A long-lived complex of thunderstorms formed over the Upper Midwest Sunday evening and dissolved over the southern Appalachians on Monday night, but was the complex a derecho?
According to AccuWeather Meteorologist Meghan Mussoline, "A derecho is often referred to as an inland hurricane, in terms of ferocious wind and torrential rain."
A derecho is capable of downing many trees along its path, as well as damaging or destroying weakly-constructed buildings, damaging roofs and overturning high-profile vehicles.
More specifically, the Storm Prediction Center (SPC) describes a derecho as a complex of severe thunderstorms that must travel along a path of at least 240 miles with widespread and long-lived, straight-line wind damage resulting from wind gusts of 58 mph or higher; the key is that the 58 mph or higher wind gusts must be consistent along much of the path of the severe storms. Straight-line winds refer to non-tornado-produced winds.
In the case of the event spanning Sunday evening, July 12, to Monday night, July 13, there were approximately 550 reports of damaging winds. However, in many cases the wind speed that caused the damage is not specifically known. Damaging winds extended along a path of about 800 miles.
According to AccuWeather Senior Meteorologist Bernie Rayno, "During the complex spanning Sunday evening to Monday night, the intensity of the thunderstorms ramped up and throttled back at times, based on official National Weather Service reporting sites."
Incidents of the criteria high wind gusts were spotty over southern Minnesota during Sunday evening.
By early Monday morning, multiple parts of southern Wisconsin and northwestern Illinois were hit with gusts topping 60 mph.
During the morning rush hour Monday, wind gusts had slipped back around the Chicago metro area, but by the midday hours on Monday, the storms were producing gusts near 60 mph once again as they approached Indianapolis.
The storms weakened moving into the Louisville, Kentucky, and Cincinnati areas.
While most of the storms were below the critical wind speed in Kentucky, West Virginia and northeastern Tennessee Monday afternoon and night, they did down trees and cause incidents of flash flooding.
The storms then weakened considerably moving into North Carolina and south-central Virginia by Tuesday morning.
A contributing factor to the scores of downed trees along the way from Minnesota to western Virginia and northeastern Tennessee may also be linked with saturated soil and waterlogged tree limbs. Under these conditions, even moderate sudden wind gusts could cause more trees to fall than with average soil moisture.
When trees come down, they often take power lines with them. Utilities reported more than 150,000 power outages along the path of the storms.
As a result of the cycling of the strength of the complex of the storms, SPC has not as of this writing referred to the event as a derecho, but rather a squall line.
This graphic shows the long-lived history (since Sunday PM) of the squall line currently moving southeast eastern KY. pic.twitter.com/7vheFunoQv— NWS SPC (@NWSSPC) July 13, 2015
Officials will continue to analyze the damage and official wind reports along the path of the squall line.
It is possible there may have been segments along the way where the criteria was achieved for a derecho.