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.

Weather Headlines

A heat dome will be expanding eastward across the USA this week into the upcoming weekend, with warm (upper elevations) to hot (lower elevations) mid-summer conditions.

Interactive Lightning And Storm Tracking Doppler

Instability driven, hit or miss, showers and downpours in thunderstorms will begin developing by later this week into next week.

While instability driven convection is mainly diurnal, favoring day-time hours, clusters of storms that develop in a ring of fire pattern can occur at any time of day or night and often favor night-time hours when they can undergo up-scale growth with increased low-level inflow.

Clusters of organized thunderstorms developing along the edge of heat will also become possible. These are often called “Ring of Fire” storms.

July rainfall totals have varied from 0.50″ or less to more than 3.00″ during this first half of the month, with much of the region running below average.

Climate System Feedback Loops

Have you ever wondered or became frustrated by, especially as a farmer or gardener, the hit or miss nature of summer showers and thunderstorms?

While part of this reason is related to the natural, chaotic nature of thermal convection, it is also due to positive feedback loops that develop between the surfaces of Earth and the overlying atmosphere.

I first started looking at feedback loops in the 1990s, as part of the seasonal variability of summer-time convection across three-dimensional terrain of the Mountain Empire.

Whether it be the long-term climate system, or a short-term weather pattern, feedback loops are critical components that function to either amplify or dampen the response to forcing mechanisms.

Feedback loops, by definition, change the sensitivity of the response to forcing.

*In this way, it becomes easier to understand how a synoptic-scale pattern, conducive to either wetness or dryness, can become amplified by a positive feedback loop which is naturally unstable in that it acts to push a set of conditions farther away from the initial state over time (that is, wet ground may become wetter and dry ground drier as time passes).

*Nothing in the climate system is ever as simple as it might appear. That is certainly true within complex, three-dimensional terrain where any given system that may be able to generate orographic forcing could potentially overwhelm a positive feedback for continuation of dryness or wetness. Orographic forcing tends to be limited in both duration and strength during the convective (warm) season, but can occasionally become a significant factor in rainfall enhancement [especially over smaller temporal-spacial scales versus the orographic forcing (cold) season (Nov-April) when widespread regional impacts are common].

A simplified (+) feedback loop runs like this, an initial increase in rainfall leads to an increase in soil moisture, which acts to increase evaporation and transpiration (evapotranspiration) from surfaces into the overlying atmosphere where water vapor increases within the troposphere.

A water vapor increase then aids production of additional rainfall (via thermal instability and associated differential heating and latent heat release) to continue this positive loop for amplification of wetness.

On the other hand, an initial decrease in rainfall leads to a decrease in soil moisture, which acts to decrease (reduce) the rate of evaporation and transpiration from surfaces into the overlying atmosphere where water vapor decreases.

A decrease in atmospheric water vapor then acts to hinder production of rainfall to continue and amplify this positive feedback for dryness.

The solid black line connecting the Latent and Sensible heat flux boxes indicate that they are connected in the surface energy balance, and while a moist surface will be dominated by the latent flux and a dry surface by the sensible flux, in reality, they may both operate at the same time with varying magnitudes dependent upon the moisture present within a given location (as suggested).

If you are a student of climatology, you already know where this is heading. This simplified feedback loop (at the top) becomes complicated by fluxes of energy driven by solar radiation and phase changes of water (surface energy budget graphic).

While the overall feedback remains positive, I have included a couple of negative couplings between soil moisture and sensible heat flux as well as between latent heat flux and temperature.

When soil moisture is present, and especially elevated, some of the incoming solar radiation from the sun (called insolation) will be diverted and used for evaporating water and for driving transpiration through vegetation. In other words, a soil moisture increase will tend to amplify the latent heat flux and diminish (dampen) the sensible heat flux such that air temperature tends to be lower when moisture is present for evapotranspiration.

When soil moisture is lacking, and especially decreasing, nearly all of the insolation from the sun will be applied to sensible heating of surfaces which then act to heat up the overlying air. So a lack of soil moisture increases the sensible heat flux, forming a negative coupling (blue arrow above graphic), and the sensible heat flux dominates the surface energy budget to cause air temperature to increase.

The bottom line, there is strong correlation between soil moisture and air temperature during summer, with dry ground tending to support hot temperatures and wet ground supporting cooler, but often more humid, conditions (hotter by daytime, as dry ground often supports cool nights, especially within the mountains)…such that positive feedbacks tend to exert significance influence upon summer-time weather conditions.

Upcoming Weather Pattern

A major weather shift has occurred during July, with a transition from cooler than average conditions (in the mean, below) during May and June giving way to above average July temperatures and a significant drying (above) of surface soils (in the mean).

NCEP/NCAR Reanalysis from the Climate Data Assimilation System (CDAS).

Although rainfall with an incoming front July 10-11 will be important, as well as rainfall next week, this generally sets the stage for the development of heat wave conditions heading into mid-July.

A heat dome core (5940 meters), recently centered southeast of the Four Corners, will be expanding eastward across the USA next week to generate unseasonably hot, mid-summer conditions.

Feedback loops, as described above, will enhance heating over locations that maintain dry surface conditions.

Soil moisture remains above average through a deep layer, but surface soil is drying significantly due to high sun angles (strong insolation) and much below average July rainfall (in the mean, some locations have observed average to above average rainfall).

A ring-of-fire convective pattern may eventually develop, otherwise, hit-miss instability driven convection will form along the mountains.

The extent of such activity, including of course, day-time cloud formations above the mountains, will determine how high temperatures get through next week and how much amplification will occur with positive feedback loops.