Tuesday, February 19, 2013

SW Upslope Flow Snow of February 2013


Morning of February 19, 2013
High Chaparral of High Knob Massif
Overnight Wet Snow Accumulation
Photograph by Darlene Fields - © All Rights Reserved.

The High Knob Landform

Although not a classic event, roaring SSW-SW winds supported a period of heavy snow into morning hours of February 19 within the favored High Knob Landform - Tennessee Valley Divide corridor of the Cumberland Mountains of far southwestern Virginia ( Cumberland Overthrust Block ).

Elevation 3300 feet
New Snow from SW Upslope Event
Snow Depth of 4.5" In High Chaparral
Photograph by Darlene Fields - © All Rights Reserved.

I have documented many SW Upslope Flow snowfall events over the years, with a couple of classic events including those of November 1977 and March 1996.

The heaviest snow, in specific, fell across upper elevations of the High Knob Massif into adjacent portions of the Tennessee Valley Divide which were not in the downstream wake of the main crest zone of the massif during this February 19 event.

February 19, 2013
Large Snow Roller Shows Wetness of Fall
Photograph Courtesy of David Shelton & WCYB Archive

While SW winds can enhance rainfall in portions of the outward protruding Blue Ridge escarpment, such as near the Meadows of Dan, it is rare for there to be much snow.  Significant to major SW Upslope Flow snowfall enhancement is unique to the High Knob Massif area in Virginia ( specifically to those locations within and adjacent of its mass and amid its great calcareous landform ).

SW air flow trajectories rise more than 3000 vertical feet in their trek from Norris Lake to the summit level of the High Knob Massif ( southwestern North Carolina is another area where orographic forcing more rarely produces snow on SW winds, verses enhancements of rainfall which are prolific at that latitude & position ).

Morning of February 19, 2013
High Chaparral of High Knob Massif
In The Clouds Following SW Upslope Snow
Photograph by Darlene Fields - © All Rights Reserved.

New snowfall totals of 3" to 6" were widespread during this event in the High Knob Massif, above 3000 feet, with 1" to 2" ( locally up to 4" of depth ) at elevations between 2000 and 3000 feet not subjected to major downsloping leeward of the massif on S-SW winds.

It is stated as "new" snowfall since some old snow remained on northern slopes of the massif, in upper elevations, from the February 15-17 snow-rime event.

Winter Majesty of Mid-February Cold Blast

February 19, 2013
Rising Above The City of Norton
Pickem Mountain of High Knob Massif
Photograph by Roddy Addington - © All Rights Reserved.

Like the very tip of a great iceberg, majestic northern slopes of Pickem Mountain rise above the City of Norton to mark the front slope of a sprawling collection of ridges and valleys comprising the remnant high country mass of the High Knob Landform ( High Knob Massif ).

This was not a classic event for the Norton-Wise area since 925 MB wind trajectories at Lonesome Pine Airport in Wise averaged around 190 degrees ( 170 to 210 degrees ), which must cut across the High Knob Massif to generate downsloping. 

As later highlighted in the climatology section, it is not atypical for portions of the Tennessee Valley Divide resting leeward of the High Knob Massif to get snow shadowed, such as at Wise 3 E and Nora 4 SSE, with different events producing different results depending upon the precise wind trajectories.

February 19, 2013 at 6:22 AM
University of Virginia's College In Wise

Morning snow covered the ground lightly in the town of Wise, with snow melting away into the afternoon beneath impressive skies.

Februray 19, 2013 at 12:24 PM
University of Virginia's College In Wise

Roddy captured the morning coating in town.

February 19, 2013
A Coating of Morning Snow In Wise
Photograph by Roddy Addington - © All Rights Reserved.

At approximately the same elevation as Wise, but some 7 air miles to the southeast where air flow on this same 925 MB trajectory sinks much less getting into Jaybird Branch at the eastern end of the High Knob Massif, JoAnna Hobbs had significantly more snow at her home.

February 19, 2013
Far Eastern End of High Knob Massif
Snow On Jaybird Branch of Guest River
Photograph Courtesy of JoAnna Hobbs & WCYB Archive

Ideally, as shown by climatology, 925 MB air flow trajectories of around 230 degrees are preferred for heavy snow in Norton-Wise, coupled with 850 MB trajectories that are also SW-WSW.

Afternoon of February 19, 2013
Looking South from Lonesome Pine Airport
Panorama Looking Along High Knob Massif
Photograph by Roddy Addington - © All Rights Reserved.

SW air flows can stream up through Powell Valley and in between the Little Stone Mountain peak, on the far right, and the large mass of the main high country to the left, and reach Wise with minimal downslope on 230 to 240 degree trajectories.

SW Air Flow Thru The High Knob Landform
Preferred SW Air Flow Trajectory Into Wise
Photograph by Roddy Addington - © All Rights Reserved.

Due to channelization amid the High Knob Landform, and between the High Knob Massif and Black Mountain, SW air flow trajectories tend to produce the strongest winds of all directions into the Norton-Wise area.

Afternoon of February 19, 2013
Lonesome Pine Airport ( KLNP ) In Wise
Awesome Skies Above Strong Surface Winds
Photograph by Roddy Addington - © All Rights Reserved.

