A "very active and complex mid-May weather pattern" is settling over states from the central United States through the Mid-Atlantic.

Severe thunderstorms and tornado watches are in place on Tuesday, with 40 million Americans on alert for strong winds, large hail and flash flooding.

There were reports of tornadoes in Nebraska and Oklahoma, and more than 150,000 customers had no power early Tuesday.

Strong tornadoes, damaging winds and hail are again threatening states across the Mississippi Valley on May 20.