Reference Early Spring 2021_High Knob Massif for an update on recent and Winter 2020-21 conditions.

Another Heavy Precipitation Event

Alert For The Potential of Heavy to Locally Excessive Rain Wednesday

If you think that it is taking less rainfall to cause rises and responses to local mountain streams, then that is correct. Why is this true?

The past 3-years have been very wet and water tables are elevated. Locations in watersheds that are above impermeable stratigraphy at depth have become most flashy with rapid responses to significant rainfall (as are karst terrain systems when conduits are full, see Recap).

Downward infiltration of surface water takes time. Once surface layers become saturated the processes of Through Flow, Interflow, and Overland Flow can contribute more rapidly to run-off into streams (a portion of this stream flow being base flow from groundwater input).

If I were responsible for the official excessive rainfall forecast below, I would make a distinction between the central (much drier) and southern (wet) Appalachians, where dramatically different rainfall regimes have been ongoing through March and previous months.

Take Elkins, in the northern mountains of West Virginia, for example, at the northern extent of the marginal excessive rainfall outlook below.

The 8.36″ total for 2021 is 0.12″ less than measured in the City of Norton during March and 1.90″ less than observed at Big Cherry Dam in March (Elkins had 8.82″ less rain in March than Big Cherry Dam).

Change To Snow Wednesday Evening

ALERT For Snow Showers And Squalls Developing From Midnight Into Morning Hours On Thursday (Reduced Visibility, Sharp Temperature Drops, And Low Wind Chills)

A transition to snow is expected Wednesday Night into the overnight and predawn hours of Thursday amid an influx of late season winter air.

A general 1″ to 3″ of accumulation will be possible, mainly within upslope locations along and northwest of the Cumberland-Allegheny Front and Blue Ridge.

More than 3″ will be possible at summit levels of the highest mountains, and in locations that may be impacted by training snow squalls known as snowstreaks (also called snow streamers or narrow bands).

Snow showers and flurries will continue into Thursday night and the morning hours of Friday amid unseasonably cold, early April air.

Due to a prime Great Lake connection, bursts of snow in squalls will be possible. With sharp temperature drops into early Thursday, this will create hazardous travel at upper elevations and within locations receiving heavier snow at lower-middle elevations in upslope corridors.

This setting is favorable for snowstreak formation, which features narrow bands of training snow squalls that become aligned along low-level streamlines.

There are many analogs, with one of the most extreme being 2 April 1992 when a snowstreak dropped more than 8″ of snow on Clintwood, causing hazardous travel.

Recap Of Flooding Rains

The corridor receiving greatest amounts of rainfall, within the High Knob Landform (Powell Valley Anticline) contains karst (calcareous stratigraphy).

An Example of Karst Flash Floods

Land Use In Karst Terrane_Temperate Karst Ecosystems

Interaction of highly heterogeneous karst + non-karstic terrain complicates this setting and is one reason that the modeling of streams in the Upper Tennessee River Basin, along the Clinch, Powell, and Holston is difficult and that forecast hydrographs for flooding often have to be adjusted more than in non-karst terrane.

A major reason for increased difficulty in forecasting, is that subterranean drainage systems in karst terrain can often not be directly correlated to surface basins.

Although the above corridor had the highest observed rain gauge totals, the entire area experienced heavy to locally excessive rain amounts.

Although there is no stream gage on the South Fork of Powell, draining Big Cherry Basin downstream of Big Cherry Dam, gages on the Powell River at Big Stone Gap and on Big Stony Creek (labeled Stoney Creek above) can often be used to judge levels when the lake is in overflow.

Forecast downward stream-level trends will be changed in the next couple days as more heavy rain impacts the watersheds.

Reference Early Spring 2021_High Knob Massif for an update on recent and Winter 2020-21 conditions.

ALERT For Heavy Rain And The Potential For Strong-Severe Thunderstorms During Thursday Into Early Hours Of Friday

Waves of rain, with embedded thunderstorms, will develop from Thursday mid-day into the afternoon. Additional showers and thunderstorms will occur Thursday Night into early Friday.

Update At 3:30 PM on Thursday (25 March 2021)

A general 0.75″ to 1.25″ of rain has fallen with this initial wave on headwater creeks from the High Knob Massif into Black Mountain. Those living and driving along streams will need to remain alert for strong water level rises as rain-thunderstorms redevelop.