No signs of snow were left by afternoon hours of February 19 at Lonesome Pine Airport, with little snow sticking there ( less than 1" ) in comparison to other nearby locations at similar elevations.

Plot of 190 degree Air Flow Trajectory Path
Air Flow Came Across High Knob Massif To LNP

To illustrate how specific and important air flow trajectories are, a plot of the 190 degree trajectory path coming into Lonesome Pine Airport in Wise finds that it must travel across a large section of the High Knob Massif before reaching the Airport.

This is in contrast to places along the Tennessee Valley Divide located 7 to 9 air miles east of the Airport.

Plot of 190 degree Air Flow Trajectory Path
Air Flow From Clinch River Valley To Sandy Ridge

Wendy Carico reported 4" of morning snow depth at her home amid one of the highest sections of Sandy Ridge to the northeast of Coeburn, with an elevation about 100 to 200 feet higher than Lonesome Pine Airport in Wise.

This small elevation difference was clearly not the main driving force in snowfall differences observed between Lonesome Pine Airport and the Fairview to Banner Mountain section of Sandy Ridge.

Snowfall differences were driven by the pathway along which a mean 190 degree air flow trajectory had to travel in order to reach both sites along the Tennessee Valley Divide.

To reach the Town of Wise and Lonesome Pine Airport air flow had to first cross the High Knob Massif, with both moisture extraction and downsloping to its lee acting to hold back snowfall amounts.  By contrast, farther east, air flow was able to rise from the Clinch River Valley and bypass the massif into eastern-southern portions of Sandy Ridge ( northeast of the Town of Coeburn ).

During SW Upslope Snowfall events with deep air flow trajectories around 230 degrees ( 220 to 240 degrees ), the City of Norton and Wise Plateau can get more snow than portions of Sandy Ridge which then rests in the downstream wake of the High Knob Massif.

February 19, 2013
Looking Across The Orchard On Long Ridge
Photograph by Wayne Riner - © All Rights Reserved.

Just under 1" ( 0.7" ) of new snow fell on Long Ridge, at Nora 4 SSE, during this SW Upslope Flow event ( 3.6" fell during the February 15-17 period ).

February 19, 2013
Long Ridge of Tennessee Valley Divide
The Pond Nestled Amid Northern Slopes
Photograph by Wayne Riner - © All Rights Reserved.

Since Wayne & Genevie have been recording 
on Long Ridge it has been very interesting and instructive to observe how SW Upslope Flow snow events vary there depending upon incoming wind trajectories, with 900 to 850 MB layer winds appearing most critical to their temperatures and snowfall amounts during these events.

As I have written previously, one would not typically think about such a lofty mountain ridge setting as being under downslope flow ( as you would agree via the beautiful drive along Long Ridge ).

However, indeed, that is the case with very significant moisture variations there when air must first stream across the High Knob Massif ( as it does during many rain and some snow events ).

Morning of February 19, 2013
Snow Plastered Bird Houses On Long Ridge
Photograph by Wayne Riner - © All Rights Reserved.

Blowing snow was a notable factor during these recent February snow events along all of these lofty, exposed mid-upper elevation mountain ridges & plateaus, making both measuring snow and collecting it in rain gauges difficult!

February 2013
Blowing Snow On Long Ridge
Photograph by Wayne Riner - © All Rights Reserved.



SW Upslope Snow Climatology
High Knob Landform Corridor 
A Few Examples For Consideration

( 2006 to 2009 Period )
Mean 925 MB Wind Vector Composite
For Five SW Upslope Flow Snow Events

Looking at the most recent five SW Upslope Flow snowfall events, prior to this one, finds that strong SSW-WSW air flow dominated the 925-850 MB layer ( close to a layer mean SW trajectory ).

( 2006 to 2009 Period )
Mean 850 MB Wind Vector Composite
For Five SW Upslope Flow Snow Events

SW Upslope Flow snow events have historically been characterized as being "surprise" falls of snow, with 925 MB temperatures nearly always being forecasted to be above freezing in all but a few exceptions ( e.g., March 1996 and December 2008 ) associated with cold air advection episodes and anomalous warm advections ( e.g., November 1977 ).

The 925 MB level is typically around
the elevation of the Town of Wise.

( 2006 to 2009 Period )
Mean 925 MB Temperature Composite
For Five SW Upslope Flow Snow Events

Temperatures in the 900-850 MB layer are also typically forecasted to be near or above freezing, or to be rising above freezing over time with warm air advection ( a few notable exceptions below ) amid events not driven by gradients in cold advection.

A key feature in warm air advection regimes is that temperatures aloft not rise strongly above 0 Celsius 
( 32 F ) beyond the 825-800 MB level ( i.e., too much warm advection at higher levels will prevent the formation or change precipitation types ).

( 2006 to 2009 Period )
Mean 850 MB Temperature Composite
For Five SW Upslope Flow Snow Events

While cold air advection ( CAA ) on SW flow has occurred during these events, it is often a setting of warm air advection ( WAA ) into an air mass with at least initial evaporative cooling potential forecasted to be overwhelmed by WAA over time
( classic "surprise" snow setting ).

Since SW Upslope Flow snow events typically occur in either CAA or WAA regimes, examples below will be classified according to regime type.


