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ALERT For Zones Of Freezing Rain During The Predawn To Post-Sunrise Period Of Early Friday (13 December)

Update At 3:00 AM_Friday_13 December 2019

Freezing rain is falling at Clintwood 1 W with 32 degrees. The temperature has risen from a late Thursday evening minimum of 28 degrees. Please use extreme caution if walking on porches, sidewalks or traveling overnight into early Friday for freezing rain in valleys such as those along streams like Georges Fork Creek, Pound River, McClure River, Cranes Nest River, Russell Fork River and Fryingpan Creek (to note a few) within Dickenson County, and similar sections in dissected terrain of Wise County and Buchanan County.

*While widespread freezing rain is expected along and east of the Eastern Continental Divide, there will also be zones of freezing rain within the complex mountain terrain along the Cumberland-Allegheny Front, especially in valleys sheltered from SSE-SSW winds. These valleys are ones which still have snow cover along slopes this afternoon (12 December).

The potential exists for freezing rain, at least briefly, on adiabatic upslope cooling along the High Knob Massif early Friday (mainly above 3000-3300 feet).

Additional areas of freezing rain will also be possible for a period early Friday in other locations that will not be under an official Winter Weather Advisory.

Be certain to check your local conditions before traveling!

Mesoscale Discussion For Forecasters

Due to this being a predawn-early morning event, centered around the AM commute, it would be best if the entire area was placed under an Advisory For Freezing Rain (which I am unofficially doing with the above alert). Counties west of the Eastern Continental Divide could then be dropped from longer-lived advisories currently in effect for eastern zones.

Thursday evening temperatures will be able to drop to and below freezing in many mountain valleys with dry low-level air in place. This cooling will be enhanced by snow cover that continues along slopes within the locations of greatest concern. There is also concern that this evaporative cooling could have more widespread effects, with exposed middle elevation plateaus-ridges west of the Eastern Continental Divide being least likely to have freezing rain. (A low-level inversion is currently in place due to snow cover, with the Thursday PM max reaching 38.8 degrees at the official NWS Cooperative station in Clintwood to contrast with middle 40s at exposed middle elevation sites in Norton-Wise).

**Temperatures Thursday afternoon have been much cooler, for example, at the City of Norton Water Plant versus the Norton Elementary School Weatherbug site, exemplifying the nature of complex terrain variations that models simply can not handle but forecasters must understand and take into account.

Cloud bases will not lower until after midnight. As precip begins, with isentropic upglide driven by positive theta-e advection, evaporative cooling will develop an inversion layer at low-levels that could keep warming winds from mixing downward into the low-level freezing air.

While this will be most widespread along and east of the Eastern Continent Divide, of course, with more classic cold air damming, it will also develop locally within the complex terrain along the western Appalachians where a downslope component of air flow will develop and initially blow across the top of these local inversions until vertical mixing from orographic waves and general atmospheric turbulence can enhance mixing to valley floors. By then, however, most of this initial precipitation driven by isentropic upglide on theta-e advection may be over, helping aid mixing with downslope induced breaks for a time during Friday.

Two Significant Storm Systems In Five Days

The first of two storm systems brings the freezing rain threat early Friday, then a wave of rainfall into Saturday. Rainfall amounts will act to keep water levels elevated. Although flooding is not currently expected, some high water issues can not yet be ruled out given so much recent wetness.

Big Cherry Lake Dam is now overflowing its spillway so future water level rises along the South Fork of Powell River will be more significant for locations downstream in Crackers Neck, East Stone Gap into Big Stone Gap.

Whitewater creeks draining the High Knob Massif are already elevated. There is increasing concern for high water issues, especially into early next week when heavy precipitation amounts are likely, such that the entire mountain area will be at risk.

An influx of colder air Saturday will help to slow run-off and may cause a change to snow. Some accumulation will be possible at upper elevations. Stay tuned for later updates regarding snow levels.

A second storm system follows quickly, with a trough of low pressure already over the area by early Monday. Copious moisture transport from the Gulf of Mexico will threaten the mountain area with heavy to excessive rainfall amounts. A flood potential will exist and must be closely monitored.

While some frozen precipitation may also be a concern, initially the main focus at this point will be on the potential for high water issues early next week during Monday into early Tuesday.

The Bottom Line

*Freezing rain will be widespread along and east of the Eastern Continental Divide Friday with moderate to significant icing developing in a hybrid classic / in-situ cold air damming setting as the anchoring high pressure retreats over time (not anchored).

*Freezing rain will be possible during the predawn-early morning hours of Friday in locations along the western Appalachians, with most favored locations being mountain valleys (beneath a inversion enhanced by evaporative cooling) and upslope locations in the High Knob Massif (on adiabatic upslope cooling). Middle elevation ridges and plateaus, exposed to mixing on SSE to SSW winds, will be least likely to have freezing rain. Everyone must check their local conditions before walking or traveling outside!

The 6:15 PM air temperature is 31 degrees (12 December 2019) at the official National Weather Service Cooperative Station in Clintwood to indicate that mountain valleys in complex terrain will be colder than models predict.

Locations such as Clinchco, Haysi, McClure, Nora, Birchleaf, Coeburn, Darwin, and many others, will have temps drop below freezing prior to precipitation onset.

*Rain will elevate stream levels into Saturday and although flooding is not currently expected, some local high water issues can not be ruled out.

*A possible transition to snow is being monitored Saturday, with sticking at this time appearing to be most favored at upper elevations.

**A major storm system early next week is now expected to bring significant precipitation with the potential of flood and flash flood threats. Some wintry types will also be possible. Stay tuned for updates.