Story highlights A storm rips most of the roof off a school in Lake City, Iowa

Flood rescues take place in Denton County, Texas

A tornado in Delmont, South Dakota damages some 20 buildings

(CNN) Across the United States, 33 million people face the risk of severe weather Sunday.

The greatest potential for severe storms will be from parts of southeast Oklahoma into northeast and east-central Texas.

The storms could spark tornadoes, particularly over northeast Texas.

Large hail could also slam parts of the region.

Flood rescues were under way in Denton County, Texas, on Sunday, just outside the Dallas-Fort Worth area. Aerial images from CNN affiliate KTVT showed severe flooding and people being lifted and then carried to safety in a helicopter.

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