ALERT For Heavy, Wet Snow Accumulations At Highest Elevations Tuesday Into Tuesday Night With A Chance For Heavy Snow And Mixed Precipitation Into Middle Elevations

A southern tracking storm system will spread significant precipitation across the mountain area on Tuesday. Rain and mixed precipitation is currently expected to rule the low-middle elevations, below 3000 feet, with increasing amounts of heavy, wet snow at elevations above 3000 ft.

Update_1:30 PM on Tuesday_31 March 2020

*Very heavy snow has fallen and accumulated at the summit level of the High Knob Massif, covering roadways despite recent above freezing conditions. Travel is being discouraged along Route 619 and adjoining roads toward the summit level.

Although not as classic as some well documented past events, the bright banding signature did take its position along the High Knob Massif with heaviest snowfall along and southeast of the edge of this zone.

Snow that falls next will be associated with the comma-head of the upper level low, with dropping snow levels this evening as temps fall (but generally remaining at upper elevations with respect to sticking).

The Potential Exists For A Crippling Fall Of Heavy, Wet Snow At High Elevations In The High Knob Massif (Fast & Furious Fall)

Temperatures in upper elevation basins fell sharply through Midnight, with dewpoints in the 20s at the summit level where mass convergence is expected.

Some routes currently expected to be impacted include State Routes 619, and Routes 237, 238, 704, among some others, due to rates of snowfall greater than melting on above freezing surfaces.

The most favorable location for heavy snow will be the upslope zones along the High Knob Massif where air flow trajectories and the forecast thermal profile is favorable for formation of a *TIM Circulation (with a rain-snow line setting up on the northwestern flank of the massif in a Doppler radar indicated bright-banding zone).

*The major difference is this is now mid-spring. If a setting like this had developed during the winter I would have been predicting a massive fall of snow. Up to 6″ or more could still occur if a TIM Circulation develops at the upper elevations, such that this is a situation worth watching.

Otherwise, highest peaks on the Blue Ridge southwest to the Great Smokies will be favored, mainly at elevations above 4000 feet.

Snow levels will drop Tuesday afternoon into Tuesday night and the early hours of Wednesday morning, but sticking is expected to mainly be restricted to elevations above 3000 feet [(+/-) 500 vertical feet of error potential].

Longer-Term Persistence

The longer-term pattern is wanting to trend right back into the same type of regime that has made this year so wet, with more heavy to potentially excessive rainfall if this indeed becomes the observed pattern.

Weather Headlines

*A weakening line of strong-severe thunderstorms across Kentucky & Tennessee will approach the mountains late Saturday into early hours of Sunday.

*Strong and gusty SSW-WSW winds Saturday night into Sunday will shift W-WNW with cooler air by late Sunday into Monday.

*Rain overspreads the mountain region during 31 March into 1 April (Tuesday-Wednesday) as air turns sharply colder (rain may end as snow at high elevations).

Colder Pattern Opens April 2020

A colder pattern will open April 2020 across the eastern USA as a negative North Atlantic Oscillation is joined by a negative Eastern Pacific Oscillation.

Note that this is opposite to these observed teleconnections during Winter 2019-20.

Cold weather across the eastern USA is favored during negative phases of the NAO & EPO oscillation patterns versus mild conditions during their positive phases.

Negative phases of the NAO and EPO should result in colder air during early April (in the mean).

As noted above, this would be nearly opposite to the observed winter pattern (below). The difference, with increasing sun angles-insolation and limited snow and ice coverage, air temperatures will be much below the average for this time of year but well above what they would have been during the same pattern in winter.

An ALERT For Elevated Stream Levels Will Continue Into Thursday (26 March 2020)

A general 1.63″ to 2.05″ of rainfall pushed steep creeks draining the High Knob Massif to near and above flood stage. High water levels will continue into Thursday. Extreme caution is advised.

Big Stony Creek crested 0.6 feet (7″) above flood stage at 2:15 AM on 25 March, with roaring water levels continuing along this creek, South Fork of Powell, Little Stony Creek, Clear Creek, Stock Creek, Cove Creek and many others that drain the border area of Wise-Scott-Lee-Harlan counties.

