Monitoring A Flood Potential

*An Enhanced Flash Flood Risk Is Expected Thursday Into Friday (30-31 July 2020)

3:00 PM Update_Friday_31 July 2020

Thunderstorms developing over central portions of Kentucky and Tennessee represent the next wave to impact the mountains into late afternoon-evening.

A southwest to northeast corridor with a heavy to excessive rainfall potential will be present into this evening and night.

A boundary from the Kentucky Foothills into the mountains along the Virginia-Kentucky border will need to be closely monitored for the potential of enhancement and training.

Antecedent Conditions = Run-off & Saturated Soils

Heavy rains have Big Cherry Lake overflowing, and water gushing through South Fork Gorge, following a dry first-half of July that dropped the lake level 2 feet below its spillway (this rise represented more than 75 Million gallons of water gained in the lake, and much more within the basin_not including overflow).

Reference the High Knob Landform for more information.

A total of 5.62″ of rain fell at Big Cherry Dam during the 27-31 July period, including 1.66″ of overnight rain since midnight this morning (31 July).

A potential for locally strong-severe thunderstorms, in addition to the flood potential, will continue through this weekend.

Previous Discussions (Below)

Following Thursday afternoon convection, that dropped 0.98″ of rainfall in Clintwood and up to 1.75″ at Big Cherry Dam in the High Knob Massif (where orographic feeder clouds were present…see images), I am now monitoring the potential for new convection and heavy rains to develop into the overnight as orographic forcing increases on southwesterly inflow.

Doppler radar tends to under-estimate rainfall amounts when orographic pilatus clouds are present and capping the High Knob high country. Why? Because radar detectable rain drops falling aloft out of seeder clouds (be them thunderstorm towers or other nimbus types) become enriched by falling through thick terrain capping clouds = increased rain volume with more rain reaching the surface than detected by Doppler radar.

The problem at hand, then becomes one of a SW low-level jet of unseasonably strong winds (given this is the convective season when organized winds are lacking in the mean) that will push upon the terrain.

The high-resolution NAM is nearly identical to the latest 18z European Model in forecasting this first wave of enhanced winds, with additional jets of enhanced low-level wind predicted at times through the upcoming weekend.

Since air can not go downward into the solid ground, the only option will be an increase in surface-based upward vertical motion. Not be a problem in a stable air mass, but with any instability in a high (rich and tropical) precipitable water air mass there is the potential for trouble.

The mountains are not merely static entities, as they counter atmospheric pushing with a torque (in this case, it is a Positive Mountain Torque force).

The Bottom Line…Remain ALERT for the potential development of heavy rains into the overnight and Friday (day-time).

A surge in precipitable water values to well above 2.00″ in combination with SW-W low-level inflow and convergence along the mountains is expected to enhance the potential for heavy rain and thunderstorms Thursday into Friday.

The location of any flooding can not be known for certain until actual rain-thunderstorms develop.

A moisture rich air mass is expected to combine with a stalling frontal boundary, and waves of low pressure, to support repetitive rounds of showers & downpours in thunderstorms during coming days.

Interactive Lightning And Storm Tracking Doppler

A main focus is expected to develop by late Thursday into Friday (30-31 July), with any accumulated rain prior to this period acting to increase soil moisture and the flooding risk.

While the NAM Model is faster than the European Ensemble Mean, there is general agreement in a heavy to excessive rainfall potential developing this week.

Part of a stormy pattern likely to continue during the first week of August 2020.

The core of a strong heat dome retrogrades into the southwestern USA as a positively tilted upper trough deepens over the eastern USA.

This locks in a stormy, wet weather pattern.

Weakening of the western extent of the Bermuda High ridge could be an ominous development (should it occur as now predicted) for tropical activity, and at the least an aid to the feed of deep, tropical moisture into the eastern USA.

All Conditions Are Connected

It is no surprise to residents that far southwestern Virginia and southeastern Kentucky are among the foggiest area’s in the United States, with nocturnal fog formation along river valleys nearly every night from summer into autumn within the ecologically diverse river basins forming headwaters of the Upper Tennessee, Upper Cumberland, and Upper Ohio river systems.

The moisture visible as fog may disappear from human sight, with a phase transition from liquid to vapor, but energy represented remains and more often than not helps to fuel development of day-time showers and thunderstorms.

This is all part of a positive feedback loop that plays a simply vital role in the summer precipitation and air temperature regime.

Afternoon air temperatures across Virginia during this time (below) varied from 60s in the High Knob Massif, and City of Norton, to around 100 degrees in the Tidewater of far eastern Virginia.