Cold Air Advection Type
SW Upslope Flow Snowfall

Example 1 --- March 19-20, 1996

March 19, 1996 At 7:00 PM ( 00z March 20 )
850 MB Geopotential Height & Temperatures

A truly classic example of cold air advection and snowfall on orographically enhanced SW Upslope Flow occurred during March 1996, in association with a powerful synoptic-scale low pressure system.

A blizzard-like fall of heavy snow developed during afternoon hours of March 19 on strong SW winds from the Norton-Wise area southwestward, in contrast to only light snow showers and flurries in Pound and Clintwood ( on downsloping SW flow ).

March 19, 1996
ETA Model Initialization At 7:00 AM

March 19, 1996
Development of Blizzard On SW Winds
ETA Model Initialization At 7:00 PM

By 1700 hours ( 5 pm ) March 20, when low-level winds began veering NW, ground accumulations ranged from a mere 1" or less in deep valleys north of the High Knob Massif - Tennessee Valley Divide to more than 10" across upper elevations amid the High Knob Massif to Black Mountain corridor 
( with MUCH deeper drifts ).

Snow depths of 5" to 8" were measured in the Norton-Wise area, with wind blown drifts of 12" to 18" reported around Lonesome Pine Airport.  A huge contrast between Clintwood & Norton-Wise that highlighted the powerful upslope-downslope couplet.

March 1996
Virginia NCDC Climate Summary
Daily Snowfall For March 16-31, 1996
National Climatic Data Center - Page 23

Although a shift into NW Upslope Flow acted to complicate this event late on March 20 into March 21, with final snowfall totals reaching 18" to 25" in the High Knob Massif from High Chaparral across the main crest zone, the SW Upslope Flow period made it a great winter storm event.

Although snow depths decreased daily during this late season event, reports from early on March 21 revealed the significant contrast across the area.

By dawn of March 21 mean ground accumulations varied from 3"-5" around the communities of Pound, Clintwood, Birchleaf, and Breaks Village, 
to 9" in Big Stone Gap, 8"-12" in the town of Wise, 10"-14" in High Chaparral, and 12"-20" on Eagle Knob of the High Knob Massif where observer Dennis Salyer reported snow drifts of 5 to 6 feet in the gap between High Knob and Eagle Knob. 

Note that snow depths for Wise are officially recorded at 5 PM daily, with morning depths reported by Roy L. Wells, Jr., being greater than those on page 23 of the March 1996 NCDC report
( morning reports via personal communication with Roy ).


Example 2 -- December 1, 2008

December 1, 2008
Flatwoods of High Knob Massif
Heavy Wet Snow Falls Upon The Flatwoods
Public Photograph - WCYB-TV Archive

A more recent example of SW Upslope Flow snow with cold air advection developed at the start of December 2008, to mark the third SW Upslope Flow snowfall event of 2008.

November 30, 2008
NAM Model Initialization At 7:00 PM

December 1, 2008
NAM Model Initialization At 7:00 AM

Decmber 1, 2008
NAM Model Initialization At 7:00 PM

While no snow accumulated across northern Wise County, and most all of Dickenson County north of the Tennessee Valley Divide, it became deep across the High Knob Massif. 

By 8:45 a.m., December 1, Darlene Fields measured 4.5 inches of snow depth in the High Chaparral community of High Knob.  At around the same time, Wayne & Genevie Riner measured 2.2 inches on the Long Ridge of Sandy Ridge at Nora 4 SSE.  By contrast, nothing was on the ground in Clintwood, Pound, and Clinchco-Birchleaf with downslope flow.

Snow continued to fall with sticking and accumulation on a cooling upslope flow rising upward through the High Knob Landform and into adjacent portions of the Tennessee Valley Divide 
( i.e., Black Mountain, Wise Plateau, Sandy Ridge ) during the daylight hours.

Meanwhile, melting, non-sticking snow of a lighter nature ruled downslope locations to the north and northeast of the High Knob Massif with little to no accumulation during the afternoon in places such as the South Fork of Pound, Darwin, and McClure.

By 3 p.m., December 1, Darlene Fields measured 
7 to 8 inches of snow depth in her High Chaparral yard at 3300 feet above sea level.  At the same time, around 3 inches of depth was on the ground amid the Head of Powell Valley at the home of Elizabeth & Addison Stallard ( 1940 feet above sea level ).

A wind shift toward the west-northwest continued lighter snow showers and flurries into the early hours of December 2, with some accumulation in other locations.


Final Snowfall Totals
December 1-2, 2008

Clintwood 1 W:  0.7"

( FAA Radar Station )
Black Mountain, Ky.:  1.9"

Wise 3 E:  2.2"

Long Ridge of Sandy Ridge:  4.1"

City of Norton Water Plant:  4.9"

High Chaparral of High Knob Massif:  8.0"

Eagle Knob of High Knob Massif:  10.0"


The snowfall distribution by elevation 
within the High Knob Landform was:

< 1900 feet…less than 3 inches

1900 to 2700 feet….3 to 5 inches

3000 to 4223 feet….6 to 10 inches.


