ALERT For Convective Snow Bursts With Accumulating Snow Through The Evening (January 4) Along With Snow Showers And Flurries

Local bursts of heavy snow, with snow squalls, will continue into the evening. Rapid drops in visibility will occur in wind driven snow to create hazardous conditions for travelers.

Slick conditions will develop on roadways in areas impacted through this evening into the overnight hours of Sunday (January 5). The greatest general coverage will be in W-NW-N flow upslope locations.

New Year And Decade_Weather Headlines

*The first significant storm of 2020 will begin impacting the mountain region Thursday into Friday (January 2-3).

A deep trough developing over the eastern USA will transport Gulf of Mexico moisture across the region with a heavy rain potential along and just west of the Appalachians.

*Heavy rainfall amounts with the potential for strong rises on streams is being monitored for late Friday into early Saturday (January 3-4).

The 51-member European Ensemble group is predicting amounts varying between 1.00″ and 3.00″ at the Wise gridpoint, such that the axis of heaviest rainfall is yet to be determined. It is most likely to be located over the Tennessee Valley and western slopes-foothills of the Appalachians given a westerly component to flow streamlines.

*Much colder air with rain showers changing to snow showers is expected Saturday afternoon into Saturday evening. Initial cold air transport on SW flow will shift NW with snow showers and possible squalls Saturday night into early Sunday (January 4-5).

Stay tuned for better resolved snowfall amounts and positioning as the event comes into view of high-resolution terrain models.

*A cross-barrier jet is expected to generate high winds along the Blue Ridge during Saturday night into Sunday (January 4-5).

*Milder conditions are expected January 6-7 as flow shifts southerly in advance of a second system.

*A second disturbance in NW flow aloft is being watched for more rain and snow during the January 7-8 period.

*A milder than average pattern is currently being indicated for the January 9-14 period.

Hemispheric Flow Pattern_No Winter Lock-down Seen Yet For Eastern USA

A review of the 2019 upper air flow regime finds persistent troughing across the western-central portion of the USA, keeping the Mountain Empire and Appalachians within a relatively mild, wet pattern (with above average total precipitation but below to much below average snowfall).

The year of 2019 will end having produced the least snowfall of the past 26 years at upper elevations in the High Knob Massif, with amounts only around 37% of the longer-term average.

A southerly to easterly anomaly to the flow pattern within the surface to 500 MB sector contributed to anomalous wetness during 2019 (and previously during 2018).

It is not surprising that this pattern wants to repeat, and that is what models are showing as the general flow pattern into mid-January.

The Winter Season Of 2019-20

Despite many factors favorable for persistent winter, that has not yet materialized as the polar vortex has become strong and hemispheric flow has featured limited blocking since the beginning of Meteorological Winter and a stronger zonal than meridional component to the flow field (below).

In other words, the polar jet stream has not been highly amplified and southward dips have been transitory, progressive and short-lived.

Although not at record strength, the zonal mean zonal wind flow has been stronger than average (in the 70-90% percentile) following a notable weakening in early December (above).

Temperatures have fallen to record cold levels within the stratospheric polar vortex (above), in the 50-90 degree North zone, indicating that the vortex has been stronger than average.

Although seemingly not intuitive, a strong polar vortex favors warmer than average conditions across middle-latitude continents as it promotes more zonal (west to east) than meridional (north to south) flow across the hemisphere.

A recent upward trend in temperatures above the North Pole, toward the long-term average (gray line above), appears to be associated with a reflective disturbance that will allow for some amplification of the polar jet stream into the eastern USA in the coming week, however, this will be progressive in nature and a setting which is favorable for cross-polar flow will not develop to help lock winter into the region.

The MJO is also working against USA winter, with a forecast movement into phases 4-6 which are mild phases for the USA during winter.

This certainly does not mean that no snow and cold air will occur, but for those looking for the development of a prolonged, persistent wintry pattern it is not favorable in the short-term.

These are reasons for a revision of the Climate Prediction Center forecast for January 2020, which predicts wetter & milder than average conditions across much of the southeast USA.

There are some signs of changes beyond mid-January, but those are merely trends to be followed (like analogs of past patterns).

Why may the use of analogs be less effective today? Could this be due to the forcing response of a atmosphere that is different from recent decades in terms of its chemistry and water vapor (sensible-latent heat) content?