Main-stem rivers will also continue to be elevated into Thursday, at near flood stage, with falling levels through the end of this week. Caution Is Advised.

**An ALERT for strong SSW-WSW winds may be needed for mid-upper elevations during Thursday night into early Friday with strong warm air advection.

Former Statements

A Heavy To Locally Excessive Rainfall Potential Is Being Monitored Through Wednesday Morning (March 25)

Weather Headlines

*Another wave of significant rainfall, with thunderstorms possible, will impact the mountain region from afternoon hours of Tuesday into early Wednesday.

*Residents living and driving in low-lying, flood prone locations will need to closely monitor conditions. Stay tuned to NOAA Weather Radio and your favorite media sources for possible watches or warnings.

*A marginal risk of strong to severe thunderstorms has been shifted southward by SPC for Tuesday (24 March). This remains to be seen and could still be adjusted back northward. Regardless, embedded thunderstorms and another wave of significant rainfall remains likely.

**Tuesday presents concern for development of a squall line or scattered area containing strong-severe thunderstorms across southern Kentucky-Tennessee. An eastward movement needs to be monitored for the southern Appalachians into Tuesday night.

Precipitation Totals
1-23 (AM) March 2020

Black Mountain Summit
7.54″

*Big Cherry Lake Dam
7.41″

Eagle Knob Summit
7.32″

High Chaparral-Robinson Knob
7.08″

Clintwood 1 W
5.57″

*The 2008-2019 average for the entire month of March is 6.36″, such that month-to-date precipitation is currently running 2.69″ above this average.

Reference 2019-20 Precipitation Totals For More Values.

Looking Into April 2020

It is more of the same through this week, with intervals of rain-thunderstorms and up-down temperatures amid a general milder than average and, of course, wetter than average regime across the eastern USA.

A -NAO pattern (above) is now showing up in the ensemble mean modeling for the end of March and first week of April 2020. This will generate a notably cooler weather pattern.

Changes are finally showing up in a more consistent manner within the modeling as an ongoing stratospheric warming continues above the North Pole with transition from winter to spring across the Northern Hemisphere.

Temperatures at 30 MB above the North Pole (above) have been much colder than average during most of the 2019-20 Winter Season (gray line = climatological mean).

A pattern featuring a colder trend, of some duration yet to be determined, is becoming more likely by the end of March into April 2020 as a powerful polar vortex that has dominated the Northern Hemisphere winter season finally weakens (releasing lingering arctic air)#.

#Weakening has been slow given the anomalously strong polar vortex. The magnitude of the final warming and impacts to middle latitudes remains to be seen, but changes of some form are likely across North America.

The European Ensemble and GFS Ensemble means are in general agreement with a negative trend developing in the North Atlantic Oscillation. This -NAO will favor colder temperatures across the eastern USA.

The PNA has been persistently negative during the 2019-20 winter, but a notable trend toward neutral (European Ensemble mean) and positive (GFS Ensemble, above) is now being predicted heading into early April 2020.

These trends will favor a period, at least of some duration, of colder conditions relative to average.

More accumulating snow may also occur, at least for the mid-upper elevations, which would fit April climatology.

Weather Headlines

ALERT For Dropping Cloud Bases And Dense Fog From Upper Elevations Into Middle Elevations On Northerly Upslope Flow And Cold Air During Saturday

*Dropping cloud bases on northerly upslope flow is expected to impact locations along and north to northwest of the High Knob Massif and Tennessee Valley Divide as air turns much colder through Saturday.

Saturday Update_2:00 PM: Low clouds are engulfing a wide area on upsloping northerly winds in wake of soaking rainfall. Afternoon air temperatures vary from 30s at upper elevations (gust wind chills in 20s) in the High Knob Massif to the middle 40s at lower elevations to the north.

Early afternoon temperatures are in the low-mid 50s toward the south in the Tennessee Valley, from TYS to TRI, with good visibility under downslope flow.

Caution for high stream levels through Saturday on headwater creeks draining the High Knob Massif-Black Mountain corridor in wake of significant rain.

