Weather Headlines
*ALERT Continues For NW Flow Snow Into Tuesday Morning (December 3)
Update As Of 5:00 PM Monday ( 2 December 2019)
Steady light snow, and occasional heavier snow showers and local squalls, will continue with Great Lake moisture transport through Monday night before ending around mid-morning Tuesday.
Generally 1″ to 3″ of new snow is expected along the upslope side of the mountains into Tuesday AM, with higher amounts possible at highest elevations where blowing-drifting will also continue. Snowfall totals of less than 1″ are expected below 1200 feet (in lowest valleys along the upslope side of the mountains).
A general 2″ to 3″ of snow accumulation occurred above 3000 feet in the High Knob Massif by 8:30 AM Monday. Several more inches accumulated into the afternoon, with 4″-12″+ depths in blowing & drifting on High Knob and Eagle Knob by afternoon.
Locations below 1500 feet elevation had melting during the daylight hours Monday, but sticking will occur again Monday night into Tuesday morning at elevations above 1200-1500 feet on the upslope side of the mountains.
Previous Forecast & Discussion
A upper air disturbance, with very cold air aloft, will cause heavy snow, amid whiteout bursts, to develop Monday with low visibilities at times as temperatures continue to drop on developing WNW-NW upslope flow.
Past climatology and analog settings with these types of parameters have produced intense snow squalls in whiteout conditions. Those traveling, and otherwise expected to be outside should be prepared for periods of near zero visibility. The potential for local thundersnow exists.
Some snow squalls may develop a convective nature, with conditional symmetric instability (CSI) or slantwise columns of elongated clouds capable of supporting this despite low clouds tops (relative to typical convection) in the vertical.
Upslope snow showers will then continue into Tuesday.
Wind chills will also be a factor, especially at higher elevations, where single digits and 10s are expected Monday into early Tuesday, with WCF variations from 10s to lower 20s in more sheltered valleys at the lower elevations to around 0 degrees in gusts on highest peaks.
Final Snowfall Forecast
3″ to 5″ below 2500 feet
(locally higher amounts)
5″ to 7″ above 2500 feet
(locally higher amounts)
6″ to 10″+ above 3000 feet
**(bare ground to 1-2’+ drifts)
1/2″ to 3″ in downslope locations
(Clinch, Powell, Holston valleys)
Target snowfall of 5″ at 2500 feet (+/-) 1″ error potential, suggesting 4″ to 6″ possible at elevations of Norton-Wise with locally deeper depths developing where snow squalls are most intense and where blowing-drifting occurs.
**Blowing and drifting is expected to cause large snow depth differences at highest elevations in the High Knob Massif, with variations from near bare ground to 1-2+ feet in drifts. The potential for some roads, such as Route 237 at Davenport Gap, 238 at High Knob-Eagle Knob Gap, and State Route 619 in Little Mountain Gap, to become impassible will exist due to drifting.
Preliminary Snowfall Forecast
1″ to 3″ below 2500 feet
(locally higher amounts)
3″ to 6″ above 2500 feet
(locally higher amounts)
4″ to 8″ above 3000 feet
**(bare ground to 1-2’+ drifts)
1/2″ to 2″ in downslope locations
(Clinch, Powell, Holston valleys)
Target snowfall of 3″ at 2500 feet (+/-) 1″ error potential, suggesting 2″ to 4″ possible at elevations of Norton-Wise with locally deeper depths developing where snow squalls are most intense and where blowing-drifting occurs.
**Blowing and drifting is expected to cause large snow depth differences at highest elevations in the High Knob Massif, with variations from near bare ground to 1-2+ feet in drifts. The potential for some roads, such as Route 237 at Davenport Gap, 238 at High Knob-Eagle Knob Gap, and State Route 619 in Little Mountain Gap, to become impassible will exist due to drifting.
ALERT For Strong WNW-NW Winds At Middle To Upper Elevations, Above 2500 Feet, With Gusts of 30-40+ MPH Through Monday
Blowing and drifting snow will be a factor at upper elevations where strong WNW-NW winds will continue through Monday into Tuesday. Riming is also expected, with the extent of orographic feeder cloud formation ultimately acting to influence snowfall totals beneath and just downstream of them as snowflakes become rimed to increase total snow volume (*).
*This has been an observed and interesting part of my climatological research in the High Knob Massif, and has been documented in other orographic belts of the world. My predicted snowfall totals are higher above 3000 feet, in part, due to the expected riming of snowflakes falling through and being blown just downstream of orographic pilatus (cap or feeder) clouds forming on WNW-NW upslope flow. Riming of flakes acts to increase the amount of snow that accumulates on the ground over time, with rime drop from trees later adding additional moisture content to the ground snow cover.
A composite of more than 80 models, including all ensembles, is predicting snowfall variations from a Trace to 6″ at the Wise gridpoint. All these are based upon a snow density of 10:1 (snow to water ratio of 10″ of snow per 1.00″ of liquid equivalent).