Weather Headlines

Reference History Of Christmases Past

*A nasty pre-Christmas storm system will pass mainly south of the Cumberland Mountains

*Unseasonably mild weather conditions will dominate the Christmas Holiday period

*A transition back to a colder, wintry pattern will occur as the new year arrives by January 2020

NASTY Pre-Christmas Storm

Although this storm has been very well predicted in the modeling, the northern extent of the precipitation shield has been highly variable in the modeling.

With that noted, a review of more than 100 models of the most recent runs (includes ensemble members) places the northern extent somewhere between the Kentucky-Virginia and Tennessee-North Carolina statelines. Rain amounts within the southern Appalachians will be greatest in southwestern North Carolina.

Interactive Doppler Radar

While a few wet snowflakes can not be ruled out for crestlines of the Smokies, Great Balsams and Black mountains, it will mostly be a cold rain as the main pocket of cold air passes farther south. Wind driven rainfall and low clouds engulfing the high country of these southern mountain ranges will be the greatest impacts for those traveling, hiking, and hunting from Sunday through Monday (22-23 December 2019).

The greatest flood threat will be across the low country of the Deep South, especially the Georgia and Carolinas. Strong rises along streams draining the eastern slopes of the southern Appalachians will be possible.

National Weather Service Watches-Warnings-Advisories

The European Model, which has consistently been farther north, brings light rain as far north as Breaks Interstate Park on the Virginia-Kentucky stateline.

Clearly, heaviest rainfall (as always anticipated) will impact a large corridor from parts of Mississippi east across Alabama, Georgia, the Carolinas and Florida.

Unseasonably Mild Christmas Pattern

A broad region of upper-level ridging will generate unseasonably warm conditions during the period between Christmas and New Year’s.

Most of the continental USA will have unseasonably mild-warm conditions during this upcoming forecast period.

Pattern Change By Early January 2020

A change back into a colder, wintry pattern is currently expected as the new year begins. This also has support from past analog years featuring an analogous pattern to that of Christmas Day 2019.

Although the hemisphere pattern this year is very similar to the analogs, local conditions with respect to snow cover was certainly not identical with variations highlighted in the History Of Christmases Past section.

One would certainly be hard pressed to find a more similar upper air match across the Northern Hemisphere to that featured between analog years (above) and the forecast for this year (below) on Christmas Day.

Although every pattern and year is different, this does spark interest in seeing what came next in the analog years for their winter seasons.

Winter seasons with a Christmas Day pattern most similar to 2019 featured high-latitude blocking as a dominate circulation mode, associated with negative phases of the Arctic and North Atlantic Oscillations.

The repetitive appearance of high-latitude blocks near Greenland and in the Gulf of Alaska have already been observed this season, driven by warm sea surface temperature anomalies and other factors.

To have these blocks reappear in similar locations during coming weeks would not be surprising and has been an expected trend already highlighted in the major features to watch for Winter 2019-20.

Previous Discussion

The potential for a nasty storm system to impact the Mountain Empire is increasing for late this weekend into early next week, with more and more models beginning to follow the early trend of the GEM (Canadian) Model and JMA (Japanese) Model, and some ensembles, that have displayed a more northward track.

The Canadian and Japanese models, and many of their ensemble members, have been consistent in bringing the precipitation shield with this southern stream storm far enough north to impact much of the Mountain Empire.

While the GFS Model and its ensembles have consistently kept the precipitation shield well to the south, the NAM Model and European have recently come into better agreement with the Canadian and Japanese models.

While impacts are likely to be greatest toward the south, versus the north, with a sharp gradient and cut-off on precipitation amounts to the west and north, exact impacts remain to be worked out for given locations.

It is important to realize that 100-200 air miles is large from the perspective of a given point, but for global-scale models such as the European, GFS, Japanese (as examples) this is only a small difference.

Past climatology of similar systems suggests that the precipitation shield will likely reach northward to the mountains of southwestern Virginia and far southeast Kentucky where orographic gravity waves (mountain waves) become prolific and weather systems “feel” the underlying topography (as exemplified by the famous Mountain Empire of the southern Appalachians).

While a cold rain will be the primary mode for most, the potential for wet snow can not be ruled out for highest elevations in the southern Appalachians (e.g., Mount Mitchell and backbone of the Great Smokies).

Stay tuned for later updates.