Big Stony Creek crested 1.0 foot (12″) below flood stage this morning (21 March), in wake of soaking rainfall, to continue a trend of elevated stream levels throughout the month of March 2020.

Precipitation Totals
1-21 (AM) March 2020

Black Mountain Summit
7.01″

*Big Cherry Lake Dam
6.71″

Eagle Knob Summit
6.72″

*The 2008-2019 average for the entire month of March is 6.36″, such that month-to-date precipitation is currently running 2.40″ above this average.

Reference 2019-20 Precipitation Totals For More Values.

Previous Statements

*A chance of showers and thunderstorms Thursday, amid unseasonably mild air, will give way to likely activity on Friday in advance of a strong cold frontal boundary.

*Strong SW winds will develop Thursday night into Friday, beginning at upper elevations and mixing downward into middle and lower elevations.

Wind gusts of 30-40+ mph will begin Thursday night at upper elevations and mix downward into middle-lower elevations during Friday. Local wind gusts to 50+ mph will be possible.

*A heavy to excessive rainfall potential will exist Friday, with strong rises on streams more likely in locations that received up to 1.00″+ of rainfall into early Thursday.

Much like late Wednesday afternoon into early Thursday (19-20 March), SW flow will add orographic lift to help enhance totals along the Cumberland Mountains (1.11″ of rain was observed at Big Cherry Lake Dam into early Thursday, with 1.01″ on Black Mountain).

Wintry Feeling Conditions Expected Saturday With Cold Daytime Temps

*Much colder air surges into the mountain area Saturday on northerly upslope flow, with low cloud bases and the threat for more dense fog at middle to upper elevations (rime levels will drop from high summits downward into the 2500-3000 foot elevation zone over time).

A brief intrusion of drier air into early Sunday will allow coldest temps to develop in mountain valleys (coldest conditions Saturday will be at highest elevations) with potential for strong low-level inversion formation (coldest conditions in valleys with rising temps on high ridges into Sunday morning).

An exception may be in some lower elevation locations where clearing over wet ground may allow for dense fog formation. Stay tuned for later updates.

*Additional waves of significant precipitation will be likely from late in the weekend through next week. Interaction with colder air at times may need to be monitored for the potential of some frozen forms, especially to the north and northeast.

Weather Headlines

Caution for low cloud bases and dense fog will continue through tonight into Monday at middle-upper elevations along and north to northeast of the High Knob Massif and Tennessee Valley Divide.

Orographic lifting of northerly low-level flow will continue producing low cloud bases into Monday, with a shift toward more ESE to SSE flow potentially lifting bases at middle elevations while upper elevations at highest elevations continue to be engulfed.

*Rain into early Sunday will cause rises on mountain streams, as a wet early Spring 2020 pattern continues.

Caution is advised for ponding of water in low-lying locations and for rises on already elevated streams draining the High Knob Massif.

Recent elevated stream levels resulted from a general 1.49″ to 1.91″ of rainfall across upper elevations during 12 March into early on 13 March 2020.

*Waves of rainfall will continue through the upcoming week to next ten days, with potential for 3.00″ or more from western slopes of the Appalachians westward.

The spread on 90 different ensemble members across the European, GFS, and Canadian (GEM) model groups varies generally between 3.00″ to 8.00″ .

Pattern Change_Late March-April?

A Northern Hemispheric pattern change of some form is likely to occur by late March into April as a major change in the Polar Vortex above the arctic and North Pole marks the climatological final warming which occurs during the annual transition from winter to spring.

The 2019-20 winter season has been dominated by much below average air temperatures above the arctic region, a signature of a powerful, strong polar vortex, which has enhanced warming across middle latitudes (i.e., it has been a major player in the mild winter observed).

This resulted in a very strong polar night jet, centered above 65-70 degrees North, along the perimeter of the stratospheric polar vortex [note 60+ meter per second (134 mph) westerly jet core above].

Major warming aloft above the arctic region is forecast to weaken the polar night jet and to change stratospheric winds to easterly (above) during the next 1-2 weeks.

