An ALERT For Cold Night-time Conditions In Mountain Valleys Continues, With Large Vertical Temperature Spreads Between Ridges & Valleys
While foggy river valleys and exposed mountain ridges-plateaus will remain well above freezing, a very dry air mass will continue to support both large diurnal temperature spreads and also large vertical differences between ridges and mountain valleys.
Colder valleys will continue to drop into the 30s, with local below freezing conditions expected through mid-week ( Wednesday AM of October 4 ). Reference observed minimums reported along the Appalachians on Sunday AM of October 1 in my discussion below.
Overnight Into Monday Morning
Partly to mostly clear ( high clouds ). Breezy to gusty SE winds of 10-20 mph, with higher gusts, through early to mid-overnight on mid-upper elevation mountain ridges before decreasing during the predawn. Large vertical temp spread between colder valleys and milder ridges-exposed plateaus. Temps varying from 30s in colder valleys to the mid-upper 40s to around 50 degrees on exposed ridges and plateaus ( except for a rapid temperature drop to around or below freezing in high valleys during the predawn-early morning ). Local areas of dense river valley fog along major rivers. Wind chills in the upper 30s to lower 40s in gusts along high mountain crest lines.
Monday Afternoon
Partly sunny ( high clouds ). Light SE to SSE winds. Temperatures varying from low-mid 60s in upper elevations to the low-mid 70s.
Monday Night Into Tuesday Morning
Partly-mostly clear ( high clouds ). Light winds, except SSE-SSW at 5-10 mph on mid-upper elevation mountain ridges. Large vertical temperature spread between colder valleys and milder mountain ridges. Temperatures varying from upper 20s to upper 30s in mountain valleys to the upper 40s to lower 50s along mountain ridges-exposed plateaus. Fog along major river valleys.
Tuesday Afternoon
Mostly sunny ( some high clouds possible ). Generally light winds. Temperatures varying from low-mid 60s in upper elevations to the lower-middle 70s.
Tuesday Night Into Wednesday Morning
Mostly clear. Light winds, except SSE-SSW at 5-10 mph, with some higher gusts, on mid-upper elevation mountain ridges. Large vertical temperature spread between colder mountain valleys and milder ridges. Temperatures varying from 20s to middle 30s in colder valleys to the upper 40s to lower 50s on exposed ridges-plateaus. Areas of fog along major rivers.
Weather Discussion ( WxDiversity )
Autumn has arrived in the mountains. This combined with a recent streak of dryness will enhance diversity in weather conditions, both from day-to-night and also ( as typical of this time of year in the mountains ) between ridges-valleys.
Rugged Stone-Pickem Mountain of High Knob Massif – September 30, 2017
The following temperatures are mostly NWS Cooperative sites, with a few Weatherbugs added ( note Weatherbug sensors are typically mounted much higher than the standard 4-6 feet height of official thermometers ).
Western North Carolina Jefferson: 31 degrees Boone: 35 degrees Spruce Pine 4 ENE: 39 degrees
West Virginia Bartow: 29 degrees Hacker Valley: 31 degrees Frost 3 ENE: 32 degrees White Sulphur Springs: 32 degrees Lewisburg: 33 degrees Beckley: 34 degrees
Areas of valley frost were widespread across Wise and Dickenson counties, away from river valley fog, early October 1 and no doubt were also in many other places located within the mountains. Coldest high valley sites, outside of Burkes Garden, of Canaan Valley and the Big Cherry Basin in the High Knob Massif are not even yet included in this list ( that data yet to be collected ).
Valley frost is typically observed when air temperatures at standard NWS sensor height of around 5 feet reach the middle 30s since air temperatures right at ground level are generally colder. Frost occurred in the Clintwood area, for example, with a MIN air temperature of 34.8 degrees at the 5 foot sensor height.
Brilliant Color In Flag Rock Recreation Area – September 30, 2017
Another interesting feature is wind chills along the high mountain crest lines. This is being caused by a compression of thicknesses by night + a pressure gradient around a High currently centered over Pennsylvania.
If anyone doubts that nocturnal wind chills are a factor, then just drive up to the High Knob Lookout and that doubt will be quickly eradicated. Winds relax into the day as thicknesses increase.
NAM 12 KM Model Mean Sea Level Pressure-Streamlines at 8 PM October 1
This type of setting is not ideal for coldest conditions in the higher mountain valleys; however, a relaxation during pre-dawn and sunrise periods is enough to allow for rapid temp drops in the favored frost pockets ( where temps often drop below surface dewpoints reported at any given time ).
Black Mountain Mesonet at 12:55 AM Monday – October 2, 2017
Wind chills in the upper 30s to lower 40s are being observed along high mountain crest lines, especially in the frequent gusts. This low-level jet setting weakened into morning hours of October 1 and will likely weaken again into this morning ( October 2 ).
If the High center slides a little more southwest, allowing the nocturnal pressure gradient to weaken, conditions will actually become more favorable for cooling within the high mountain valleys into Tuesday and Wednesday mornings. Therefore, I have continued the alert for cold nights since no official advisories have been issued.
European Model 51-Member Ensemble Mean 500 MB Height Anomalies: Days 0-4
While there is little to no chance of rain this week, there are signs of important changes ahead that could break this dry streak out in the 5-10 day period ( below ) as eastern USA ridging ( above ) breaks down.
European Model 51-Member Ensemble Mean 500 MB Height Anomalies: Days 6-10
Meanwhile, it will be wise to avoid any outdoor burning.