Previous Statements

Caution For Low Temperatures And Wind Chills Into Thursday Morning

Temperatures in the 10s and low 20s will combine with breezy to gusty NW-N winds to generate bitter conditions into Thursday morning across exposed mountain ridges and plateaus. Cooling in valleys will accelerate toward morning as drainage of cold air increases.

Mountain valleys are “milder” than exposed ridges and plateaus this evening, with less wind to generate chill factors, but cold air drainage will increase overnight into morning as winds decrease and clouds diminish.

Wind chills in the 10s on exposed middle elevation ridges and plateaus, with single digits at upper elevations (sub-zero chills in gusts on highest peaks), will continue into the overnight before winds diminish in the predawn.

Rain will change to light snow and snow showers into Tuesday afternoon and evening. Accumulations from a dusting up to 1″ will be possible below 2500-3000 feet, with 1″ to 2″ in upper elevations above 3000 feet (*).

*A burst of snow will be possible in upslope locations along-north of the High Knob Massif and Tennessee Valley Divide between sunset and midnight with the approach of a upper wave and the initial low-level 850 MB thermal minimum on NW-N upslope flow.

Slick conditions will develop on roads at upper elevations with persistent snow and snow showers.

The development of slick conditions will also be possible on middle-lower elevation roadways (especially secondary routes) after sunset within locations receiving more persistent snow showers (upslope locations) and a possible burst of heavier snow between sunset and midnight Tuesday.

Rime formation is expected at elevations above 3000 to 3300 feet, depending upon the exposure, during Tuesday afternoon into Tuesday night with orographic clouds obscuring the high country in sub-freezing air.

Temperatures Will Continue To Drop Into Wednesday Morning.

Air temperatures will drop into the 20s Tuesday night into Wednesday morning, with 10s at highest elevations. Wind chill factors will range in the 10s to near 20 degrees, except for single digits at upper elevations (locally below zero on highest peaks).

Caution Advised Around Whitewater Creeks Draining The High Knob Massif

High stream levels and swift water on creeks draining the High Knob Massif will continue through tonight into Wednesday, (18 December) following a general 1.50″ to 2.00″ of rainfall.

Former Alerts

ALERT For The Potential Of Flash Flooding And Flooding Late Monday Into Tuesday

A general 0.50″ to 1.00″ of rain fell overnight into Monday morning, from the High Knob Massif northward to Clintwood and Breaks Interstate Park, with embedded thunderstorms. Additional rain, with possible downpours, will become likely Monday night into the overnight-predawn hours of Tuesday.

Interactive Doppler Radar

*The latest HRRR (High Resolution Rapid Refresh) model is showing the potential for 1.00″ to 2.00″ rain amounts into Tuesday morning and is attempting to resolve the orographic component. Models struggle to handle both convection and orographics.

850 MB Streamline Flow Field

Atmospheric conditions over the mountains are more stable than locations to the west and southwest of the Appalachians, but there is a notable orographic component on moderate-strong SSW-WSW flow streaming from the Gulf of Mexico (sea level) upward into the Cumberland Overthrust Block and High Knob Landform.

Residents living or driving along streams or into low-lying, typically flood prone locations should remain alert for changing conditions through tonight into early Tuesday.

Water gushing out of the High Knob Massif in wake of recent rainfall and snow-rime melt is expected to continue through Tuesday, with potential for significant rises in water levels if additional downpours develop into Tuesday.

Strong SW Winds Monday Night Into Tuesday AM Will Shift NW With A Plunge In Temperatures And Wind Chills During Mid-Morning Into Tuesday PM

Update: 3:00 PM Monday_16 December 2019

Wind speeds will increase again Monday Night into Tuesday as a strong cold front approaches and pushes across the mountains. A wind shift from SW to NW is expected between 7-10 AM Tuesday, with a sharp temp drop through mid-morning into Tuesday afternoon.

Former Discussion & Updates

*ALERT For Strong S To SW Winds Developing Monday Into Tuesday AM

*Strong S-SW winds will develop late tonight into Monday morning as a low-level jet streak develops and surges up along the Appalachians with a strengthening pressure gradient in advance of a major storm system.

While wind gusts of 30-40 mph will become common, gusts of 50-60+ mph will be possible, especially at the highest elevations, during the predawn to post-sunrise period of Monday.

Sheltered valleys which decouple this evening from boundary layer flow will initially have calm conditions prior to recoupling with turbulence and mountain waves that will mix out the nocturnal inversion to generate rapid short-period rises in temperature (**).