Specific Point Measurements to 5 PM include:

Birchleaf 1 SW: 0.38″

Clintwood 1 W: 0.41″

Norton (Downtown): 0.70″

Big Cherry Dam: 0.84″

Black Mountain: 0.87″

Eagle Knob: 0.88″

High Chaparral-Robinson Knob: 1.20″

Breaks in the overcast behind this first wave of rain are beginning to create an unstable atmosphere that will become favorable for severe thunderstorms, with possible tornadoes, by late afternoon into this evening.

The SPC Supercell Potential at the Wise Gridpoint increases to 45% by 8 PM, to 60% between 11 PM and 2 AM Friday, then falls sharply to less than 20% after 4-5 AM Friday.

Rain cooled air north of a warm frontal boundary will help limit severe potential during the day. While rotating storms will be possible along the warm front in converging air streams, the main risk will be in the warm sector (between the warm front and cold front) where shear, lift, and instability will be greatest.

The greatest threat for rotating supercell thunderstorms will exist Thursday evening into the overnight hours of Friday (between sunset and 5 AM Friday).

Parameters for rotating storms and tornadoes look to extend farther north and northeast than last week. The Storm Prediction Center may upgrade the region by tonight or early Thursday.

An Enhanced Risk of Severe Thunderstorms with possible Tornadoes has officially been extended northeast into southwestern Lee County, Va., and Bell County, Ky., however, the entire area will be at risk tonight.

The potential for heavy to locally excessive rainfall amounts also exists, with ponding and flooding of low-lying and poor drainage areas, as well as strong rises on streams.

This marks the beginning of a stormy 5-7 day period that could generate widespread 3.00″ to 5.00″+ rainfall totals across the southern Appalachians.

Please monitor NOAA Weather Radio and your favorite media sources for possible watches and warnings that may be needed.

MRX NOAA Weather Radio

Previous Discussion

Nice early spring weather conditions are expected in the short-term, and while some showers will be possible Wednesday the next chance for significant rain and thunderstorms will hold off until late week.

The main focus for the second half of this weekend into early next week will be on more significant rainfall, with heavy additional rain amounts.

Reference Early Spring 2021_High Knob Massif for an update on recent and Winter 2020-21 conditions.

ALERT For Elevated Stream Levels And Ponding Of Water In Flood-Prone Low-Lying Areas Into Early Friday

While locally strong-severe thunderstorms remain possible this afternoon, residents and travelers will need to closely monitor stream levels due to a general 2.00″ to 2.50″ of rain that has fallen on headwater creeks from the High Knob Massif into Black Mountain.

Any additional downpours, if prolonged or training, could quickly trigger flash flooding.

An influx of cold air tonight into early Friday will change rain to snow at upper elevations, with 1″ to 3″ possible (mainly above 3300-3500 feet).

A pocket of cold air associated with an upper-level low currently over western portions of Kentucky-Tennessee will support a transition to snow at high elevations into early Friday. Travel along routes 619, 238, 237, 704, 160 and others that run above 3300-3500 feet could become slushy overnight into early Friday morning.

Previous Discussion

Heavy rain with the potential for strong to severe thunderstorms is being monitored for Thursday (18 March 2021).

Conditions favorable for a major outbreak of severe thunderstorms are coming together over the Deep South and lower Mississippi River Valley. Strong-severe thunderstorms will move northeast into or toward the Appalachians Wednesday night into the early hours of Thursday.

An outbreak of severe thunderstorms, with tornadoes, across the Deep South on Wednesday will spread into the Appalachians on Thursday with a continued chance for strong to locally severe storms.

Strong to severe thunderstorms will be ongoing early Thursday from the main outbreak to the southwest, and will need to be closely tracked.

Additional development will be possible along the mountains into the afternoon.

Potential for heavy to locally excessive rainfall will occur and depend upon where bands of thunderstorms and downpours develop.

Stay tuned to NOAA Weather Radio and your favorite media sources for the possible issuance of watches and warnings.

Thunderstorms To Snow?

A rapid influx of cold air aloft and toward the surface will create a potential for rain to end as snow during the predawn-morning hours of Friday, especially at upper elevations where accumulation can not be ruled out above 3000 to 4000+ feet.