My actual forecast is going above the mean of 3″ due to strong orographic forcing and the tapping of moisture from the Great Lakes, with lower snow densities (higher ratios) than indicated by models. In addition, a pocket of very cold air aloft will aid initial snowfall with blizzard-like bursts expected. Riming of snowflakes falling through orographic clouds at upper elevations will further enhance snowfall amounts above 3000 feet (**).
**Past climatology and analogs of similar settings support a more aggressive forecast than indicated by the mean of forecast models. While every event is different, and results vary, this can be a powerful tool for prediction of these types of winter events.
Past documented history of snowfall forecasts made since 2014 on the Appalachian Climate Center is verification of this fact, along with additional forecasts made going back to the early 1990s.
Caution For High Water Levels On Steep Creeks Draining The High Knob Massif Into Monday ( 2 December 2019 )
High water levels continue with ROARING creeks draining the High Knob Massif. Extreme caution is advised due to slick banks and high velocity flows.
Caution is also advised near the main-stem rivers. Although warnings are in effect for minor flooding, I think subterranean conduits within the High Knob Massif and along the Clinch and Powell river valleys will be able to take in more water than models predict given near to below average autumn precipitation.
Forecasting precise stream flows are very difficult in this karst area, with Wise, Scott and Lee counties collectively containing the highest cave density in Virginia. This is one of the greatest karst landscapes in North America, with an array of complex conduits at elevations varying from well over 3000 feet in the High Knob Massif to around 1000 feet on downstream valley floors (*).
*A karst landscape possesses a very dynamic, heterogeneous character with large and unknown subterranean features that are continuously changing through space and time, making hydrological predictions more difficult than within non-karstic terrain.
Previous Alerts and Discussions
*ALERT for ponding of water in low-lying locations and along roads, with the potential for rapid water level rises on streams during intervals of heavy rain and downpours.
An average of 3.00″ of rain had fallen across the High Knob Massif as of 4:00 AM on 1 December, with strong rises on creeks draining the upper elevations. Big Stony Creek was only 1/2 foot below flood stage as of 4 AM.
Please use extreme caution near creeks, and monitor water levels through Sunday morning.
*ALERT for strong SSW-WSW to W winds Saturday into Sunday (Nov 30 to Dec 1), beginning at upper elevations early on Saturday and expanding across the area Saturday Night into mid-AM Sunday.
A strong precipitation generated inversion, with cooler temperatures at low elevations versus high elevations, has kept stronger winds (15-30+ mph) along high ridges with calm conditions in lower elevation valleys. This scenario will change Saturday night and (especially) Sunday when vertical mixing will develop and become enhanced by turbulence.
Strongest winds are expected Sunday in most locations as mixing increases in a narrow dry slot, with efficient downward transfer of high momentum air. Strong winds will continue Monday into Tuesday at upper elevations, and in cross-barrier flow along and just lee of the Blue Ridge, in cold air advection on WNW-NW flow.
Reference 112619 Forecast for the previous event.
The next in a series of storm systems will spread rain, with a chance of thunderstorms, across the mountain region Saturday into early Sunday.
Reference Winter 2019-20 for a look ahead at main weather players.
There remains some disagreement in models regarding where bands of heaviest rains will fall, with the NAM and GEM (Canadian) groups being farther north while the GFS and ICON groups are farther south, as examples, with the ECMWF (European) members split among themselves.
As noted below, the latest run of the NAM Model group has trended southwest, more in line with the GFS-ICON models.
Predicted air flow vectors and orographics, combined with past climatology, suggests heavy rainfall will be likely in the mountain area, especially in favored orographic lifting zones along and northwest of the Cumberland-Allegheny Front.
The Cumberland Front typically functions as a weather change boundary, which becomes especially notable during winter when NW flow snowfall tends to be heaviest along and northwest of this boundary. It is otherwise focused along and just northwest of higher elevations of Clinch Mountain and the Blue Ridge (Mount Rogers-Whitetop southwest to the Smokies).
A dry slot of air will cause skies to break around mid-morning Sunday as the warm air advection rain shield lifts out of the region, with windy and relatively mild conditions before clouds increase as the comma-head of this large mid-latitude cyclone begins to wrap into the region to bring cold air advection and a redevelopment of rain and snow showers. A rapid change to snow at high elevations early Sunday evening will occur, with quickly dropping snow levels into middle elevations during the evening.
Cold air aloft generates vertical instability and while perturbation of low-levels with orographic lifting favors snow bursts in NW flow lifting zones, waves generated by the main mountain barriers, local hills, or just local instability, will be able to trigger snow burst development in typical downslope dominated locations.
Due to a pocket of very cold air aloft with an upper air disturbance, bursts of snow will not be restricted to just the typical NW flow orographic belts.