Mountain Area Forecast ( Jan 11-13 )

Alert For Significant Frozen Precipitation And The Development Of Extremely Bad Road Conditions By Friday Evening With A Sharp Temperature Plunge Expected Initially Along And North To West Of The High Knob Massif & Tennessee Valley Divide During The 6:00 PM To Midnight Period Friday ( Including Far Western Lee County ) – Followed By Continued Drops And Very Low Wind Chills Into Saturday Morning

A wind shift to a northerly upslope direction, with influx of cold air, is currently timed for late Friday into Friday evening.  Due to initially shallow cold air advection a transition from rainfall to freezing rain will become possible first at mid-upper elevations within the lifting zone along and north of the High Knob Massif and Tennessee Valley Divide and Pine Mountain.  As the cold air deepens vertically a transition into sleet and snow will occur across the area.  Bitterly cold air and dangerous wind chills are expected to develop as temperatures drop into the single digits and teens at mid-upper elevations into Saturday morning.

Bitterly cold conditions will continue through Saturday with daytime temperatures expected to hold in the 10s to around 20 degrees at lower-middle elevations, and in the single digits to around 10 degrees at upper elevations, in locations along and north of the High Knob Massif & Tennessee Valley Divide.

ALERT For Ponding Of Water Along Roads And Strong Rises On Creeks With Upslope Rains Within The High Knob Massif-Black Mountain Corridor Into Friday In Advance Of The Temperature Plunge

As of 1:00 AM Friday more than 2.00″ of rain had fallen within upslope locations of the High Knob Massif and Black Mountain corridor of far southwestern Virginia.  Due to partially frozen ground, especially across northern slopes, run-off on creeks will need to be closely monitored in watersheds of the Cumberland, Clinch, and Powell basins.

Lingering ice along some creeks could also be problematic.  Folks living and driving along streams should remain on alert as periods of heavy rain continue to fall through Friday afternoon.

**A break in rainfall will be possible Friday morning before new rains, heavy at times, redevelop in advance of a strong cold front during Friday afternoon.  Remain alert for strong rises on streams.

Overnight Into Thursday Morning

Mostly cloudy ( mid to high clouds ).  Large vertical temp spread between sheltered valleys & gusty mountain ridges and exposed plateaus.  SE-S winds 5-15 mph, with higher gusts, along mountain ridges and plateaus below 2700 feet.  SSE-S winds 15-30 mph, with higher gusts, along mountain ridges above 2700 feet.  Temperatures varying from around 30 degrees in colder, sheltered valleys to the mid 40s to low 50s on windy mountain ridges.  Rising valley temperatures overnight into morning in most locations.

Mid-Morning Thursday Through The Afternoon

Mostly cloudy with rain & showers developing into the afternoon.  Windy.  SE-S winds 10-30 mph, with higher gusts.  Temperatures varying from the lower 50s to lower 60s ( coolest at highest elevations ).

Thursday Night Into Friday Morning

 Rain likely with a chance of thunder.  Downpours possible, especially in upslope locations within the High Knob Massif-Black Mountain corridor.  SSE-S winds 5-15 mph with higher gusts along mountain ridges-plateaus below 2700 feet.  SSE to SSW winds 15-25 mph, with higher gusts, along mountain ridges above 2700 feet.  Low cloud bases with widespread dense fog at upper elevations.  Areas of fog at middle to lower elevations.

Friday Afternoon

Periods of rain.  Chance of thunder.  Downpours likely.  Winds shifting SW to NW at 10-20 mph, with higher gusts.  Sharply dropping temperatures toward sunset into early evening.  Temps in the upper 40s ( highest elevations ) to upper 50s prior to falling into the 30s shortly after sunset.  Widespread fog at upper elevations with orographic clouds, then dropping cloud bases into middle elevations behind the frontal passage; otherwise, local areas of fog.

Friday Night Into Saturday Morning

Rain changing to freezing rain, sleet, and snow.  Snow heavy at times after midnight.  Turning bitterly cold.  Winds NW-N 10-25 mph, with higher gusts.  Temperatures plunging into the 10s to around 20 degrees by morning, with single digits to 10 degrees at highest elevations, along and north of the High Knob Massif-Tennessee Valley Divide.  Milder amid downslope areas of the Clinch, Powell, Holston valleys ( 20s to around 30 degrees ).  Wind chills falling through the 20s into the 10s and single digits, except to below zero at upper elevations.  Riming upper elevations.

Saturday Morning Through The Afternoon

Mostly cloudy.  Bitter.  Snow showers & flurries.  NW-N winds 10-20 mph, with higher gusts.  Temperatures near steady in the 10s to around 20 degrees across lower-middle elevations, along-north of the High Knob Massif-Tennessee Valley Divide, and in the single digits to around 10 degrees across upper elevations.  Riming at upper elevations. Wind chills in the single digits above and below zero, except -10 degrees below zero or colder at highest elevations.

A prolonged period of winter cold, with additional snowfall chances, is being monitored through next week ( Jan 14-20 ).

 

Weather Discussion ( Huge Change )

Thursday Evening Update

My previous forecast holds with this update, with new additions being to highlight persistent rains that have dropped locally more than 1.50″ in the past few hours within portions of Wise & Harlan counties.

*More than 2.00″ of rainfall as of 1:00 AM Friday.

The latest run of the NAM terrain model has done a nice job of picking up this band, being forced by a combination of low-level lift and upper-level vertical motion into the right entrance region of a high altitude jet streak, as well as the enhancement with rise along the Blue Ridge in the extreme southern portion of Virginia and western North Carolina.

