Weather Headlines
*Cold and blustery, wintry air will be felt Thursday as skies become mostly sunny during the afternoon with a surge of increasingly dry low-level air.
A notable northwest to southeast temperature gradient will be present, with much colder conditions along and west to northwest of the Cumberland-Allegheny Front.
Widespread Dense Fog At Upper Elevations Into Early Thursday
Former ALERT
ALERT For Areas of Dense Fog
Due to the amount of fog developing at lower elevations I have changed the alert to be more general through tonight into Wednesday. Persistent low clouds in orographic upslope flow will continue to produce dense fog at high elevations into early hours of Thursday.
*Rain with a chance of thunderstorms spreads back across the mountain area Tuesday into Wednesday-Thursday (January 14-16).
Heaviest rain totals are expected to extend from the Cumberland Mountains south across the Tennessee Valley and southern Appalachians. Those living and driving along streams and in low-lying, flood prone locations will need to be alert for ponding and water level rises given saturated ground and recent rains.
*Colder air pushes into the mountains Thursday into Friday morning. The coldest air will be in lower levels of the atmosphere with an inversion aloft. The coldest low level air will be felt in locations along and north of the Tennessee Valley Divide and High Knob Massif.
Short-Term Forecast
Overnight Into Monday Morning
High clouds. Large vertical temperature spread between sheltered mountain valleys and gusty middle to upper elevation mountain ridges. Winds WSW-WNW 5-20 mph, with higher gusts, on mid-upper elevation ridges. MIN temperatures varying from 28 to 34 degrees in colder valleys to the mid 40s to low 50s. Wind chills in the 30s along upper elevation mountain ridges.
Mountain valleys decoupled from boundary layer winds, especially at lower-middle elevations, will be much colder than exposed mid-upper elevation ridges which will hold in the middle 40s to lower 50s.
Monday Afternoon
High clouds. Unseasonably mild with temperatures varying from upper 40s to the middle-upper 50s. Winds SE to S at generally less than 10 mph below 2700 feet. Winds S-SW at 5-15 mph, with higher gusts, at upper elevations.
Monday Night Into Tuesday Morning
Increasing and lowering clouds with rain developing toward morning. A chance of thunderstorms. Windy across mountain ridges and exposed plateaus. Winds SSE-SSW at 5-15 mph and gusty below 2500 feet. SSW-SW winds 15-25 mph, with higher gusts, on mountain ridges above 2700-3000 feet. Unseasonably mild with temps in the upper 40s to middle 50s, except colder in sheltered valleys into the evening prior to rises. Cloud bases lowering to obscure high mountain ridges into Tuesday morning.
Isentropic upglide with increasing warm air advection will increase the coverage of rain, with the chance of elevated convection, across the southern Appalachians and Tennessee Valley into Tuesday morning. Temperatures in sheltered mountain valleys will be able to decouple initially from PBL flow, with drops into Monday evening prior to rising temperatures into the overnight with enhanced mixing and development of showers and possible elevated thunderstorms with downward momentum transfer.
Update_Tuesday Afternoon
Periods of light rain and drizzle. SW-W winds 5-15 mph, with higher gusts along mid-upper elevation mountain ridges, becoming NW-N. Dense fog (lowering cloud bases) widespread at mid-upper elevations along and north of the High Knob Massif-Tennessee Valley Divide (areas of fog at low-middle elevations elsewhere). Temperatures mainly in the upper 40s to upper 50s.
Tuesday Night Into Wednesday Morning
Low clouds. Dense fog at upper elevations, with cloud bases on middle elevations along and northward of the High Knob Massif-Tennessee Valley Divide. A chance of showers, with thunder possible into morning. Variable winds at 5-15 mph with higher gusts on mountain ridges. Temps in the 40s to lower 50s (coolest high elevations).
Wednesday Afternoon
A chance of showers and thunderstorms. SW winds 10-20 mph, with higher gusts, especially along middle to upper elevation mountain ridges-plateaus. Temps varying from the lower 50s (upper elevations) to the lower 60s, except locally warmer with downsloping into northern portions of Wise, Dickenson and Buchanan counties. Dense fog continuing at upper elevations in the High Knob Massif with orographic upslope flow (temps around 50 degrees along highest ridges with wind chills in the lower 40s).
The chance for showers, with possible local thunder, will increase into Wednesday evening and the early overnight hours of Thursday along and ahead of a cold frontal boundary that marks the beginning of cold air advection.
Wednesday Night Into Thursday Morning
Showers likely with a chance of thunderstorms, then turning colder with a chance of scattered light showers-drizzle. Dense fog at upper elevations becoming freezing fog along highest ridges by morning. Wind shifting NW at 8-18 mph with higher gusts. Sunrise temps dropping into the 30s (20s at upper elevations above 3500 feet) by mid-morning to mid-day (milder within downslope locations in the Clinch, Powell, Holston valleys).
Cold air advection will be ongoing into Thursday morning and MIN temperatures are currently expected to occur through mid-morning, with readings holding near steady (rising slightly or falling slightly) during Thursday afternoon (20s at upper elevations and 30s to around 40 degrees at lower-middle elevations along and northward of the High Knob Massif-Tennessee Valley Divide). Gusty NW winds will make conditions feel significantly colder (wintry air), especially on mountain ridges & exposed plateaus.
Low clouds may linger in NW flow upslope locations through mid-morning prior to being overwhelmed (evaporated) by increasing dry air advection, thus I have now worded my updated Thursday afternoon forecast (made Wednesday afternoon) as “becoming” mostly sunny as a wintry cold air mass is felt.
Thursday Afternoon
Becoming mostly sunny & colder. Temperatures near steady (rising slightly then falling) in the upper 30s to lower 40s in locations along and north of the High Knob Massif and Tennessee Valley Divide (20s to lower 30s at highest elevations). Milder in valleys with downsloping into the Clinch, Powell, Holston river basins. NW winds 8-18 mph with higher gusts. Wind chills in the upper 20s and 30s, except 10s to mid 20s at upper elevations.
Cold air transport will continue near the surface into early hours of Friday as warm air advection begins aloft (increasing high clouds), and on highest mountain ridges into sunrise. Turbulent mixing will oppose decoupling, but frictional terrain drag below an increasing inversion within the 850-775 MB layer should allow many valleys to decouple with developing drainage flows.
Increasing high clouds aloft will hinder OLR into the predawn-sunrise period, such that cooling conditions will not be ideal despite the advected presence of very dry (very low dewpoint) air within cold air drainage source locations of the higher mountain terrain during Thursday night into Friday AM.
Thursday Night Into Friday Morning
Mostly clear early with increasing high clouds. NW-NE winds 5-15 mph, with some higher gusts, on mid-upper elevation mountain ridges. Temperatures widespread in the 20s, except 10s possible in colder mountain valleys and on coldest mountain ridges. Wind chills in the 10s to lower 20s on mountain ridges and exposed plateaus, except single digits in gusts on coldest ridges.
**An alert for High Winds will be likely for Saturday (January 18)