A powerful storm system hit the Midwest on Sunday, trailinglightning, hail, high winds and heavy rain into Monday morning.

The Storm Prediction Center (SPC) preliminarily registered 67tornado touchdowns in several states, as well as straight-linewinds of more than 85 mph and baseball-sized hail. Six people werekilled, including at least five in Illinois.

According to Aon Benfield, Nov. 17 provided “highly conducive”atmospheric conditions for explosive thunderstorm development.Illinois, Indiana and Kentucky bore the brunt of the storm'sdamage, which the SPC says at its peak threatened more than 53million people in 10 states. The National Weather Service alsoissued high-risk weather alerts for Ohio and Michigan over theweekend.

SLIDESHOW: Tornadoes, Severe Storms Devastate Midwest

“This is very unusual for this time of year,” Jeff Ormond,Allstate spokesperson for the insurer's regional office overseeingOhio, Michigan and Indiana, told PC360. “Usually theregion would see like weather in the warmer months of May throughSeptember. We did have tornados in Henryville, Indiana and Dexter,Michigan in March 2012, which was very unusual at that timetoo.”

Illinois saw the worst devastation, says Aon. National WeatherService (NWS) meteorologists recorded an EF-4 twister thatcompletely wiped a farmhouse off its foundation, killing an elderlyman and his sister. An EF-2 tornado tracked across Grundy and Willcounties, and a deadly tornado in the Peoria region destroyed homesand an apartment complex. Other touchdowns destroyed homes,structures and mobile homes.

Local news reported hospitals were treating patients for severestorm-related trauma injuries.

Thunderstorms, hail and thousands of power outages in theChicago area grounded flights at O'Hare and midway internationalairports and delayed an NFL game between the Baltimore Ravens andChicago Bears at Soldier Field. Fans were evacuated during thegame's first quarter.

At least 12 counties in Indiana reported tornado damage, withstates of emergency declared in Howard County, home to the towns ofKokomo, Russiaville and Greentown. Severe damage was also reportedin Lafayette, Washington and Lebanon cities. One tornado inLogansport caused gas leaks that prompted several complexes to beevacuated.

Eight Kentucky counties reported tornado touchdowns. The GreatLakes were also affected, primarily by straight-line winds thatleft 253,000 customers without electricity in Michigan. Similarwinds of up to 70 mph were felt in Missouri, Ohio andWisconsin.

Meteorologists from Weather Underground and the Weather Channelsay tornadoes are possible in the Ohio Valley and Midwest earlyMonday, with subsiding but still-strong winds to move into theNortheast by evening.

It is too early to tell the scope of the losses, says LorettaWorters, spokesperson for the Insurance Information Institute, butexpects that “hundreds of homes and businesses were damaged ordestroyed.”

“Insurers stand ready to help policyholders with the recoveryprocess. Claims adjusters have already begun to helppolicyholders file claims,” said Jim Whittle, chief claims counselfor the American Insurance Association, in a statement. “AIA'smember companies have begun communicating with policyholdersthrough a variety of channels, including Facebook andTwitter.”

Ormond says Allstate has mobile claims units en route to severalof the affected locations, where agents have seen severe windstormdamage.

Until Sunday, economic and insured losses from severe weather in2013—the quietest U.S. tornado year since 1988—were slightly belowthe 10-year average, records the Storm Prediction Center.

U.S. thunderstorms cost $14.9 billion insured losses in 2012,reports Munich Re. The U.S. experiences more tornadoes than anyother country worldwide.