As Winter Storm Titan burst through the Ohio and Mississippi Valley Sunday, the region become ground zero for a rare winter phenomenon.

According to the National Weather Service, thundersleet was reported in at least six states: Arkansas, Illinois, Kentucky, Missouri, Oklahoma and Texas.

(MORE:Sleet and Freezing Rain Are NOT The Same)

"Thundersleet is amazing because it requires such a specific, rare combination of elements to happen," weather.com meteorologist Nick Wiltgen explained. "There has to be warm, unstable air aloft to form updrafts that lead to thunderstorms – but there also has to be a deep layer of cold air near the ground so that raindrops will freeze into ice pellets (sleet) before hitting the ground."

"That combination doesn't happen very frequently," Wiltgen continued. "It was especially amazing to see it happen over such a large area Sunday in places where millions of people live."

Oklahoma

The sleet accumulations were also unusually heavy. NWS measured as much as six inches on the ground in parts of Carroll and Montgomery County, Tenn. Photos surfacing on social media show sleet drifts more than 11-inches high.

Tennessee

Arkansas

Although the storm's worst impacts are forecast to end Monday night, cold temperatures will linger to create slick conditions across the Carolinas into the Mid-atlantic Tuesday morning. You can get more updates at this link on weather.com.

MORE ON WEATHER.COM: Winter Storm Titan

A snowplow clears snow on Capitol Hill in Washington, Monday, March 3, 2014. The National Weather Service has issued a Winter Storm Warning for the greater Washington Metropolitan region, prompting area schools and the federal government to close for the wintry weather. (J. Scott Applewhite/AP Photo)