One day after a derecho caused widespread damage and power outages in Nashville and surrounding areas, a second burst of high winds Monday night toppled more trees and left at least an additional 10,000 Middle Tennessee residents in the dark.
About 80,000 customers remained without power around Nashville on Tuesday morning because of the back-to-back high-wind events.
The arcing derecho that struck Sunday, unleashing 70-to-80-mph gusts over much of Tennessee, turned out be just Mother Nature’s opening act.
Monday night featured another band of severe storms. Strangely, however, the worst winds didn’t arrive ahead of the leading edge of the storms as they usually do. Instead, they accompanied an area of lighter rain behind the storm. That is where the wake low had formed.
How does a wake low form?
Before a band of strong thunderstorms approaches, the warm air just ahead of the storms rises into the clouds. Rain-cooled air crashes down on the backside, contributing to an increase in surface air pressure and a drop in temperatures. But when a wake low forms, additional mechanisms are at play.
When a pocket of air sinks toward the ground, increasing air pressure warms it. But as this happens, the moisture in the air parcel evaporates and robs heat from its surroundings, in turn cooling the air through evaporative cooling. These effects oppose each other.
Most of the time, the precipitation-cooled air that reaches the ground is still colder than the surrounding air. That’s part of the reason we get a chilly breeze behind a summer storm. With a wake low, however, the warming induced by an air parcel’s sinking trumps evaporative cooling, leading to a warmer — and less dense — pocket of air reaching the surface. That creates an area of lower pressure just above the ground.
Since air flows from high pressure toward low pressure, a rush of air surges toward the wake low and can bring strong gusts to the surface.
An impressive event in Nashville
The National Weather Service office in Nashville tweeted that Monday night’s wake low was unusual. “It is quite rare to experience one this intense,” the office wrote.
The area of enhanced winds this evening is the result of what is known as a "wake low." A wake low sometimes forms in the wake of a thunderstorm complex like we had earlier. It is quite rare to experience one this intense.
— NWS Nashville (@NWSNashville) May 5, 2020
“We had a 43-mph wind gust at Nashville International Airport at 10 p.m.,” said Scott Unger, a meteorologist at the Nashville office. “Same thing at Clarksville. We were mostly in the 45-to-50-mph range. A couple employees wrote in and reported the same thing. I know it probably finished off a couple of trees.”
The office even opted to issue a short-fused high-wind warning, between 9:35 p.m. and midnight, cautioning that “damaging winds will blow down trees and power lines. Widespread power outages are expected.”
Detecting a wake low
The data from Nashville International Airport resolves the wake low’s passage remarkably well. Between 9 and 10 p.m., surface air pressure dropped from 1016.3 millibars to 1006.9 millibars — a 9.4-millibar fall. That’s an exceptional fluctuation to have in one hour; at the time, winds were gusting up to 43 mph.
Air pressures slowly rebounded to their previous levels afterward.
“That’s what typically happens,” Unger said. “It can happen after a [mesoscale convective complex].” An MCS is a system of thunderstorms often on the order of hundreds of miles long.
In addition to the pressure and wind signatures, it’s sometimes possible to spot a wake low using radar, since the processes that give rise to them eat away at falling rain.
“On radar, you can see the rain eroding very quickly on the backside,” Unger said. “We had a big shield of rain across the north.”
It’s been a long week for Nashville weather-wise, but Music City should soon catch a break. Sunshine dominates Wednesday and Thursday, with only an outside chance of a couple thunderstorms Friday.