Locally less snow fell across portions of the Wise Plateau, where downsloping leeward of the High Knob high country became more of a factor ( During earlier events, on January 22 and February 29 of 2008, the Long Ridge section of Sandy Ridge also got caught within the downslope from the High Knob Massif.  In this case, the SW flow streamed across the lower eastern end of the massif and there was not as much downsloping into the Long Ridge area ).



Warm Air Advection Type
SW Upslope Flow Snowfall

Example 1 --- November 27, 1977

November 27, 1977 At 1200 UTC
850 MB Geopotential Height & Temperatures


A classic example of SW Upslope Flow snow with orographic enhancement, amid an atmosphere cold enough to support widespread snowfall, was observed during November 1977.

Snowfall developed with isentropic upglide of warmer air over an arctic dome, and became enhanced by orographic forcing as strong low-level southwesterly winds pushed into the High Knob Massif and adjacent high terrain along the Tennessee Valley Divide.

Public snow depth reports of up to 16" were received from Big Stone Gap, in lowlands of the Powell River Valley, with 11" to 14" depths reported from the Norton-Wise area northeast to Hazel Mountain along the Tennessee Valley Divide ( * ).

Although no reports are available for the High Knob Massif from this event, it is almost certain ( based upon many events since this time ) that snow depths were much deeper, especially above 3000 feet.

Snow Depths Reported In Virginia
Outside The SW Upslope Zone
November 27-28, 1977

Blacksburg 3 SE:  2"

Big Meadows:  2"

Monterey: 2"

Roanoke: 2"

Burkes Garden: 7"
( 12.5" of total snowfall )

Clintwood 1 W:  7"

Mountain Lake Biological Station:  7"
( 7.0" of total snowfall )

*An outstanding signature of this type of event is the difference in snow depths between Clintwood, on downsloping air flow with SW winds, and the Powell River Valley where 11" was reported in Pennington Gap and 16" in Big Stone Gap on SW winds upsloping through the High Knob Landform toward its remnant massif.


Example 2 -- January 22, 2008

January 22, 2008 at 10:00 AM
Heavy Snow Falling In Big Stone Gap
Sheila Durham Photograph - WCYB Archive

A typical forecast before WAA Events:

VAZ001-002-005-006-008-222115-
LEE-WISE-SCOTT-RUSSELL-WASHINGTON
INCLUDING THE CITIES OF...JONESVILLE...WISE...
GATE CITY...LEBANON...ABINGDON
400 AM EST TUE JAN 22 2008

TODAY...A CHANCE OF A LIGHT WINTRY MIX OF RAIN...FREEZING RAIN...SLEET OR SNOW EARLY IN THE MORNING...THEN OCCASIONAL RAIN SHOWERS IN THE LATE MORNING AND AFTERNOON. ONLY MINOR ICE AND SNOW ACCUMULATIONS POSSIBLE IN A FEW AREAS DURING THE EARLY MORNING HOURS. HIGHS IN THE UPPER 30S TO
LOWER 40S. SOUTHWEST WINDS 5 TO 15 MPH.
CHANCE OF PRECIPITATION 80 PERCENT.

One problem prior to my climatic research on 
the High Knob Landform is that SW air flow trajectories were never fully recognized as being orographically enhanced flows with distinct upslope-downslope couplets in Virginia 
( i.e., like NW Upslope Flows ).

During CAA types when air is cold enough from bottom to top in the atmosphere to support snow, these SW Upslope Flow events result in distinct areas of enhanced snow amounts relative to what is forecasted and relative to locales outside of the main SW Upslope Flow zone.

Likewise, they result in areas of reduced snowfall
amounts via associated inverse forcing or downsloping.

During WAA types, when air may only be marginally cold and seemingly unsupportive of snow, SW Upslope Flow snowfall events result in unforecasted falls that often have fast hitting, high impacts upon travel.

A second, major problem is that SW Upslope Flow snowfall events occur less often than NW Flow events and are poorly handled by forecast models, especially those of the WAA type.

There are many cases of enhanced snow on SW Upslope Flow in the High Knob Massif area that may not be fully developed, or typical, such that snow accumulates to the floor of Powell Valley and the Powell River Valley.

When SW Upslope Flow of the WAA type becomes fully developed, accumulating snow levels reach the floors of Powell Valley and the Powell River Valley.

January 22, 2008
Warm Bullseye Over Heavy Snow Zone
NAM Forecasted Surface Temperature At 6 Hours

For example, during the January 22, 2008 event the NAM Model was forecasting a warm bullseye right over the SW Upslope Flow corridor that had the greatest low-level cooling.

In reality, moderate to heavy snow continued into much of the afternoon as adiabatic cooling on the upslope flow maintained a column of freezing air through the vertical depth of the precipitating atmosphere ( a nearly isothermal layer at lower elevations = BIG flakes ).

January 21, 2008
NAM Model Initialization At 7:00 PM

January 22, 2008
NAM Model Initialization At 7:00 AM

January 22, 2008
NAM Model Initialization At 7:00 AM

Snow depths of 3" to 4" were widespread from the floor of Powell Valley into the City of Norton and windward slopes of the High Knob Massif.

This event had a high impact upon the U.S. 23 and Alternate 58 corridors in Wise and Lee counties, as well as the extreme northern portion of Scott County.