Although still cold (-28 Celsius at core), a 50 degree Celsius temperature rise is expected to occur above the North Pole as part of the final warming and break-up of the polar vortex with transition from winter into spring.

A release of lingering cold air may help to change the pattern at mid latitudes, with a colder trend developing during late March into April. It is possible that a 2-3 week period of colder conditions will develop, although, both the length and focus of colder conditions remain to be resolved in the modeling.

This does not produce a great amount of change in the wet, milder than average pattern through this coming week, but begins to by final days of March.

That more cold air is coming would certainly be in line with climatology, and more snow (especially at mid to upper elevations) will also be possible IF the pattern actually changes.

The lowest temperatures observed so far during March in classic Appalachian cold air collecting basins have varied from 4.5 degrees in Canaan Valley to 5.9 degrees in Big Cherry Lake Basin (12 degrees in Shady Valley and 17 degrees in Burkes Garden).

The 5.9 degree MIN on the morning of 1 March was the coldest reported along the length of the central-southern Appalachians as fresh snow cover allowed enhanced cooling within the high basin. Even lower temps would have occurred if some high clouds had not been streaming across the area aloft.

The 4.5 degree MIN in the northern Canaan Valley frost pocket occurred on the morning of 8 March when conditions in the Big Cherry Basin were much less favorable for extreme minima.

Cold March temperatures are nothing atypical, of course, with 0 degrees (F) being reached in Big Cherry Basin on 16 March 2017. Temps again dropped to near 0 degrees last March, during 2019, but fell only to 12 degrees in March 2018 (a heat wave).

The coldest March temperatures on record occurred during March 1960, with -10 degrees F below zero in Burkes Garden and -19 degrees in Canaan Valley.

My records do not extend back to 1960, but I recorded a -15 degree below zero MIN over 8″ of fresh snow on the morning of 3 March 1980 at Clintwood 1 W (**).

**This suggested that the MIN would have likely been colder than -20 (F) below zero in Big Cherry Basin during March 1980 (based upon typically observed differences between Clintwood and Big Cherry during recent years).

***It is interesting to note that the all-time coldest March temperature in the Tri-Cities, TN occurred during March 1980 with -2 degrees (F) below zero. That is the only March temperature in the record period to reach 0 degrees or below at TRI (1938-Present).

Recent March MINS at TRI reached 15 degrees in 2017, 24 degrees in 2018, and 19 degrees during both 2019 and 2020.

Weather Headlines

ALERT For The Potential Of Strong-Severe Thunderstorm Development With A Heavy Rainfall And Strong Wind Potential Through Thursday Night Into Early Hours Of Friday

*A chance of showers and thunderstorms Thursday PM will give way to likely activity Thursday night into the early hours of Friday. Some storms could be strong to locally severe, with downpours and strong winds.

Stay tuned to NOAA Weather Radio and your favorite media sources for the possibility of watches and/or warnings which may be needed.

A spring-time storm pattern is upcoming for the mountain region, with periodic waves moving through a developing longwave trough in the upper air that deepens over time.

Each shortwave passing through the longwave trough will tend to strengthen the mean trough and bring rain, with thunderstorms becoming increasingly likely as air turns unseasonably warm for mid-late March.

An increasing threat for heavy-excessive rainfall amounts will arise as time passes.

*Additional waves of rainfall are expected during late week into this weekend and next week. These waves are trending heavier in modeling, with a heavy to excessive rainfall potential being monitored once again. The exact placement of heaviest rain bands remains to be resolved, with the greatest threat region stretching from the lower Mississippi and Tennessee-Lower Ohio valleys across the southern Appalachians and northern Gulf Coast states.

Reference Persistence In Longer-Term of My 030520 Forecast

The ridge-trough-ridge pattern of February is rearing its ugly face once again, with a much more amplified pattern predicted by the 51-member European Model ensemble group by late this week into next week.

The current tilt of the mean trough favors heaviest rain amounts being along and west of the Appalachians, with orographic enhancement on westerly component flow into the western slopes of the mountain range as observed during February in 2020, 2019, and 2018.