Update: 3:00 PM Monday_16 December 2019

A 14 degree temperature rise occurred between 12:10 AM and 1:20 AM [19 degree (F) total change to 3:40 AM] as the nocturnal temperature inversion was dissipated by the onset of turbulent mixing and strong SW winds. Peak wind gusts reached 40 to 50 mph during the predawn in the Wise to Pound-Clintwood area.

**I have named this the Appalachian-chinook effect, where a large vertical temperature gradient develops between mountain valleys and exposed ridges and plateaus with development of strong nocturnal temperature inversions and decoupling of boundary layer flow. Turbulent flow then develops with rapid temperature rises over short-time periods as the inversion (typically nocturnal) is mixed out by turbulence.

Update: 3:00 PM Monday_16 December 2019

Stream levels are already elevated from recent rainfall and snow melt, with whitewater creeks draining the High Knob Massif gushing during Saturday (14 December).

A heavy to excessive rainfall potential will exist with the next major storm system, with southward trends in the placement of axes of heaviest rainfall on models during the past 24 to 48 hours.

Counties along the Virginia-Kentucky border are coming into a higher risk with this southward trend that will be complicated by orographically forced lift as well potential for convection to enhance rainfall amounts upon already wet surfaces.

Around 33% of the European Model ensemble group now includes VA-KY border counties in the band of heaviest rainfall amounts versus no members prior to 24-hours ago.

Stay tuned for updates.

Update: Christmas Bird Count 2019

Reference Early Winter_High Knob Massif for more scenes.

Around 50 bird species were identified during another year with bad weather conditions. A light mixture of sleet and snowflakes at the summit level of High Knob gave way to rime formation around mid-day (as the air temp dropped below freezing).

A burst of moderate-heavy, wind driven snow developed around 2:30 PM Saturday (14 December), and although accumulations were generally 0.5″ or less it added insult to already nasty conditions (orographic clouds and wind chills in the 10s).

Weather Headlines

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.

*ALERT For The Development Of Accumulating Snow Late Tuesday Afternoon Through Tuesday Night

Update: 9:00 PM on Tuesday_10 December 2019

A broad snowband, with embedded mesoscale banding, has dropped a general 2-3″ of snow on the Norton-Wise to Pound-Clintwood corridor as of 9:00 PM (snow is still accumulating).

State Route 83, from Pound to Clintwood, is extremely hazardous with portions of U.S. 23 also becoming snow covered (from Pound to Pound Gap, along portions of Indian Creek Mountain outside the Town of Wise and between Norton and Powell Valley Overlook).

VDOT has been out working main roads, but snowfall rates have been too high for them to keep pace. Please allow time for VDOT to work roads overnight into Wednesday AM before traveling, if possible.

Update: 3:00 PM on Tuesday_10 December 2019

The first snow flakes began falling on High Knob around noon, with sticking between 1-2 PM (28 degrees at 2:22 PM).

A more concentrated area of snow is expected to move into the mountain area from 4 PM through sunset, with sticking snow levels dropping to include Norton-Wise and much of the adjoining area with a snowband.

Interactive Regional Doppler Radar

Expect roadways above 3000 feet to become snow covered and slick, with the potential for sticking on lower elevations roads (especially secondary roads) due to the increase in snowfall rates with dropping temperatures on upsloping NW winds.

Previous Discussion

*While a general transition from rain to sleet and snow will occur within an elongated band, the potential for a relatively narrow axis (mesoscale band) of heavy snow exists inside of this band. The location of this may not be known until it begins development, but locations in this mesoscale band could have a rapid deterioration in conditions despite the current lack of recognition.

Rain is expected to change to sleet and snow between 1 PM and 4 PM Tuesday, then to all snow by before or just after sunset.

A change to frozen precipitation may occur before 1 PM at highest elevations, with rapid temperature drops expected between 10 AM and 1 PM in places along and west-northwest of the High Knob Massif and Tennessee Valley Divide.

*Snowfall Forecast

2″ or less below 1500 feet

2″ to 4″ from 1500-3000 feet

3″ to 5″ above 3000 feet

*Target snowfall: 3″ at 2500 feet (+/-) 1″ error potential,
suggesting a possible 2-4″ in the Norton-Wise area, with
higher amounts should a mesoscale banding axis of snow
develop over the area (lower amounts with no banding).