The upslope-downslope couplet was active with only 0.5" of snow at Nora 4 SSE on Long Ridge and 1.5" in Wise which were again in the downslope wake of the High Knob Massif.

No snow to only a dusting occurred in downslope locations across much of the Russell Fork & Levisa Fork basins from Pound to Clintwood, Haysi, and Grundy.  In fact, this was mostly a non-event for the entire region outside of this favored SW Upslope zone.


Example 3 -- February 29, 2008

February 29, 2008
Heavy Snow Falling In Norton At 1600 Hours
Photograph Courtesy of Mr. Smith - WCYB Archive

NWS Forecast Discussion Before Event:

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE
400 AM EST FRI FEB 29 2008

SHORT TERM ( TODAY AND TONIGHT )
SHORT WAVE AND COLD FRONT TO MOVE ACROSS THE AREA
IN THE SHORT TERM. MODELS IN DECENT AGREEMENT WITH DEPICTION OF THIS...ALTHOUGH GFS SLIGHTLY FASTER WITH MOVING PRECIP INTO THE AREA. 

WILL GENERALLY FOLLOW A COMPROMISE SOLUTION...WHICH WILL BRING BULK OF PRECIP IN THIS AFTERNOON AND EARLY TONIGHT. WILL DEPICT BAND OF CATEGORICAL POPS ACROSS THE AREA...IN WESTERN ZONES LATE THIS AFTERNOON AND ELSEWHERE EARLY TONIGHT. PRECIP TYPE SHOULD BE PRIMARILY RAIN. HOWEVER...MODEL THERMAL PROFILES SHOW FREEZING LEVEL DOES NOT GET OVERLY HIGH...SO THERE MAY BE SOME SNOW OVER THE HIGHEST MOUNTAIN PEAKS. WILL MENTION IN FORECAST. SNOW LEVEL WILL LIKELY LOWER AS COLDER AIR MOVES IN TONIGHT BEHIND THE FRONT...BUT BY THEN THE PRECIP WILL BE RAPIDLY DIMINISHING. WILL GENERALLY GO SLIGHTLY ABOVE MAV TEMPS MOST LOCATIONS IN THE SHORT TERM.


Forecast models prior to this event indicated that the vertical temperature profile would remain at or below freezing from around the 850-825 mb level on up through the atmosphere, such that at least some snow was expected to fall upon the highest mountain summits.

February 28, 2008
NAM Model Initialization At 7:00 PM

February 29, 2008
NAM Model Initialization At 7:00 AM

February 29, 2008
NAM Model Initialization At 7:00 PM

The morning began with a light coating of snow from passage of an overnight warm front, with 0.1" to 0.5" of accumulation being the general range from Clintwood and Wise south into the 
High Knob Massif.

An overnight minimum of 19 degrees at Clintwood 1 W climbed to 25 degrees by around sunrise, as the warm front passed with its light dusting.  

Increasingly gusty S-SSW winds were observed during the 0700-1200 hour period, in wake of the warm front.  A period of partial clearing allowed mid-morning sunshine and rapid temperature rises into lower-middle 40s across lower elevations of northern Wise and Dickenson counties 
( warming enhanced by downsloping winds ).

Temps struggled, by contrast, to reach mid-upper 30s in middle elevations ( 2000-3000 feet ) of the High Knob Landform - Tennessee Valley Divide corridor, from the Wise and Sandy Ridge plateaus southwest through the City of Norton into 
Lee County, on upsloping S-SW winds 
( colder at highest elevations ).

February 29, 2008
850 MB Vector Wind Composite Mean

Although James & Carol Bolling, who live on Little Mountain of the High Knob Massif just south of Big Cherry Dam, reported that the first snowflakes began falling by 1000 hours at their elevation of 3430 feet ( 4.0 air miles SW of High Knob peak ), 
it was early afternoon before any precipitation of significance spread across the area.

Snowfall became heavy across the upslope cooling zone by 1400-1500 hours, with low visibilities, as rain fell on downsloping into the Russell Fork and Levisa Fork basins from Pound to Grundy. 

Northern Scott County of High Knob Massif
Lorens Camp of Osborne Ridge At 5:00 PM
Photograph courtesy of Christy Lawson - WCYB Archive

The Bolling family measured 2.5" of snow on their truck by 1540 hours, with 6.5" drifts, as driving SW winds blew across the Big Cherry Basin.

Snowflakes were huge, indicative of a nearly isothermal temperature profile, with hazardous road conditions developing from Norton-Wise southwest into Powell Valley, Appalachia, Stonega, Keokee, and adjacent communities in Wise, Scott, Lee counties.

Numerous accidents were reported in the impact zone.  A white pickup truck rolled over onto its side to block a portion of U.S. 23 just outside of Wise, with several vehicles and a school bus sliding off State Route 706 of High Knob, between Tacoma and Robinson Knob, amid this "surprise" snow.

Darlene Fields, of High Chaparral, drove through the above conditions on Route 706.  She reported that small trees were drooped across the road by the heavy snow.

February 29, 2008
Heavy Snow On The Wise Plateau
Photograph courtesy of Summer Smith - WCYB Archive

This event exemplified what happens when SW Upslope Flow becomes fully developed amid a WAA regime, with 1" to 4" depths accumulating upon the floor of the Powell River Valley and Powell Valley in Lee and Wise counties.