Reference Huge Weather Swings In Early March for more details on the ridge-trough-ridge pattern that looks to become longer-lived again. Reference my 020620 Forecast for a review of past flooding and the repetitive nature of this wet 2020 pattern. In addition, please reference my 013020 Forecast for more information on the “big picture” of this pattern.

Deep moisture transport from both the Pacific Ocean and Gulf of Mexico will combine with unseasonable warmth for March to set the stage for heavy to excessive rainfall, including potential for strong-severe thunderstorms at times which could add convective downpours.

A pattern change across the Northern Hemisphere may (with emphasis on may) finally be upcoming as the final warming occurs with breakdown of an anomalously long-lived and strong polar vortex. This breakdown is typical and occurs every year over the arctic region.

Impacts from this may not be felt in the southeastern and eastern USA until the final days of March, and more likely impacts will be most pronounced during April 2020. Stay tuned for later updates as timing and impacts become better defined.

Diversity Of Nocturnal Temperatures

*A large vertical temperature spread develops tonight into Sunday morning as light winds and clear skies allow strong surface cooling to contrast with significant warm air advection across mid-upper elevation mountain ridges, generating a strong vertical temperature inversion that increases into early on 8 March.

A huge and seemingly always present (since beginning of time) problem in operational meteorology is the lack of recognition of diversity across complex terrain, with this aspect especially evident on nights featuring formation of nocturnal inversions which are more the norm than exception in complex terrain settings.

The vast majority of forecasters (including those in the NWS and Weather Channel) use much too narrow (tiny) nocturnal temp ranges to represent complex terrain.

Actual field research finds 15-30 degree (F) temp ranges to not be uncommon over short distances, and while official forecasters may be reluctant to use such large spreads their existence needs to be better represented in real-time forecasts for complex terrain.

This fact is becoming more evident to everyone as more and more people share personal weather data, and is no longer merely an aspect restricted largely to the research community. Here are merely a few examples.

Birchleaf of Russell Fork Basin

The Birchleaf weather station of Jonathan Owens, former broadcast meteorologist, lies 500 vertical feet above the Russell Fork River. This site is typically 3-5 degrees F or more milder than valley locations in the mid-upper basin through evening hours, but as the inversion inside Russell Fork Basin deepens to 500 vertical feet (152 meters) or so within the lower basin it tends to trend colder and become nearly as cold as Clintwood 1 W by morning. Clintwood 1 W is an official NWS Cooperative station within a lower elevation cold air drainage of the Russell Fork Basin (not currently listed with live data).

Flatwoods Mountain_KY_Dorton Near Pine Mountain

Contrasting mountain ridge sites for both Russell Fork Basin and Norton Valley stations, can be found by scrolling to Dorton in Pike County, KY and to Black Mountain in Harlan County, KY on nights featuring the development of nocturnal inversions (like tonight).

Norton Valley_Downtown Norton Virginia

This weather station in downtown Norton tends to be one of the coolest (on average) for any city in the Old Dominion on radiational cooling nights. The City of Norton lies on the divide between the Clinch and Powell river basins, nestled amid the northern base of the High Knob Massif, with cold air drainage from both the northeast end of Black Mountain (along Guest River) and the high country of the High Knob Massif. So while this site tends to be significantly milder at times than valleys embedded in the adjacent high country of the High Knob Massif, it still can have impressive cooling despite being under two-way drainage into both the larger-scale Clinch and Powell river basins.

Lonesome Pine Airport_Wise Plateau

A classic contrast to the Norton Valley on radiational nights with warm advection can be obtained by comparing the temperature observed on the exposed Wise Plateau at nearby Lonesome Pine Airport. It is a critical forecast mistake to confuse KLNP with Norton under these conditions (observed on many nights throughout a year). Amazingly, locations only a short distance from Lonesome Pine Airport can be much colder within depressions that are embedded within top of the plateau (such as the location of J.J. Kelly High School), with frost formation at times when air temperatures at KLNP are in the 40s and 50s.

