120817 Forecast

Mountain Area Forecast ( Dec 8-10 )

ALERT For Accumulating Snow Friday Into Friday Night – Especially Along & South of the High Knob Massif and Tennessee Valley Divide

An increasing gradient of snowfall is likely from northwest to southeast across the area, with limited snow along and north to northwest of a Pennington Gap to Clintwood line where little snow is expected to accumulate through Friday Night ( less than 1″ ).
A zone along the Tennessee Valley Divide, including Norton-Wise, Banner Mountain, Sandy Ridge, and adjacent communities could see 1″ or more of snow, with heavier snow more likely from the High Knob Massif south & southeast across the Tennessee Valley toward the Blue Ridge ( general 1″ to 4″ ).  A rare setting where more snow may fall in the Tri-Cities than in Norton-Wise. 
The heaviest snow, with 4-8″+, will be likely along and east of the Blue Ridge and Tennessee-North Carolina border.

Only a slight shift in the track of this system could change the placement of these amounts, but the general increase in snowfall from northwest to southeast will hold.

Friday Morning Through This Afternoon

Cloudy with morning flurries giving way to developing snow, especially along and south of the High Knob Massif and Tennessee Valley Divide.  Light winds.  Temperatures varying from upper 10s to lower 20s highest elevations to the upper 20s to lower 30s.

Friday Night Into Saturday Morning

Cold with periods of snow, especially along and southeast to east of the Virginia-Kentucky stateline into the overnight before tapering to flurries.  Light winds becoming W-NW.  Temperatures varying from low-mid 10s to low-mid 20s, coldest at highest elevations.  Wind chills in single digits within highest elevations.

My Mesoscale Discussion

Moisture From A Southern System And Front Over The Southeastern USA Will Transport Moisture Into Cold Air In Place Over The Southern Appalachians During Friday Into Early Saturday AM With Widespread Snow Accumulations.  A Sharp Cut-off Will Be Likely Along The Western & Northwestern Side Of This System.

This is an evolving situation with only a small shift in the track of the main moisture band having large implications on the snowfall potential across the Mountain Empire.  As of late Thursday Night a small but notable westward shift was occurring in models, and I expect this could continue due to the formation of the Right Rear Quadrant Entrance Region Of A 250-300 MB Jet Streak.  Upper air divergence beneath this region could allow snowfall to build west to the High Knob Massif & Tennessee Valley Divide and will need to be closely monitored during the next 12-24 hours.

GFS Model 300 MB Forecast At 7:00 PM Friday – December 8, 2017

The GFS Model is farther west than the 00z European with the core of this developing Jet Streak, and the NAM 12 KM Model is also a little farther west than the European despite having a moisture field farther east.  Thus, given this is a developing Jet Streak, there is significant potential for a continued westward shift in accumulating snowfall across the Tennessee Valley toward the Virginia-Kentucky stateline during Friday into Friday Night.

**Cross-isobaric ageostrophic circulation developing within the right-rear entrance region of the jet streak will be the synoptic mechanism capable of forcing a westward shift; albeit small, in the snowband extent.  This often has some natural interaction with the three-dimensional terrain.  Past climatology of similar systems dictates that locations along and southeast of the VA-KY border are typically at highest risk for expansion beyond what most models tend to indicate.
In this case, only time will tell exactly where the western edge of 1-3″+ of sticking snow sets up Friday into Friday Night.  New runs of models overnight into Friday will be important to monitor.
NAM 12 KM Model 300 MB Forecast At 2:00 AM Saturday – December 9, 2017

The coldest air is along the west side of the Appalachians such that any moisture reaching this area will have a lower snow density ( higher snow to water ratio ) than locations farther east and southeast.  Drier air is also over this area, thus the battle will be on between lift and drier air.  If the synoptic-scale lift becomes strong enough it will allow the snowband to build west more than models currently show, while if the synoptic-scale lift is weaker then the snowband will remain farther east.

Odds for 3-6″ of snow are high for locations along and east of the Tennessee-North Carolina border, with 1-3″ toward the west.  At this time the westward extent remains in play and could cover the Tri-Cities, and could reach as far west-northwest as the Virginia-Kentucky line.

NAM 12 KM Model 500 MB Vorticity Forecast At 7:00 AM Saturday

A second system will drop into the developing upper air trough and phase with southern energy to form a large storm for the East Coast.  Note how close it is to phasing over the southern Appalachians.

NAM 12 KM Model 500 MB Vorticity Forecast At 7:00 PM Saturday

A tap of some Great Lake moisture will combine with this to generate snow showers, flurries, and possibly a burst or two of snow Saturday Night into Sunday AM, with best accumulations expected along the front range of the Cumberland-Allegheny mountains.

A general 1″ to 2″ of dry snow will be possible along and north to northwest of the High Knob Massif & Tennessee Valley Divide during Saturday Night into Sunday AM.  The main unknown, will this be on top of any prior snow from the southern system?