ALERT For Dense Fog At Mid-Upper Elevations Along And North Of The High Knob Massif-Tennessee Valley Divide From Late Tuesday Afternoon Into Wednesday
*A prolonged period of dense fog ( low clouds ) will develop as colder air invades the front range of the mountains from late Tuesday into Wednesday on upsloping northerly winds. A cold, damp & raw feeling day is expected Wednesday when air temps will struggle in the 30s to lower 40s in locations along and north of the High Knob Massif & Tennessee Valley Divide, with milder conditions leeward of the mountains into river valleys of the Clinch, Holston and Great Valley.
Monday Night Into Tuesday Morning
Low clouds. Turning cooler during the evening, especially along and north of the High Knob Massif-Tennessee Valley Divide. Then rain developing overnight, with a chance of thunder. Downpours likely. N-NE winds at 5-10 mph, with some higher gusts, into the early overnight shifting SSE at 5-15 mph, with higher gusts, by morning. Temperatures falling into the upper 40s to low-mid 50s.
Tuesday Morning Through The Afternoon
Rain, heavy at times, through the morning. Thunder possible. Rain tapering to showers into the afternoon. Winds SSE-SSW shifting to NW-N by mid-late afternoon below 3200 feet at generally less than 10 mph. SSE winds shifting W-WNW at 5-15 mph, with higher gusts, at upper elevations above 3200 feet. Temperatures in the 50s to around 60 degrees, turning chilly during mid-late afternoon ( milder south into the Great Valley ).
Tuesday Night Into Wednesday Morning
Low clouds. Light showers or drizzle. Turning colder. Another period of dense fog likely at mid-upper elevations along and north of the High Knob Massif-Tennessee Valley Divide. NNW-NNE winds at 5-10 mph, with some higher gusts. Temps falling into the 30s to around 40 degrees. Wind chills dropping into the 20s to lower 30s at the highest elevations.
Wednesday Morning Into The Afternoon
Low cloud bases with areas of dense fog at mid-upper elevations along & north of the High Knob Massif and Tennessee Valley Divide. Cold & damp. Drizzle possible. Winds N-NE at generally less than 10 mph. Temperatures varying from the lower 30s to lower 40s ( coldest at the highest elevations ). Some breaks in the overcast possible leeward of the High Knob Massif-Black Mountain. Wind chills in the 20s to around 30 degrees at high elevations.
Weather Discussion ( Wet & Colder )
A mild, wet beginning to November will transition into a wet and cold pattern into mid-week. Locally heavy rain amounts will be possible Tuesday during this transition.
Folks living along and north of the High Knob high country and adjacent Tennessee Valley Divide need to get use to this view as low cloud bases become a persistent feature by late Tuesday into Wednesday, on upsloping northerly air flow.
The interaction of abundant low-level moisture with colder air will combine with lifting on upslope flow to generate a widespread low cloud deck into Tuesday Night-Wednesday, especially in locations ( of course ) on the rising air side of the Cumberland-Allegheny Front range.
Forecast models have been struggling to pin down where heavier rains will fall through Tuesday, but amounts of more than 2.00″ ( when including Monday ) are certainly possible in some locations ( around 1.30″ fell on Eagle Knob of the High Knob Massif Monday ), especially in those zones which have been ( and are being now ) pre-conditioned by condensation on rising air ( e.g., the lifting zone of the High Knob Massif-Black Mountain corridor to include the City of Norton & Towns of Wise, Appalachia, Big Stone Gap etc…).
A wave of low pressure developing over Texas, as seen above at 7 AM Monday, will spread a new wave of rain across the mountain region into Tuesday morning.
This will mark the beginning of a colder trend; however, due to upper air blocking over the Aleutians-Bering Sea sector, the brunt of the cold will dive into New England later this week as general -PNA ( negative Pacific North American oscillation ) is maintained.
The current 6-10 day trend is for near to a little above average temperatures locally, with above average temps forecast across much of the nation between the Rockies and Appalachians ( below ) by the 51-member ensemble mean.
Statistically, and in theory, a 51-member MEAN forecast should be more accurate than any individual model forecast. While this is not always true, more often than not the use of the MEAN of a large group is best when doing longer-range outlooks.
A notable exception to the above being when atmospheric changes climatologically proven by history to alter a particular pattern can be picked out by a forecaster in advance of later model shifts. An example of what I mean by this will be cited below with respect to the potential for changes heading toward Thanksgiving Week and early December that are only being hinted at by the best models.
Beyond the 6-10 day period there are increasing signs that high latitude blocking will begin changing its orientation, with a cross-latitude bridge of positive height anomalies forming from Northern Europe into Eastern Siberia along with a notable and increasingly negative trend in both the AO and NAO teleconnections, which could signal a more important pattern shift down the road ( especially if the current -PNA dissipates, which is NOT yet being forecast ).
Building of bitter air across northwestern Canada and parts of Alaska during next week in combination with formation of blocking near Greenland, can be preliminary indicators of Arctic Outbreaks into the continental USA. Stay tuned for later updates on this evolving pattern.
Reference my 110417 Forecast for details about why blocking over the Aleutians-Bering Sea is important to SE USA conditions. Note that cold can occur given this mean pattern, with 0 degrees being observed in high valleys of the High Knob Massif as late as March 16 this year; however, the type of pattern observed during Winter 2016-17 was not favorable for sustained cold-snowy weather.