At least 650,000 customers in the Midwest, mainly in Michigan, were without electricity Wednesday after a powerful line of thunderstorms swept through with damaging winds. Stretching for more than 400 miles from central Michigan to eastern Kentucky, the squall line produced widespread winds of 60 to 70 mph, with a few gusts topping 80 mph.

More than a quarter-million Michigan residents remained in the dark Thursday morning. The line of storms fit the definition of a derecho: a fast-moving line of violent storms that produces widespread damage along its path. Derechos come in varying shapes and sizes but frequently leave swaths of wind damage that can stretch several states in length.

Winds gusted to 75 mph in Grandville, Mich., and 76 mph in Sweetser, Ind. A 77 mph gust was recorded in Clark County, Ky., while Miami, Ind., recorded the day’s strongest winds, with an 80 mph gust.

The Sandusky State Theatre in Sandusky, Ohio, was heavily damaged when strong winds peeled off its roof and brought down several brick walls. The iconic business has been a landmark in the Lake Erie shoreside town since 1928.

Farther east, an apparent tornado touched down in Beaver County, Pa., shortly after 9:30 p.m. Wednesday. Weather spotters reported power flashes, and radar detected a brief debris signature in the area, indicating debris present in the atmosphere.

A well-predicted windstorm

The storms prompted the National Weather Service’s Storm Prediction Center to declare a Level 4 out of 5 “moderate risk” for severe weather, advertising the potential for widespread severe winds.

The storms met the criteria needed to be called a derecho — including producing extensive winds along a path several hundreds of miles long. On radar, several curved “bows” spanned segments along the line, a structure commonly exhibited by derechos. A few small-scale, weak vortices occurred in between. Most derechos feature a “northern bookend vortex” a bit more prominently.

On the other hand, despite nearly 500 reports of damaging winds, only eight were of “significant” gusts eclipsing 75 mph. However, this specific wind-gust criterion is not unanimously agreed to in the meteorology community.

But despite lacking some elements of a classic structure, Wednesday’s storms were a bona fide low-end “serial” derecho. Serial derechos usually form ahead of a cold front, with a swooping jet stream overhead providing the wind energy needed to generate powerful gusts.

How Tropical Storm Cristobal’s remnants helped prime the atmosphere for strong storms

Apropos of Wednesday’s storms, some of the energy needed to trigger storms came from the remnants of Tropical Storm Cristobal, which merged with a mid-latitude cyclone over the Great Lakes and southern Canada after making landfall Sunday in Louisiana.

Moreover, Cristobal also dragged north a ribbon of exceptional tropical moisture. Detroit’s morning weather balloon launch Wednesday set a record for the most precipitable water observed at that location through June 15, with records dating to 1949.

‘Derecho season’ in full swing across the United States

Wednesday’s low-end derecho was the third derecho to sweep across the United States in the past week.

On June 3, a localized, progressive derecho developed in western Pennsylvania and surged east on a crash course with Philadelphia. Four were killed by widespread damaging winds that reached 80 mph in the Philadelphia area. Farther east, several gusts over 90 mph were measured in Ocean County, N.J.

A more widespread serial derecho raced from central Colorado and Utah northeastward across Wyoming, the Sandhills of Nebraska, and into western South Dakota on Saturday. A 99 mph wind gust was clocked in Great Divide, Colo., with a 110 mph gust measured atop a mountain in Winter Park, Colo.

A number of other significant gusts between 75 mph and 90 mph accompanied the vicious storms as they blasted northeastward.

Capital Weather Gang’s Jeff Halverson contributed to this report.