Wise Plateau_Hurricane Road Cold Air Site

The Hurricane Road weather station appears to be an example of the above, and will be interesting to follow as a contrast to nearby Lonesome Pine Airport (as I have noted so many times in the past, the KLNP sensor appears to possess a +1 C (+1.8 F) degree error which should also be taken into account for greatest precision). Other sites listed may possess errors as well, but nocturnal temps are often more accurate than daytime temperatures when small solar radiation shields and other placements can increase temperature errors. In general, these sites are of high quality by night.

Mountain Lake Biological Station

The Mountain Lake Biological Station is located in an upper elevation sink featuring the only natural mountain lake in Virginia (even though it has mostly been dry in recent years due to seepage). During January 1985, when an official NWS Cooperative site was here, it recorded the lowest official temperature on record in Virginia with -30 F (-34 C). Colder, unofficial, temperatures have been documented in the High Knob Massif area.

Mountain Lake_Bald Knob

Bald Knob of Salt Pond Mountain is the highest summit surrounding Mountain Lake and can exhibit significantly milder readings at times. It will be interesting to see if the upper elevation sink of the Biological Station can fully decouple into the overnight as warm air advection begins into the summit.

Shady Valley_TN_Lower Valley

The Shady Valley Basin drains to the northeast, with this weather station located near its northeastern outlet. While the floor of Shady Valley is middle elevation, per my specific classification system, it is not far beneath the 3000 foot (914 meter) contour and like the Bark Camp Basin of the High Knob Massif can effectively be considered upper elevation given it is influenced predominately by elevations above 3000 feet on cold air drainage, outgoing longwave radiation (OLR) nights.

Northern Canaan Valley_WV_Frost Pocket

This central Appalachian site in the northern portion of the Canaan Valley Basin is within a classic cold air generating basin, and is part of ongoing research into frost pockets of the southern-central Appalachians. This upper elevation site, at 3150 feet (960 meters) has the potential to eventually break the state record MIN for West Virginia (-34 F or -36.7 C), along with other spots in Canaan Valley, where despite temperature recording since 1945 the lowest MIN ever documented reached “only” -27 F (-32.8 C). An aid to cooling here, other than its obvious elevation and latitude, is a large sky view factor (lack of trees).

Cabin Mountain_WV_Dolly Sods

A contrasting site for this study is the nearby weather station within Dolly Sods, on Cabin Mountain, located 885 feet (269.8 meters) higher than the northern Canaan Valley frost pocket. When wind speeds here are around 14 mph (6.3 m/s) or less the northern valley can typically decouple under clear conditions with strong radiative flux divergence.

Other classic sites not currently linked up with live data are found within Burkes Garden and the Big Cherry Lake Basin of the High Knob Massif. Cooling at all sites tends to be enhanced, of course, by snow cover which acts to block radiative heat flux from the ground that works to compensate for OLR that cools overlying air.

**The Big Cherry Lake Basin of the High Knob Massif will eventually break the all-time record MIN for Virginia and likely did so just prior to recording in February 2015 (temp recording began on a regular basis during 2016).

Unlike within the western USA, the most extreme temperature minima are even more restricted to fresh snow cover events in southern-central Appalachian cold air basins versus places like Peter Sinks, Utah where very dry air at high elevations can support extreme temperatures throughout the year (although, even there the coldest temps also occur most naturally above a snowpack).

Differences between central and southern Appalachian frost pocket sites are enhanced when snow is not present on the ground at all sites [being present to the north, like tonight (7 March), and limited to non-existent within the southern extent of the mountain range]. Many other factors are, of course, involved, and this is assuming ideal conditions at all locations which rarely occur.

It is very important to note that the above is not meant to criticize forecasters, but instead to push for an advancement of our knowledge and ability which is greater than being publicly displayed.

The extension of this existing knowledge possesses applicability beyond that to humans, with naturally occurring complex terrain differences acting to regulate essentially all aspects related to floral and faunal (flowering time, migration, breeding, etc.) activities across the natural world.

Weather Headlines

ALERT For Accumulating Snow From Friday Afternoon Into Saturday Morning

Moisture transport from the Great Lakes will combine with WNW-NW-N upslope flow to generate snow showers, snow flurries, and locally heavy snow squalls Friday into Saturday morning. Accumulations are expected to begin at the highest elevations early Friday, with sticking snow levels dropping through middle into lower elevations during Friday afternoon and evening.