ALERT Continues For Strong S-SW Winds Tonight Into The Overnight Of Tuesday

SW-WSW wind gusts of 30-40+ mph will continue at middle-upper elevations into the overnight hours. Wind gusts to 50+ mph will remain possible at high elevations in the High Knob Massif.

ALERT For Development Of Strong S-SW Winds Tonight Into Monday Morning At Higher Elevations (2700-3000+ Feet), Mixing Downward To Include Middle & Lower Elevations Monday

An increasing pressure gradient in advance of an arctic cold front and storm system will begin to develop strong winds in upper elevations Sunday night into Monday morning, with downward mixing across middle-lower elevations during Monday into Monday night.

Wind gusts of 40-50+ mph will be likely at upper elevations into Monday AM, with 25-40+ mph gusts becoming possible at middle to lower elevations along the Cumberland Mountains by later Monday into Monday evening.

Mountain waves developing with air flowing across the High Knob Massif were a signal of upcoming weather changes, along with high altitude, ice crystal clouds aloft.

Potential for snowband development continues to increase for Tuesday (December 10), with models and ensembles coming into agreement with development. The intensity and axis of heaviest snowfall will continue to be defined, with an ALERT for snow looking more likely.

Mathematics of this states that it is the upper-level circulation that drives the lower-level ageostrophic cross-isobaric flow, which functions to generate the convergence (pile up of air). The potential for a narrow but crippling axis of snow is certainly real, especially if the low-level convergence becomes enhanced by terrain and orographics.

The atmospheric setting is favorable for snowband formation beneath the right-rear entrance region of a strong (180+ knot) jet streak at 200-300 MB, where upward vertical motion will be generated with rising air to enhance low-level precipitation generation. The heaviest snow will develop where low-level convergence and upper level divergence phase.

Latest runs of both the NAM and GFS models have come into better agreement with the European Model and its ensemble cluster (who vary from 0″ to 8″ at the Wise gridpoint, with a mean of 3″ to 4″).

A sharp cut-off is likely to the northwest and southeast, with a narrow but concentrated and potentially crippling axis of snow somewhere in the middle. This axis is not yet defined and model shifts in placement and intensity are likely. Stay tuned for later updates.

Weather Headlines

*Frosty cold conditions will prevail in mountain valleys sheltered from gusty SW winds into Sunday morning. This will contrast with breezy to gusty winds across mid-upper elevation mountain ridges, where despite milder temperatures conditions will feel cold due to wind chills in the 20s and 30s.

*Increasing high altitude ice crystal clouds will turn sunshine hazy Sunday ahead of the next significant weather event.

*Strong S to SW winds will develop across the higher mountain ridges Sunday night into Monday morning as rain showers develop. Wind gusts of 30-50+ mph will be possible at the upper elevations. An Alert for strong winds will be likely for the higher terrain.

*Intervals of rain, and showers, are expected Monday into Tuesday in advance of a strong cold front. Local downpours will be possible, especially in upslope locations along the High Knob Landform on winds shifting to SSW-WSW.

*The potential for snowband formation is now increasing for late Tuesday into early Wednesday (Dec 10-11), with significant snow accumulations along the band. Placement of the band remains uncertain, but confidence is growing with locations along and west of the Cumberland-Allegheny Front currently most favored. Stay tuned for later updates.

Former Alert

ALERT For Lowering Cloud Bases With Dense Fog Development At Middle-Upper Elevations, And At Some Lower Elevations, Along & North Of The High Knob Massif And Tennessee Valley Divide From Midnight To 10 AM Saturday (*)

*Freezing fog, with riming, is expected at elevations above 3300 feet as temperatures drop to and below freezing in highest elevations by morning.

Low-level moisture combined with a wind shift to upsloping N-NE winds will cause cloud bases to drop in locations along and north of the High Knob Massif and Tennessee Valley Divide late Friday evening into the overnight hours of Saturday. Cloud bases are expected to drop to around the elevation of Wise (locally lower).

Update: 12:01 AM Saturday_7 December 2019

Clouds bases are in the process of dropping and reached high mountain ridges from Pine Mountain to Black Mountain into upper elevations of the High Knob Massif by midnight, with a continued drop expected through the overnight until bases reach around the elevation of the Town of Wise.

Overnight and early morning travelers will need to slow down and use caution.

