Rain will change to light snow and snow showers into Tuesday afternoon and evening. Accumulations from a dusting up to 1″ will be possible below 2500-3000 feet, with 1″ to 2″ in upper elevations above 3000 feet (*).
*A burst of snow will be possible in upslope locations along-north of the High Knob Massif and Tennessee Valley Divide between sunset and midnight with the approach of a upper wave and the initial low-level 850 MB thermal minimum on NW-N upslope flow.
Slick conditions will develop on roads at upper elevations with persistent snow and snow showers.
The development of slick conditions will also be possible on middle-lower elevation roadways (especially secondary routes) after sunset within locations receiving more persistent snow showers (upslope locations) and a possible burst of heavier snow between sunset and midnight Tuesday.
Rime formation is expected at elevations above 3000 to 3300 feet, depending upon the exposure, during Tuesday afternoon into Tuesday night with orographic clouds obscuring the high country in sub-freezing air.
Temperatures Will Continue To Drop Into Wednesday Morning.
Air temperatures will drop into the 20s Tuesday night into Wednesday morning, with 10s at highest elevations. Wind chill factors will range in the 10s to near 20 degrees, except for single digits at upper elevations (locally below zero on highest peaks).
Caution Advised Around Whitewater Creeks Draining The High Knob Massif
High stream levels and swift water on creeks draining the High Knob Massif will continue through tonight into Wednesday, (18 December) following a general 1.50″ to 2.00″ of rainfall.
Former Alerts
ALERT For The Potential Of Flash Flooding And Flooding Late Monday Into Tuesday
A general 0.50″ to 1.00″ of rain fell overnight into Monday morning, from the High Knob Massif northward to Clintwood and Breaks Interstate Park, with embedded thunderstorms. Additional rain, with possible downpours, will become likely Monday night into the overnight-predawn hours of Tuesday.
*The latest HRRR (High Resolution Rapid Refresh) model is showing the potential for 1.00″ to 2.00″ rain amounts into Tuesday morning and is attempting to resolve the orographic component. Models struggle to handle both convection and orographics.
Atmospheric conditions over the mountains are more stable than locations to the west and southwest of the Appalachians, but there is a notable orographic component on moderate-strong SSW-WSW flow streaming from the Gulf of Mexico (sea level) upward into the Cumberland Overthrust Block and High Knob Landform.
Residents living or driving along streams or into low-lying, typically flood prone locations should remain alert for changing conditions through tonight into early Tuesday.
Water gushing out of the High Knob Massif in wake of recent rainfall and snow-rime melt is expected to continue through Tuesday, with potential for significant rises in water levels if additional downpours develop into Tuesday.
Strong SW Winds Monday Night Into Tuesday AM Will Shift NW With A Plunge In Temperatures And Wind Chills During Mid-Morning Into Tuesday PM
Update: 3:00 PM Monday_16 December 2019
Wind speeds will increase again Monday Night into Tuesday as a strong cold front approaches and pushes across the mountains. A wind shift from SW to NW is expected between 7-10 AM Tuesday, with a sharp temp drop through mid-morning into Tuesday afternoon.
Former Discussion & Updates
*ALERT For Strong S To SW Winds Developing Monday Into Tuesday AM
*Strong S-SW winds will develop late tonight into Monday morning as a low-level jet streak develops and surges up along the Appalachians with a strengthening pressure gradient in advance of a major storm system.
While wind gusts of 30-40 mph will become common, gusts of 50-60+ mph will be possible, especially at the highest elevations, during the predawn to post-sunrise period of Monday.
Sheltered valleys which decouple this evening from boundary layer flow will initially have calm conditions prior to recoupling with turbulence and mountain waves that will mix out the nocturnal inversion to generate rapid short-period rises in temperature (**).
Update: 3:00 PM Monday_16 December 2019
A 14 degree temperature rise occurred between 12:10 AM and 1:20 AM [19 degree (F) total change to 3:40 AM] as the nocturnal temperature inversion was dissipated by the onset of turbulent mixing and strong SW winds. Peak wind gusts reached 40 to 50 mph during the predawn in the Wise to Pound-Clintwood area.
**I have named this the Appalachian-chinook effect, where a large vertical temperature gradient develops between mountain valleys and exposed ridges and plateaus with development of strong nocturnal temperature inversions and decoupling of boundary layer flow. Turbulent flow then develops with rapid temperature rises over short-time periods as the inversion (typically nocturnal) is mixed out by turbulence.
Update: 3:00 PM Monday_16 December 2019
Stream levels are already elevated from recent rainfall and snow melt, with whitewater creeks draining the High Knob Massif gushing during Saturday (14 December).
A heavy to excessive rainfall potential will exist with the next major storm system, with southward trends in the placement of axes of heaviest rainfall on models during the past 24 to 48 hours.
Counties along the Virginia-Kentucky border are coming into a higher risk with this southward trend that will be complicated by orographically forced lift as well potential for convection to enhance rainfall amounts upon already wet surfaces.
Around 33% of the European Model ensemble group now includes VA-KY border counties in the band of heaviest rainfall amounts versus no members prior to 24-hours ago.
Stay tuned for updates.
Update: Christmas Bird Count 2019
Reference Early Winter_High Knob Massif for more scenes.
Around 50 bird species were identified during another year with bad weather conditions. A light mixture of sleet and snowflakes at the summit level of High Knob gave way to rime formation around mid-day (as the air temp dropped below freezing).
A burst of moderate-heavy, wind driven snow developed around 2:30 PM Saturday (14 December), and although accumulations were generally 0.5″ or less it added insult to already nasty conditions (orographic clouds and wind chills in the 10s).