New snow depths of 4" to 8"+ were common 
across the High Knob Massif during this event.

Total snow depths varied widely with old snow and drifting.  A general 1 to 4 feet of snow depth was on the ground across the main crest zone from Eagle Knob into Big Cherry Basin ( deepest on north slopes & drift zones ).

Once again portions of the Tennessee Valley Divide got into the wake of the massif during this event with snowfall totals reaching only 0.8" on Long Ridge of Sandy Ridge and 1.5" at the official measuring site in the town of Wise ( Wise 3 E ).

Most of the remainder of the Mountain Empire had rain with little to no snow outside of the highest elevations where 1" to 3" were reported in places like Burkes Garden, Banner Elk, Beech Mountain, Mount LeConte, and Mount Mitchell in the 3000-6000+ foot zone. 

Differences in measured water equivalent precipitation totals reveal the significance of the upslope-downslope couplet associated with SW flows in this area.


Water Equivalent Precipitation Totals
SW Flow Event of February 29, 2008

Downslope Locations In This Event

Blacksburg: 0.05"

Wytheville 1 S: 0.06"

Bland: 0.15"

Richlands: 0.23"

Clintwood 1 W: 0.30"

Burkes Garden: 0.32"

Lebanon: 0.31"

Grundy: 0.35"

Nora 4 SSE: 0.37"


Upslope Locations In This Event

Pennington Gap: 0.55"

Head of Powell Valley: 0.65"

Appalachia Lake WP: 0.67"

Big Stone Gap WP: 0.67"

City of Norton WP: 0.70"

Robinson Knob of High Knob Massif: 0.73"
( Robinson Knob IFLOWS: 0.56" )

Stickleyville of Wallen Creek Basin: 0.76"

*Big Cherry Dam of High Knob Massif: 0.76"

*From automated IFLOWS gauge.  Otis & Nancy Ward measured 0.73" in Robinson Knob verses the IFLOWS total of 0.56", suggesting the actual total was around 0.99" at Big Cherry Dam ( 23% more than IFLOWS ).


While SW Upslope Flow snow events are certainly a unique part of Virginia climatology, and can be simply wicked at times in the High Knob Massif, 
in particular, they only rarely attempt to rival the more potent and extreme TIM Circulatons already highlighted on this website.

High Impact Snowfall & TIM Circulation of February 2012

MEGA-Disaster of December 2009 & TIM Circulation

One rival to TIM events like those documented during January 1998 and December 2009, may have been the Great Thanksgiving Snowstorm of November 1950.

If old weather charts are correct, SW Upslope Flow would have played a prominent role in that massive snow event across the High Knob Landform.




This would have been a CAA type analogous to the March 1996 episode highlighted previously, but stronger and much more prolonged in nature with respect to its SW Upslope Flow snow.  Wow!

With respect to Powell Valley these phenomena, unique to Virginia, are polar opposites.  Like Dr. Jekyll and Mr. Hyde for snow lovers.  Powell Valley can not buy snow on a fully developed TIM Circulation, while it gets plastered on a fully developed SW Upslope Flow trajectory.  Amazing!

February 11, 2013
Mountain Waves Galore
A Golden Sunset From Long Ridge
Photograph by Wayne Riner - © All Rights Reserved.

Wayne Riner Photograph Thoughts...
"There were only the sounds of nature with the occasional bird making his final song of the day and wind was seen in the bare trees."

A simply AWESOME way to end this section!

Sunday, February 17, 2013

Winter Majesty of Mid-February Cold Blast


February 16, 2013
High Knob Massif Crest Zone
Majestic Rime Glows In Afternoon Sunlight 
Photograph by Roddy Addington - © All Rights Reserved.

The High Knob Landform

Written by Wayne Browning - October 2009...
"RIME makes the sprawling high country expanse of High Knob a water capturing wonder during the cold season, as moist, bitter winds are lifted and forced to rise, cool, and condense out their loads 
of moisture across its many lofty ridges & basins."

Introduction To RIME - October 2009 Dazzle Event

February 16, 2013
Windward Slopes of High Knob Massif
Darkening Skies Against Rime Forest
Photograph by Roddy Addington - © All Rights Reserved.

Rod Addington Photography

Low clouds lifted and parted just in time for photographers Roddy Addington and Bill Harris 
to capture an array of gorgeous scenes 
( as VDOT worked slick roads ).

High Knob Massif - Afternoon of February 16
Working State Route 619 In A Rime Forest
Photograph by Roddy Addington - © All Rights Reserved.

Wondrous windward growth of rime 
never fails to amaze!

February 16, 2013
High Knob Massif Crest Zone
Rime On Windward Side of Branches
Photograph by Bill Harris - © All Rights Reserved.

Temporary breaks formed in the overcast.

February 16, 2013 at 12:47 PM
Breaks Develop Above The Rime Forest
Photograph by Bill Harris - © All Rights Reserved.

When the summit was reached, by luck of timing, rime was stunning against an interval of blue skies as the heavens aloft turned increasingly bitter to indicate clearing would be short-lived.

Temperatures aloft were plunging during afternoon hours of February 16, with 40 to 50 below zero air poised to pass overhead ( above 17,000 feet ) by early evening.