The best sticking and most widespread impacts to travel are expected along the upslope side of the Appalachians during Friday night into Saturday morning.

Update At 5:00 PM on Friday_6 March 2020

Wind blown snow showers covered roadways at highest elevations in the High Knob Massif Friday afternoon, with deteriorating conditions expected Friday evening into middle and some lower elevation locations along and north to northwest of the Tennessee Valley Divide and High Knob Massif.

Upslope snow showers will continue developing in Great Lake moisture transport as air temperatures drop.

ALERT For Much Colder Air Temperatures And Low Wind Chill Factors

*Expect air temperatures to fall into the 20s at middle elevations, and into the upper 10s at highest elevations, during Friday afternoon (with much colder wind chills).

Snowfall Forecast

A general 1″ to 2″ below 3000 feet

**A general 2″ to 4″ above 3000 feet

**Locally higher amounts possible on highest peaks, with deeper depths due to blowing and drifting snow, being in contrast to only a dusting within some lower elevation sites at elevations below 1200 feet.

Any accumulations are expected to be limited leeward of the mountains into the Clinch, Powell, and Holston river valleys on downsloping air flow trajectories.

The worst travel conditions are expected to develop Friday night into Saturday morning, with falling temperatures and wind driven snow showers.

This snowfall forecast is primarily for WNW-NW-N upslope flow locations along and northwest of the Cumberland-Allegheny Front and highest elevations of Clinch Mountain and the Blue Ridge.

*Clearing and seasonally cold conditions are expected Saturday afternoon into Sunday morning, with coldest conditions developing within mountain valleys.

High pressure and sinking air, along with light winds and mostly clear skies are expected to set the stage for good outgoing longwave radiation (OLR) and compensative turbulent heat flux divergence from overlying air heading into late Saturday afternoon-evening. This will support strong temperature falls along slopes and initial drainage into valleys and basins where radiative flux divergence will aid cooling. Any lingering snow cover will also help to support cooling, and this is primarily expected to be the case for upper elevation cold air collecting basins and draining valleys-hollows of the southern Appalachians (middle to upper elevations in the central Appalachians).

Cold air collecting basins in the central Appalachians will have higher wind speeds into the evening that may initially make decoupling more difficult into the overnight hours [e.g., the classic northern Canaan Valley, WV frost pocket site appears to have a reflection point centered near 14 mph (6.3 m/s or 12 knots), with decoupling when speeds are under this point and partial to no-decoupling of basin winds when mean speeds are above 14 mph on adjacent Cabin Mountain at Dolly Sods]. The reflection point varies for different locations depending upon topographic factors.

Persistence In Longer Term

As I recently noted, persistence is the main guide for the medium range forecast through next week with warming and wet conditions (surprising…NOT!) developing once again in wake of early weekend cold and snow.

Although not exactly the same pattern as observed during February, the trend of the ensemble means of different model groups (GFS, GEM, and ECMWF) is becoming increasingly more like the Feb pattern.

This would increase wetness through the second-third weeks of March, with unseasonable warmth raising a concern for more strong-severe thunderstorms and convective rains.

The European Model is looking especially anomalous in terms of warmth during the upcoming 5-10 day period.

This anomalously warm pattern continues through mid into late March on the latest ensemble mean of the GFS Model group (below).

Observed SSW mean 925-850 MB flow trajectories during February (near the long-term climatological mean flow trajectories) provided plenty of orographic forcing and convergence for enhanced precipitation amounts along the Cumberland Mountains, Cumberland Plateau and Tennessee Valley during the month.

Precipitation Totals – February 2020

Clintwood 1 W
9.20″

City of Norton WP
10.47″

Jonesville 3.1 WSW
10.49″

Hunter Valley
10.70″

Black Mountain Mesonet
12.39″

Big Cherry Lake Dam
13.01″

High Chaparral-Robinson Knob
13.32″

Eagle Knob of High Knob Massif
13.47″