An Alert For Local Snow Accumulation Continues Into Saturday Morning; However, Amounts Will Likely Be Much Less Than Earlier Indicated. Many Places May Have Little To No Accumulation.
A northward shift in the main snow band has developed on evening model runs, with a narrow area of heaviest impact stretching from portions of central Kentucky into southern sections of West Virginia ( along a LEX to BKW line ).
Locally, a dusting up to 2″ is now forecast with locally more at upper elevations within the high country of the High Knob Massif only if a predawn-morning corridor of moisture can develop as suggested by some short-range guidance ( otherwise, even at the highest elevations amounts will be 2″ or LESS ).
A dusting up to 2″ along and north of a Harlan, Ky., to Marion, Va., line with little to none to the south and a narrow band of heavy snow to the north across parts of eastern Kentucky into western West Virginia.
Target Snowfall of 1.0″ in Norton-Wise ( +/-) 1″ error potential, suggesting a dusting to 2″ will be possible with locally higher snow amounts within the High Knob Massif and only at highest portions of the Tennessee Valley Divide into Saturday ( March 10 ).
*A transition to rain may first occur north of the High Knob Massif and Tennessee Valley Divide where SSW-WSW winds downslope into northern Wise, Dickenson, and Buchanan counties, with the longest period of snow likely at upper elevations in the High Knob Massif and along portions of the Tennessee Valley Divide where SSW-WSW winds upslope ( the bulk of this initial event will be snow in the High Knob high country ).
European Model and GFS Model Ensemble Means suggest another period with possible significant snow will occur Sunday into Monday ( March 11-12 ). Stay tuned.
Thursday Night Into Friday Morning
Mostly cloudy with snow showers, flurries, and local squalls ending into morning. W- NW winds 5-15 mph, with higher gusts, on mountain ridges and plateaus below 2700 feet. WNW-NW winds 10-20 mph with higher gusts on mountain ridges above 2700 feet. Temperatures widespread in the 10s ( upper elevations ) and 20s ( low-middle elevations ). Wind chill factors in the 10s to around 20 degrees, except in single digits above and below zero on highest peaks.
Friday Morning Through The Afternoon
Thickening and lowering clouds ( from mid-high clouds in mid-day to low clouds by end of day ). Virga developing by late. WSW winds 5 to 15 mph, with higher gusts. Temps varying from 20s at upper elevations to the middle 30s to lower 40s. Wind chills mainly in the 10s & 20s along mid-upper elevation mountain ridges & exposed plateaus, coldest at highest elevations.
Friday Night Into Saturday Morning
Light snow & flurries. A period of heavier snow possible by the predawn-morning hours. Winds SW-WSW at 10 to 20 mph, with higher gusts, on upper elevation mountain ridges. Temps in the 20s to lower 30s, tending to rise toward morning. Wind chills in the 10s and 20s along higher mountain ridges.
Weather Discussion ( Active Pattern )
Friday Night Update
Although a dusting of new snow has occurred in upper elevations of the High Knob Massif Friday evening, the latest forecast model trends are for a much more narrow, mesoscale band of heavy snow to develop farther north than indicated by many previous runs.
*The 7:00 AM Friday run of the high-resolution WRF model was showing enough moisture for 5-6″ of snow at upper elevations of the High Knob Massif, versus the 7:00 PM run which predicts only around 0.5″ or less. Reality may rest in between, but time will tell.
GOES-16 Low-Level Water Vapor Image At 11:15 PM on Friday – March 9, 2018
Even better than Doppler, at this point, the low-level water vapor imagery from GOES-16 shows the mesoscale banding nature across Kentucky that is creating havoc with models forecasting this event ( scroll to the USA for a view ).
A fast and nearly zonal flow across the region is making it more difficult for models to resolve the impacts of a wave embedded within the flow ( shortwave seen at 700-500 MB ) near the Mississippi River as of 11:00 PM Friday.
A batch of moisture over far southeastern Missouri and western Kentucky looks to ultimately be the area to watch into the predawn to mid-morning of Saturday for a possible period of moderate-heavy snow that may form southward of the main mesoscale ( narrow ) band…especially at the middle-upper elevations ( some short-range guidance is forecasting this to also miss the area ).
It is always frustrating when model guidance makes an abrupt change during the onset of an event, but it happens and the ultimate result will not be known until another 12-hours have passed. This illustrates that Mother Nature is in control and that settings like this will likely continue to be problematic to even the best models and forecasters.
Previous Discussion
A combination of bitterly cold air aloft and terrain generated a classic snow squall day in the mountains during Thursday.
High Knob Massif Webcam – University Of Virginia’s College At Wise
*UVA-Wise graduate student Layton Gardner measured 1″ on the ground during the morning at his home on the Wise Plateau.
Vertical cloud development in wake of overnight squalls was a signal of more to come during the day, with locally intense, hit or miss snow squalls.
High Knob Massif Webcam – University Of Virginia’s College At Wise
Melting and sublimation in between squalls = a very tough day to obtain an accurate measurement unless you were a dedicated and/or veteran snow observer.
Melting and sublimation in between the squalls held the ground accumulation well under the total snowfall.
High Knob Massif Webcam – University Of Virginia’s College At Wise
The more intense snow squalls were convective and typically signaled by darkening skies and a gush of wind, with fine graupel snow that gave way to larger, often huge, flakes unless wind and downdraft motion was so strong as to break up the big flakes into fragments. Visibility typically dropped to well under 0.25 mile.
Conditions were similar in Wise with huge snowflakes, like in Clintwood, at times, and melting in between with sinking air around these intense squalls ( air converges and rises in the squall and sinks around its periphery to generate cloud breaks ). This pattern, and a clear indication of bitterly cold air aloft, was documented well by GOES-16 visible imagery.
GOES-16 Visible Image At 1:17 PM on March 8, 2018
The array of cloud “blobs” visible on this image are the many snow squalls and heavier snow showers that were developing beneath very cold air aloft. Terrain helped to focus the most intense action along and west of the Cumberland-Allegheny Front.
NAM 3 KM Model Total Snowfall Forecast To 7 AM Saturday – March 10, 2018
Snow accumulations with warm air transport systems can be problematic, such that positioning of the main band and amounts will be tricky. Evaporative cooling + upslope flow into the High Knob Massif and Tennessee Valley Divide will favor some of the heavier amounts.
GFS Model Snowfall Forecast – Friday Night-Saturday Morning
An area of enhancement on the windward side of major terrain features, and lesser amounts on the down-wind side, is trying to be picked up by models to fit observed climatology of such events.
At least, through Friday morning, models are coming toward my forecast and not away from it so time will tell. Certainly some slick conditions are looking likely regardless of exact snowfall amounts into Saturday.