February 16, 2013
Stunning Beauty of Rime On High Knob Peak
Photograph by Bill Harris - © All Rights Reserved.

There is much more to this stunning scene 
than meets the eye.

February 16, 2013
Below High Knob Meadow
More To This Scene Than Just Eye Candy
Photograph by Bill Harris - © All Rights Reserved.

Upon close inspection flakes of rime, 
much like snow, are throughout the air.

February 16, 2013
Rime On The Trees & In The Air
Photograph by Bill Harris - © All Rights Reserved.

This can be clearly seen by zooming 
in on Bill's high resolution photograph.

February 16, 2013
Rime Flakes Fill The Air On High Knob
Photograph by Bill Harris - © All Rights Reserved.

Many years ago I first got weather reports that it was snowing on High Knob out of clear skies!  In most cases this would be due to rime flaking off trees and swirling through the air like snow.

High Knob Massif
Flakes Filling The Air Amid Blue Skies
Photograph by Bill Harris - © All Rights Reserved.

In other cases, amid very cold air, ice crystals sometimes called Diamond Dust can fall out of cloudless air and glitter in sun or moon light.  

In the High Knob Massif this typically occurs as cold air that is relatively moist in surrounding valleys rises upslope where crystals form 
( sometimes with clear skies above ).

February 16, 2013
Looking Up Toward High Knob Meadow
Photograph by Bill Harris - © All Rights Reserved.

The MAX temp reached only 20 degrees on High Knob February 16 before bursts of afternoon snow developed beneath very cold air aloft.  A ferocious wind driven snow band later struck amid NW upslope flow into the overnight of February 17 ( highlighted later ).

Bill Captures The Motion Of Rime Flakes
Rime Flakes Falling In Mostly Clear Sky
Photograph by Bill Harris - © All Rights Reserved.

Although rime grows into the wind in saturated, sub-freezing air, the nature of this deposition as captured by Roddy shows how amid some bright sunlight ( insolation ) it might flake off into the blowing wind.

February 16, 2013
High Knob Massif Crest Zone
Close Up Of Rime Deposition On A Tree Limb
Photograph by Roddy Addington - © All Rights Reserved.


Recent Links of Interest

Cold & Snowy Opening To February 2013

The Fast & Furious Storm of January 2013



Caught In The Middle
But Still Wintry & Cold Enough

February 17, 2013
Chicken In The Snow On Sunday Morning
Photograph by Wayne Riner - © All Rights Reserved.

Wayne Riner Photograph Thoughts...
"The cement hen and family have seen many different weather patterns.  Sunday morning brought 11 degrees and snow."

Although caught in the middle, north and west of the best upper air support and south of the best Great Lake moisture, the High Knob Massif - Tennessee Valley Divide area still managed to collect a general 2" to 5" during this event.

A general 8" to 10" of snow was reported at the summit levels
of Mount Mitchell and Mount LeConte as the main upper air disturbance passed over the southern Appalachians on its way to produce rare Piedmont snow from Charlotte to Raleigh in North Carolina and support another Atlantic bombogenesis event.

This event came in three phases.

1 ).  A light lower elevation rain and upper elevation snow during February 15, with 0.5" to 1.5" above 3000 feet in the High Knob Massif and at lower elevations to the east-northeast.

Beginning of Rime Event
February 15, 2013 at 11:01 PM
Eagle Knob of High Knob Massif
Image by Steve Blankenbecler - © All Rights Reserved.

2 ).  A pocket of super-cold air aloft that triggered hit-miss snow showers and squalls during the afternoon of February 16, with best accumulations at mid-upper elevations where the air was more moist ( dry air hindered snow at low elevations ).

This setting was analogous to a summer shower pattern with a vertical temp drop from around 32 degrees at low elevations ( around 1500 feet ) to -40 to -50 below zero above 17,000 feet.  This steep lapse rate generated hit-miss downpours of snow with localized whiteouts for brief intervals amid the heaviest squalls.

NASA Visible Imagery Of Cold Air Aloft
Early Afternoon Development - February 16, 2013

Click consecutively on images for motion

NASA Visible At 11:02 AM - February 16, 2013

NASA Visible At 11:32 AM - February 16, 2013

Observe the formation of billowing clouds that grew vertically upward through the bitter air aloft, forming distinct cellular convective blobs indicative of steep lapse rates ( i.e., strong temp drops with height ).

