
Wayne Riner Photograph © All Rights Reserved

Morning Views
(15 December 2020)

John Varner Photograph © All Rights Reserved

John Varner, of VDOT (Virginia Department Of Transportation), captured simply awesome morning views looking across a lingering layer of low clouds from the summit of Black Mountain, with crest lines 
of the High Knob Massif along the horizon.

                            John Varner Photograph © All Rights Reserved

I have labeled a few features for reference.

                             John Varner Photograph © All Rights Reserved

There was more total snow on top of the High Knob Massif than Black Mountain, even though it did not appear that way from GOES-16 and MODIS imagery.

Snowfall-Heavy Rime Event
(17-18 December 2020)

Snow has now covered highest elevations in the High Knob Massif during nearly the entire month of December, with conditions poised to turn more severe heading into the Christmas Holiday.

17 December 2020
Eagle Knob of High Knob Massif

18 December 2020
Forest Service Route 238
Courtesy of WCYB-TV Chime In

18 December 2020
Forest Service Route 238
Courtesy of WCYB-TV Chime In

State Route 160
Black Mountain - Wise County, Virginia

Virginia-Kentucky Stateline

Along State Route 160 - Black Mountain

Beautiful Morning Sunrise

A beautiful morning sunrise over the Tennessee Valley Divide, as captured by photographer Wayne Riner, contained an array of wave clouds.

19 December 2020
Long Ridge of Sandy Ridge

Mountain Waves

Glancing advection of Great Lake moisture into morning hours of 22 December generated nice mountain waves over the High Knob Massif.

22 December 2020
Southwestern Edge of Great Lake Moisture

Lingering Snow
Ahead Of Christmas Snowstorm

23 December 2020
Along Forest Service Route 238

Although the southern Appalachians had missed the major snowstorms up to this point, snow has covered highest elevations in the High Knob Massif nearly all month, and was lingering in advance of a significant Christmas Holiday snowfall event.

23 December 2020
Looking Across High Knob Lake Basin