Mountain Area Forecast ( Jan 31-Feb 2 )
ALERT For Significant Snow Accumulations During Thursday Into Friday – Bitterly Cold Air And Wind Chill Factors Are Expected To Make This A Significant Impact Event In The Mountains
*Many school closings and schedule changes are expected for Friday ( Hey Teachers!!! ).
Significant snowfall is expected during Thursday Night into Friday Morning with a change to snow at all elevations in the 7:00 PM to Midnight period of Thursday evening.
The Potential For A Period Of Snow On SW Upslope Flow Exists For The Locations Indicated Above During Thursday Afternoon Prior To A Change To Snow At All Elevations And Sites Into Thursday Evening
The potential for SW Flow Upslope Snow is being monitored for Thursday in locations along and to the southwest of the orographic lifting zone generated by the High Knob Massif and Tennessee Valley Divide.
Main aspects that could prevent formation of this unique feature of Appalachian Climatology are development of a cross-contour ageostrophic flow that cuts across high terrain to force sinking air over valleys, instead rising, and stronger warm air advection above 850 MB than models indicate.
Locations inside the RED above will be most favored where low-level air will be forced to rise to highest levels, with areas in the black in the potential zone where air rises 1000 vertical feet or more upon valley floors between the Virginia-Tennessee border and the City of Norton.
Wednesday Night Into Thursday Morning
Mid-high clouds. Windy. SSW-SW winds 10-20 mph, with higher gusts, at elevations below 2700 feet. SSW-SW winds 15-30 mph, with higher gusts, along mountain ridges above 2700 feet. Temps varying from 20s in sheltered valleys to the mid-upper 30s. Wind chills in the 20s and 30s, except locally below 20 degrees in stronger gusts on high peaks.
Thursday Morning Through The Afternoon
Increasing & lowering cloud bases with rain, mix, and snow developing. The chance of snow-mix more likely along and southwest of the High Knob Massif-Tennessee Valley Divide, with mix or rain northeast of this corridor. SW-WSW winds 5-15 mph, with higher gusts, at elevations below 3000 feet. SW-W winds at 10-20 mph, with higher gusts, at elevations above 3000 feet. Temperatures in the upper 20s to low-mid 30s, or falling to these levels, except middle to upper 30s in downslope locations ( coldest at highest elevations ). Wind chills in the 20s to lower 30s, except in the 10s at the high elevations in the High Knob Massif.
Thursday Night Into Friday Morning
Rain or a mixture changing to snow. Snow heavy at times. Turning bitterly cold. SW to W winds shifting NW at 10-20 mph with higher gusts. Temperatures plunging into the 10s by morning, with single digits at highest elevations. Wind chills dropping into the single digits above and below zero, except to under -10 degrees F at highest elevations. Rime formation at elevations above 3000 feet.
Preliminary Snowfall Forecast ( Into Friday AM )
2″ to 4″ at elevations below 2500 feet
4″ to 8″ at elevations above 2500 feet
Target Snowfall of 4″ for Norton-Wise ( +/- ) 1″ Error Potential. This suggests a 3″ to 5″ snowfall potential for the Norton-Wise area, with higher amounts likely across the sprawling high country ( above 3000 feet ) in the High Knob Massif.
A potentially more significant winter storm will impact the mountain region this weekend into early next week.
Weather Discussion ( Interesting )
A very interesting, to say the least, weather pattern is now taking shape. Although the next wave arriving by later this weekend will be much stronger, the disturbance impacting the mountain area Thursday into Friday has potential and will have a notable impact even if the rain-snow change does not occur until after sunset Thursday.
From a research perspective the Thursday system will be very interesting to me and has potential to develop snow on a SW upslope flow. Although I have documented these SW upslope snowfall events back into the 1970s, I have a composite chart displaying 925 MB flow during a few “younger” events.
If SW upslope flow snow occurs it will only be for part of this event, with the main snow period being Thursday night into Friday morning. That is generally the case, with only a few events in the past being almost completely dominated by SW upslope flow snowfall. At the least, this type of flow will keep temperatures colder and make for a faster change to snow in the SW upslope flow corridor versus locations under downslope flow on SW winds.
Recent terrain model forecast trajectories are nearly identical, as noted below, but translating that to reality requires the model to be accurate with both flow trajectories and the magnitude of warm air advection around and above 850 MB.
Looking at new 00z data on Wednesday I think that this is trending toward mostly an all snow event for locations in upper elevations of the High Knob Massif, with snow levels during Thursday afternoon prior to the widespread change into all snow ( at all locations ) being the only real question. Places that get snow during the SW flow period will pick up general 1″-3″ amounts; therefore, I have factored that into my preliminary snowfall forecast for upper elevations.
If snow levels should drop to the floor of Powell Valley prior to the main, widespread change to snow then final snowfall amounts in such locations will tend to be near the top, or above, my current forecast ranges.
Meanwhile, I introduce a new weather station courtesy of one of my field research students, Layton Gardner, who graduated with honors from UVA-Wise in Spring 2017.
Layton is currently assisting with the High Knob Project as he prepares to later attend graduate school.
Layton’s station is less than 2 air miles from Lonesome Pine Airport and is about 100 feet higher in elevation. It should not be surprising, of course, that the air temperature runs lower ( I have only talked about this now for YEARS ).
At a slightly higher elevation than the Airport cold air drainage can certainly not be blamed, and since Layton’s station has a small solar radiation shield I highlight current data when incoming and any reflected insolation is not a factor ( on a well mixed night ).
A 4 degree F air temperature difference at current recording times ( note 39 degrees at 8:14 PM versus 43 degrees at 8:15 PM ).