When a model initializes current conditions well then this helps to improve the odds that its short-term outlook will be on target, thus the thinking does not change that very bad conditions will become likely Friday evening into the morning hours of Saturday as a major temperature drop occurs with influx of bitter air into locations along and westward of the Cumberland-Allegheny Front.

Temperatures will begin to initially plunge first in the far western portion of Lee County, and along the Cumberland Plateau in Tennessee, as winds have more of a westerly flow component during Friday afternoon.

During Friday evening, as winds develop more of a NW-N flow component, the strongest temperature drops are then expected to become concentrated along and north to west of the High Knob Massif-Tennessee Valley Divide with a period of low-level terrain blocking likely acting to increase temp differences between Norton-Wise and the Tri-Cities more than the high resolution NAM 3 KM Model is suggesting.

Temperature drops will locally be stronger Friday evening in communities such as Norton-Wise, Clintwood and Pound than in Big Stone Gap, Gate City, Fort Blackmore and Dungannon as the flow works to accentuate terrain influences.

The Bottom Line…Conditions Are Expected To Become Bad Friday Evening Into Saturday Morning As A Major Temperature Plunge Interacts With Moisture.  Places Along And West Of The Cumberland-Allegheny Front Are Likely To Experience The Worst Conditions.

Towering cliffs along the Cumberland Front, the White Rocks of present day Cumberland Gap National Historical Park, were first described by early frontiersmen such as Daniel Boone.  The bulge of high country along this front associated with the High Knob Massif was a refuge for Robert ( Bob ) Benge, who terrorized the frontier of eastern Tennessee and southwestern Virginia during the long and bloody period of 1774-1794.

Bob Benge was a  Chickamauga Cherokee war chief, the mixed-blood son of a Cherokee woman and a Scots-Irish trader named John Benge ( reference Wikipedia ), with places such as Benges Basin and the Chief Benge Scout Trail in the High Knob Massif named after him today. 

Hazardous Roads Could Develop Across The Region Due To Such A Sharp Temperature Drop, Even In Locales Which Receive Little Frozen Precipitation.

 

Previous Discussion

A huge weather change back into winter is approaching the mountains as unseasonably mild conditions into Friday give way to a sharp temperature plunge Friday Night into early Saturday.  Precipitation types during the Friday and Friday Night period could run the gamut from wind driven rain, spiced with thunder, to freezing rain, sleet, and snow.

Temperatures currently are forecast to drop below freezing beneath the 850 MB level between 7:00 PM and 10:00 PM Friday evening, as an above freezing zone lingers aloft.  This vertical temperature profile would result in freezing rain and sleet.

All eyes in the forecast community should be on soundings, with the next few model runs being critical as to helping better determine how this period will unfold.

The positions of developing surface and 850 MB low pressure centers will be important, with a somewhat farther east solution than currently shown by the NAM Model allowing cold air to advect into the western front range of the Appalachians a little quicker.  A farther west solution would tend to delay the cold air.  Climatology in this type of digging upper trough setting suggests that a leeward low track ( east of the Eastern Continent Divide ) tends to be favored.

While the current NAM Model is in basic alignment with the latest European Model and its mean Ensembles & Control in showing a pronounced orographic signature, with heaviest snowfall along and upstream of the western front range of the Cumberland-Allegheny Front, there will likely be some changes in the placement of this during the next 24-48 hours.  Stay tuned for important updates.

Bitterly cold air flooding into the mountains will be a most notable part of this event.

Although changes may occur in this forecast, the drop in 850 MB temperatures on this type of air flow agrees with past climatology of observed events.

The development of the coldest 850 MB temperatures in the time between 7:00 PM Friday ( above ) and 1:00 AM Saturday ( below ) aligns with the strongest low-level orographically forced lifting along and north of the High Knob Massif and Tennessee Valley Divide on northerly air flow.  So while the complete three-dimensional nature of this complex terrain tends to not be fully resolved, one has to think that this run of the model is picking up on part of the orographics.  This aspect, along with general alignment with the ECMWF, is the main reason I chose this model to highlight how the Friday evening period will unfold ( remembering this is a setting that may, and will likely, change in some ways ).

I had highlighted this system for a long time as one to watch, and as it gets closer the reasons are becoming more and more distinct with a huge temperature gradient acting to drive imbalances within the thermal energy, mass and momentum of the atmosphere.  Quasi-geostrophic theory says that the atmosphere will thus respond by changing the vorticity structure of its mass field ( see map below ) and by changing the temperature of air parcels by adiabatic rising and sinking motions as demanded by thermal wind balance.

By 10 PM Friday Night the 500 MB trough ( above ) has taken on a negative tilt, with cyclonic vorticity advection as well as mass divergence ( not shown ) contributing to upward motion aloft as orographics lift the lower atmosphere.

A 30 to 40 degree temperature plunge is expected to occur between 4:00 PM Friday and 1:00 AM Saturday, with more declines through sunrise on Saturday.

These are surface temperature forecasts and do not reflect colder conditions at higher elevations nor the wind chills that will really make this a notable event.

This will mark the beginning of another prolonged period of cold conditions, and if significant snow accumulates it may be able to generate readings significantly lower than are being currently predicted into next week.

Meanwhile, lets not forget about the strong winds blowing across the high country which will tend to mix downward more into middle-lower elevations during Thursday.