Stormy Weather Pattern Ahead

My Preliminary Snowfall Forecast for this early December storm was right on target, and I should not have changed it, with the main error in the preliminary forecast being within downslope locations where some places did not have any measurable snowfall at all. At any rate, as my friend Addison Stallard used to say, that shows first impressions can sometimes be correct (mostly, at any rate!).

Following a major storm that opened December, a calm period is about to turn stormy once again with a train of waves across the Pacific Ocean poised to cross the North American continent next week.

Although timing may vary, right now the main periods are December 9-11 and December 14-15 for impacts from the first two major waves visible on the globe.

A cross-polar flow (above) will transport bitterly cold air into the northern Plains and Great Lakes by the middle of next week, with coldest air likely remaining north of the Ohio River, although, local conditions will be getting plenty cold as Arctic High pressure builds southward.

The baroclinic zone generated by the main thermal zone of temperature contrast will allow the storm track to be across this region, and that will bring another soaking rain event. As cold air plunges southward the potential for rain changing to snow is being monitored for Dec 10.

Updated: 3:00 AM Sunday_8 December 2019

The potential for snowband development is increasing, with a possible frontal wave and enhanced convergence.

Dynamically, a powerful 200-300 MB jet streak will be in position for lift into its right-rear quadrant, a climatologically favorable position for enhancement of upward vertical motion (rising air) and low-level convergence for a wave to form. This had been a notable feature in the modeling for a few days, although models were a little slow to catch on!

A jet streak moving over a strong baroclinic zone with a large thermal contrast is always a flag raiser even if models do not initially place any emphasis on it. That is where one must anticipate and understand how the atmosphere should respond to such a setting in advance. That is, the human brain is still the superior forecasting tool even with fancy model graphics cranked out by their phenomenal computing power.

The 18z (1 PM Saturday) run of the European Model and it’s ensembles were closer to the GFS, a little southeast of the NAM with the main snowband axis. Many more of the European ensembles were in agreement versus a lower number of the GFS ensembles.

The 00z run of the European Model continued to show a baroclinic snowband developing during December 10, with potentially moderate-heavy snow accumulations. Nearly all the ensemble members show some snow, with large variations in amounts. Confidence in the placement and intensity of this band should become more clear by later Sunday into early Monday.

The latest run of the GEM (Canadian) Model is farthest south with the snowband (below).

The building of Arctic High pressure into the Ohio Valley and Great Lakes by Thursday (12 December) will help to dictate impacts of the next major wave that will develop over the eastern USA, however, as of early December 8 the most potent winter weather maker has become the initial wave predicted to impact the mountain region December 10 into early on December 11.

This is getting way out into the medium range time period, so you know changes are going to occur, however, what has sparked my attention is the consistent development of a deep trough for days now. When models can “see” something that far in advance it is often a sign of a major storm event, especially if they are consistent in showing development.

The Bottom Line

A stormy weather pattern will redevelop next week as arctic air makes a moves into the United States, with two significant storm systems expected next week (and more in the pipeline upstream).

Given precipitation observed recently, rain amounts will initially need to be closely monitored for the potential of water issues.

At least two periods with important winter weather potential are being watched now, centered around December 10 and December 15.

Stay tuned for later updates.

Caution Is Advised for anyone traveling across the high country of the High Knob Massif through the remainder of today into early Wednesday, with freezing fog (riming) and slick conditions on Routes 237, 238, 619, 704 and many others (*).

*It should be noted that portions of some roads may be impassible, such as Route 237 in the Davenport Gap area between Little Mountain Knob and Big Cherry Lake, with chains and a high profile vehicle required.

Prolonged riming will finally end as drier air invades the high country into this evening, with clouds Tuesday being trapped beneath a low-level inversion.

Andrew Greear, superintendent of the City of Norton Water Plant, and veteran technician Joe Carter, measured 0.38″ of water content in this snow event (4.5″ total snowfall) with 4″ of snow on the ground at 9:00 AM today (3 December).

The City of Norton Water Plant is located at the base of High Knob, at approximately 2342 feet above sea level.

Nearly twice as much snow fell at the heavily rimed summit level, above 4000 feet, where ground depths varied dramatically in strong NW winds throughout Monday into Monday Night (with 0 degree WCFs).

A covering of new snow coated many secondary roads into Tuesday morning as upslope snow and snow showers continued, with Joe & Darlene Fields showing what conditions were like on State Route 706 heading toward Tacoma (below the riming).