Damaging thunderstorms were most likely in a zone from northern Mississippi and northern Alabama to central Pennsylvania and western Maryland, Accuweather.com said. The tornado risk was highest in the mountains from Tennessee to Pennsylvania.
Storms were expected to reach the East Coast Monday evening.
Several tornadoes were reported across Wisconsin Sunday evening. Worst-hit was the town of Merrill, where numerous homes and businesses were destroyed and power was knocked out to thousands, authorities said. Some injuries were also reported.
Preliminary reports from the National Weather Service indicate tornadoes in Winnebago, Eau Claire, Adams, Marathon and Lincoln counties, The Milwaukee Journal Sentinel reported.
La Crosse, Wis., hit 81 degrees Sunday afternoon, followed by a thunderstorm and large hail, The La Crosse Tribune reported. Temperatures were expected to fall sharply through the week.