My Forecast For Today ( January 29 )
ALERT For Snow Shower & Local Snow Squall Development Overnight Into This Morning
Visibility will be reduced in heavier snow showers and any snow squalls which develop. Roads in affected locations could quickly become slick & hazardous. Remain Alert.
*State Route 619 remains extremely icy. Chains are recommended if traveling across the High Knob Massif from the City of Norton to Fort Blackmore. Hazardous conditions remain on other routes.
Overnight Into Mid-Morning
Lowering cloud bases with flurries and snow showers developing overnight into morning. Snowstreaks and snow squalls possible. Winds SW to WNW-NW at 10-20 mph, with higher gusts on middle-upper elevation ridges and plateaus. Temperatures dropping into the upper 10s to upper 20s by sunrise to mid-morning. Wind chill factors in the single digits and teens ( coldest at highest elevations ).
Snowfall Forecast For Friday
Upslope Side of Mountains
0.5″ to 3.0″
Downslope Side of Mountains
Dusting to 0.5″
Target snowfall of 1.0″ at Wise, ( +/- ) 0.5″ Error Potential. This implies 0.5-1.5″ possible in the Norton-Wise area. Extremes of a dusting up to 2-3″ will be possible across the area, with greatest snow amounts expected along upslope side of mountains with respect to WNW-NW air flow trajectories ( i.e., least snow likely ESE-SE of the High Knob Massif and Tennessee Valley Divide ).
Any snow showers & squalls tapering to flurries & ending. Cloudy skies becoming mostly sunny. Cold. WNW winds 5-15 mph, with higher gusts. Temperatures varying from 20s to low 30s high elevations to the middle-upper 30s. Milder south into river valleys of the Clinch, Powell, Holston and Great Valley of the Tri-Cities. Wind chills 10s & 20s.
Tonight Into Saturday Morning
Becoming mostly clear to partly cloudy. A large vertical temperature spread developing between colder valleys and milder mountain ridges. WNW winds shifting S to SSW at 5-10 mph along mountain ridges-plateaus below 2700 feet. SW-WSW winds 10-20 mph, with higher gusts, along upper elevation mountain ridges ( especially by morning ). Temps dropping into the 10s to low 20s in colder valleys with calm winds verses rising readings through the 30s along mid-upper elevation ridges ( mainly above 2700-3000 feet ).
*Strong SW winds of 10-20 mph, with 30-40+ mph gusts, will become possible Saturday into Saturday Night across mid-upper elevation mountain ridges as warmer air is transported into the Cumberland Mountains ( conditions will feel much cooler than generally forecast, especially at higher elevations ).
Weather Discussion ( January 28-29 )
My Afternoon Update
This afternoon update documents my first missed snow forecast of this winter season ( it will remain above and be saved for later reference ). The only current change was to update afternoon temperatures since with more sun and less cloudiness they have trended a little higher.
Even though this was to be a very weak event, a dusting to 0.5″, at the very most, is all that occurred along the upslope side of the mountains.
I typically look at a dozen or more different parameters when forecasting NW Flow snowfall. However, of them all moisture is perhaps most critical since regardless of how good other factors are you can simply not get snow without sufficient moisture.
Moisture along and upstream of the high north-northwest facing slopes and crestlines is especially important in the generation of orographic snow on NW flow. At 10:00 AM above, as one point selected, the 87% RH value came in well below the 96% value predicted by the 7 AM run of the NAM model this morning ( the GFS Model being very similar ).
The air never did saturate completely with orographic pilatus being limited and of short-duration during the morning, which limited riming. Riming is a very important factor in enhancement of snowfall across upper elevations in the High Knob Massif where it adds to snow volume, in addition to its deposition and growth upon trees and other exposed objects ( as a secondary source of moisture that is visible until its drop…the rime enhancement of snowfall is visible only via close analysis and inspection ).
A major reason, of course, for the lesser amount of low-level moisture is that a calculation of backward air flow trajectories into Wise up to 7:00 AM reveal a miss with incoming air from a continental source region instead of the Great Lakes ( the above plot set for 500 meters above ground level with a 100 meter plot streaming in from mid-continent to an even greater extent ).
Vigorous morning snow showers moved through the Charleston area of West Virginia. From space the difference is small between Wise & Charleston; however, on the ground the difference this AM was large since air flow into Charleston crossed the Great Lakes upstream ( illustrating, as has been well known here for decades, how important the Great Lakes are to NW Flow ( upslope ) snow in absence of wrap-around and all other moisture sources ).
Moisture streams off the Great Lakes are notoriously narrow in nature along the Lakes, and can remain that way downstream. Hitting any given point from 400 air miles away then becomes a challenge if flow across the Lakes is not strong and vigorous and cyclonic in nature.