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.