A Potentially Serious Flash Flood And Flooding Situation Is Expected To Continue Developing Into Mid-Day
As of 5:00 AM Thursday, more than 5.00″ of total rain have fallen at upper elevations in the High Knob Massif from Powell Mountain across Big Cherry Basin to the Robinson Knob community. Creeks are now near or above flood stage in many locations downstream of these upper elevations along the high border area of Wise-Scott-Lee and Wise-Harlan counties.
Additional locations will likely experience flooding before rainfall diminishes later today. The main-stem rivers will likely rise through tonight. Please monitor the latest levels and forecasts for these rivers.
Big Stony Creek (upstream Fort Blackmore)
Powell River at Big Stone Gap (Gage Height)
**When Big Stony Creek is above flood stage it generally is an excellent indicator for all the steep creeks draining the High Knob Massif, with analogous levels on South Fork of Powell, Little Stony Creek, Stock Creek, Cove Creek, Clear Creek, and many others that drain the high country along the Wise-Scott-Lee border area.
Big Stony Creek in northern Scott County, VA, was 2.2 feet (27 inches) above official flood stage at 2:15 AM on Thursday, 6 Feb 2002
Residents living along South Fork of Powell River, downstream of Big Cherry Lake Dam, on the Wise County side of the High Knob Massif will need to remain alert for potential of significant flooding with a large overflow ongoing over the Dam
A large plume of rainfall downstream will be lifting northeast across the mountain area overnight into the daylight hours of Thursday.
*As of 5:00 AM on 6 February, the rainfall total had reached 5.07″ at Big Cherry Lake Dam (2008-2019 February average precipitation was 6.87″).
*More than 0.50″ of rain recently accumulated within less than 30 minutes to trigger the current spike in water levels.
Due to antecedent wetness, with 12.22″ since the beginning of January and 20.74″ since the start of December, the concern for flash flooding through Thursday is well above average. This also includes the possibility of mud and rock slides.
ALERT For Heavy To Excessive Rainfall Amounts During The Period From Late Tuesday Through Late Thursday (February 4-6)
Waves of rain, moderate to heavy at times, are expected throughout the next several days within an anomalously mild, moist atmosphere for this time of year. Folks living and driving along streams and in low-lying, flood prone locations will need to remain alert for water level rises and ponding of water.
**Stay tuned to NOAA Weather Radio and your favorite media sources for possible warnings.
Locations with greatest impacts will depend upon where the heaviest run-off develops, as waves of rain continue to move across the mountain landscape.
Although the greatest current flood potential is in the Clinch, Powell, and Cumberland river basins, where the greatest precipitation amounts have accumulated in the past 36-hours, flooding within adjoining basins (e.g., Russell Fork, Levisa Fork, Holston, etc.) will also become possible.
While orographic forcing will increase amounts within the High Knob Massif-Black Mountain corridor, as a low-level jet forms into Thursday, increasing upslope flow, the possibility of convection superimposed upon the general rainfall pattern will complicate as well as enhance the flash flooding potential in locations impacted by thunderstorms.
Weather Headlines
*Colder air is being monitored for possible significant snowfall accumulations by Friday into this weekend (centered on 7-8 February) as additional waves move east of the Appalachians prior to warming once again next week to anomalous levels. This would add to any flood potential next week.
*A pattern conducive to additional heavy to excessive rainfall is being monitored for next week, through the middle of February (reference updated charts below).
Big Picture Pattern
In the big picture, the upper air flow regime makes all the difference regarding how much precipitation falls as rain versus snow, and how much falls in total. This regime is dictated by ocean and land temperature contrasts which generate pressure differences that develop and drive weather systems across planet Earth.
Reference Mid-Winter 2020_High Knob Massif for updated precipitation amounts from the City of Norton into the high country of the High Knob Massif.
This big picture regime is then modulated by orographics within three-dimensional, complex terrain as exists here amid the majestic Appalachians.
With February knocking on the door, it is time to review the past two years when February produced excessive precipitation amounts (mostly rain, very little snow).
The past two Februaries had very similar upper air patterns, especially across the central-eastern USA.
Adding the two patterns together produces the above, featuring a notable SW upper air flow into the eastern USA with transport of both Pacific and Gulf of Mexico-Atlantic moisture. This flow regime included westerly component atmospheric trajectories.
Precipitation totals at Big Cherry Lake Dam reached 14.37″ in February 2018 and 12.50″ during February 2019, with locally higher amounts in the high country. Could February 2020 produce a trifecta?
The pattern ahead through the first week of February 2020 becomes eerily similar to that of the past two Februaries, so that means wetness increases!
Current Interactive 500 MB Flow Field_North America
Many ensemble members of both the European and GFS model groups are predicting heavy-excessive rains to fall in waves through the middle of February. This trend has only increased in recent days, as anomalous warmth for February returns next week to set the stage for more waves of heavy-excessive rainfall.
Consecutive model runs see the flood potential that is analogous to February 2018 and February 2019 in the upper air flow regime (as outlined above).
The Monday evening runs of both the European and GFS models are similar with prediction of significant snowfall during Saturday (8 February). Expect both timing and amounts to vary on future model runs.
If the mean upper trough axis remains positively tilted, as shown in the composite of the past two Februaries, it will push heaviest precipitation into western slopes of the Appalachians where orographic forcing will act to enhance amounts.
If the mean upper trough axis becomes negatively tilted it would favor heaviest amounts into the eastern slopes of the Appalachians, and locally along the southeastern side of major mountains like the High Knob Massif.
A comparison between daily accumulated precipitation observed in Big Stone Gap (VA) and Boone (NC) during February 2018 and February 2019 illustrates this very well, with the two months producing a total of 25.39″ under westerly component (upslope) flow in Big Stone Gap, VA versus 9.07″ in Boone, NC under downsloping.
Lovers of snow, of course, do not want to see this type of pattern. Instead, patterns like observed during 2010 and 2015 are the ones desired for deep snowpack formation.
Such patterns can easily produce between 50″ to 100″ of snow within a single month in favored upslope locations along the Cumberland-Allegheny Front.
To contrast with the patterns of 2017, 2018, 2019 and 2011 when the southern Appalachians were robbed of February snow. The current snow drought being very impressive from the perspective of recent decades.
If you are a student of weather, then learn how to read these maps and what they mean as you can see what an impact they equate to regarding sensible conditions.
Regressing the February snowfall trend during the past 6 decades, using the Burkes Garden NWS Cooperative site, finds a notable decrease (the 1897-Present trend shows an increase, but are early years as accurate?).
Measuring precipitation accurately can be difficult, especially snowfall at higher elevations, with many factors potentially acting to skew reported amounts away from reality. A topic for another day!