NASA Visible At 12:02 PM - February 16, 2013

NASA Visible At 12:32 PM - February 16, 2013

NASA Visible At 12:45 PM - February 16, 2013

NASA Visible At 1:45 PM - February 16, 2013

NASA Visible At 2:02 PM - February 16, 2013

NASA Visible At 2:15 PM - February 16, 2013



Doppler Radar Patterns
Hit-Miss Snow Showers & Squalls
Afternoon-Early Evening of February 16, 2013

Click consecutively on images for motion

Doppler Radar Base Reflectivity At 4:00 PM

Doppler Radar Base Reflectivity At 4:10 PM

Doppler Radar Base Reflectivity At 4:19 PM

Doppler Radar Base Reflectivity At 4:29 PM

Doppler Radar Base Reflectivity At 4:39 PM

Doppler Radar Base Reflectivity At 4:49 PM

Doppler Radar Base Reflectivity At 4:59 PM

Doppler Radar Base Reflectivity At 5:08 PM

Doppler Radar Base Reflectivity At 5:18 PM

Doppler Radar Base Reflectivity At 5:27 PM

Doppler Radar Base Reflectivity At 5:40 PM

Doppler Radar Base Reflectivity At 5:49 PM

Doppler Radar Base Reflectivity At 5:59 PM

Doppler Radar Base Reflectivity At 6:09 PM

Doppler Radar Base Reflectivity At 6:19 PM

Doppler Radar Base Reflectivity At 6:28 PM

Doppler Radar Base Reflectivity At 6:38 PM

Doppler Radar Base Reflectivity At 6:48 PM

Doppler Radar Base Reflectivity At 6:57 PM

Doppler Radar Base Reflectivity At 7:07 PM

Doppler Radar Base Reflectivity At 7:17 PM


3 ).  The third and final phase of this event featured a surge of bitter air and some NW upslope flow bands of snow during the overnight hours of February 17 ( with hazardous driving conditions ).


Snowstreaks With Heavy Snow
February 17, 2013

Click consecutively on images for motion

Doppler Radar Base Reflectivity At 12:07 AM

Doppler Radar Base Reflectivity At 12:17 AM

Roads across northern Wise and Dickenson counties became slick around and just after Midnight before 
a concentrated snowstreak ( band of heavy snow ) developed and moved south across Wise County into Norton-Wise and the High Knob Massif after 2:00 AM.  Only heaviest snow is represented by these images.

Doppler Radar Base Reflectivity At 12:26 AM

Doppler Radar Base Reflectivity At 12:36 AM

Doppler Radar Base Reflectivity At 12:46 AM

Doppler Radar Base Reflectivity At 12:55 AM

Doppler Radar Base Reflectivity At 1:07 AM

Doppler Radar Base Reflectivity At 1:15 AM

Doppler Radar Base Reflectivity At 1:25 AM

Doppler Radar Base Reflectivity At 1:34 AM

Doppler Radar Base Reflectivity At 1:44 AM

Doppler Radar Base Reflectivity At 1:54 AM

Doppler Radar Base Reflectivity At 2:03 AM

Doppler Radar Base Reflectivity At 2:16 AM

Doppler Radar Base Reflectivity At 2:25 AM

Doppler Radar Base Reflectivity At 2:35 AM

Doppler Radar Base Reflectivity At 2:45 AM

Doppler Radar Base Reflectivity At 2:55 AM

A general 1-2" of snow accumulated amid the snowstreak, with locally more in places that got repeated hits by multiple squalls.

The CAM at UVA in Wise captured the heavy snow as it moved across the town of Wise, which previously had missed much of the snow during the first two phases of this event ( February 15-16 ).


University of Virginia's College In Wise
February 17, 2013 ( 2:29 AM to 2:52 AM )

Click consecutively on images for motion

Observe how distant lights disappear as 
the snow becomes moderate-heavy.

Snow Beginning To Fall In Far Distance
University of Virginia's College In Wise At 2:29 AM

University of Virginia's College In Wise At 2:31 AM

University of Virginia's College In Wise At 2:33 AM

University of Virginia's College In Wise At 2:34 AM

University of Virginia's College In Wise At 2:35 AM

University of Virginia's College In Wise At 2:36 AM

University of Virginia's College In Wise At 2:37 AM

University of Virginia's College In Wise At 2:38 AM

University of Virginia's College In Wise At 2:39 AM

University of Virginia's College In Wise At 2:40 AM

University of Virginia's College In Wise At 2:41 AM

University of Virginia's College In Wise At 2:42 AM

University of Virginia's College In Wise At 2:43 AM

University of Virginia's College In Wise At 2:44 AM

University of Virginia's College In Wise At 2:45 AM

University of Virginia's College In Wise At 2:46 AM

University of Virginia's College In Wise At 2:47 AM

University of Virginia's College In Wise At 2:48 AM

University of Virginia's College In Wise At 2:49 AM

University of Virginia's College In Wise At 2:50 AM

University of Virginia's College In Wise At 2:51 AM

University of Virginia's College In Wise At 2:52 AM

Additional light snow and snow showers 
fell into mid-morning on February 17.

February 17, 2013 at 8:49 AM
Along the Tennessee Valley Divide
Snow Falling Upon The Long Ridge Orchard
Photograph by Wayne Riner - © All Rights Reserved.



High Knob Massif
Winter Beauty Shots

February 16, 2013
Beauty of Rime & Rime Flakes In The Air
Majestic RIME of High Knob Crest Zone
Photograph by Roddy Addington - © All Rights Reserved.


High Knob Massif
Special Effects of Light On Rime
Photograph by Roddy Addington - © All Rights Reserved.

( Before the Snow Squalls )
Afternoon of February 16, 2013
Looking Across The High Knob High Country
Photograph by Roddy Addington - © All Rights Reserved.

Little Stony Basin of High Knob Massif
Special Effects of Winter 2012-13
Photograph by Roddy Addington - © All Rights Reserved.

Just a big ole...

WOW!