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.