Almost 50 million Americans from Wisconsin to Ohio and Kentucky were facing the threat of severe thunderstorms and possible tornadoes Monday, forecasters said.
Damaging winds, heavy rain and lightning were likely for many parts of the Midwest, according to the Weather Channel.
Tornado sirens sounded in several counties in Wisconsin, while central Illinois was a greatest risk of twisters. However, the larger threat was from thunderstorms and localized flooding in areas that have already seen heavy rainfall.
A severe thunderstorm watch was in place until 10 a.m. CT (9 a.m. ET) for southern Wisconsin, northern Illinois and northwestern Indiana — including Chicago and Milwaukee — the Weather Channel said. Over 47 million people were in areas of "slight risk or worse," it said.
"The threat could stretch as far northeast as central Ohio," lead forecaster Kevin Roth said. "Altogether quite a few states will be affected."
He added: "We can't rule out tornadoes but damaging winds and lightning are the main threat."
The severe weather was predicted to move southeastwards during Monday and into Tuesday, affecting Kentucky and Tennessee through to the Carolinas and northern Georgia, Roth said.
Meanwhile, the South was sweltering in under summer temperatures. Dallas could top 100 degrees Monday, Roth added.
Kansas City was also under a heat warning.