What makes Kentucky's devastating tornadoes so rare

At least 22 reported tornadoes ripped across six states overnight, devastating communities and killing dozens of people, including possibly over 70 in Kentucky, during what is typically a quieter time in the United States for the storms.

Tornadoes can happen any time of year, though the greatest threat is typically in spring and summer, according to the National Oceanic & Atmospheric Administration, with the peak season on the earlier side for more southern states. So far this year, the days that reported the highest number of tornadoes were in March, May and July, a National Weather Service tally shows.

That's what makes the latest activity so rare, experts say.

"Something like this is an unusual event for the month of December. It's typically our quietest month for tornadoes, especially in Kentucky," ABC News meteorologist Rob Marciano told "Good Morning America" on Saturday.

It's unclear if climate change could play a role in the activity, he said.

"There's no evidence that climate change has any impact on the strength of severe storms or tornadoes," he said. "That said, to get a tornado this strength and magnitude or length -- in December -- is incredibly rare."

An average of 24 tornadoes were reported in the U.S. during the month of December from 1991 to 2010, according to data from NOAA's Storm Prediction Center. Last year, there were 30 preliminary tornado reports in December.

At least 22 tornadoes alone were reported Friday night through Saturday morning in Arkansas, Kentucky, Illinois, Mississippi, Missouri and Tennessee. A powerful supercell thunderstorm traveled more than 200 miles, from Arkansas to Kentucky, and likely spawned several massive tornadoes.

Kentucky is combing through the wreckage of likely the deadliest tornado system in state history, with at least four tornadoes reported in western Kentucky.

Among the at least 70 people believed to have been killed in western Kentucky, dozens were at a candle factory in Mayfield, according to Gov. Andy Beshear.

"This has been the most devastating tornado event in our state's history," Beshear said during a press briefing Saturday. "And for those that have seen it, what it's done here in Graves County and elsewhere, it is indescribable. The level of devastation is unlike anything I've ever seen."

At least six people also were killed in southern Illinois, when an Amazon distribution warehouse in Edwardsville was ravaged by a tornado and collapsed, officials said.

In Arkansas, two people were reported dead from a tornado, according to Gov. Asa Hutchinson.

The deadliest tornado in Kentucky history was March 27, 1890, when 76 people died, according to the National Weather Service. Beshear said he anticipates the current death toll will exceed 100.

In more recent memory, Kentucky's only recorded F5 tornado killed more than 30 people on April 4, 1974.

One of Kentucky's most violent storms to have occurred later in the year was a multiple-vortex tornado that destroyed over 150 buildings in Hopkins County on Nov. 15, 2005, according to NWS. No fatalities were reported.

ABC News' Kelly McCarthy and Dan Peck contributed to this report.