Snowplows were sent out on Denver's streets early Monday while the threat of tornadoes and severe thunderstorms loomed over the nation's heartland, forecasters warned.
A Mother's Day storm dropped more than a foot of sloppy, wet snow on parts of Colorado and Wyoming, with the National Weather Service issuing a winter storm warning for some areas of both states into Monday morning.
"The [tornado] threat area today goes from southern Michigan into Minnesota and then southeast into Missouri," The Weather Channel's Kevin Roth said. "Iowa and Wisconsin could also be threatened."
Powerful thunderstorms produced tornadoes as they moved across Nebraska on Sunday caused damage in several towns and rural areas in the east of the state.
Officials said the storms damaged homes and businesses in or near Sutton, Garland, Cordova and Daykin, and knocked out power to 18,000 utility customers.
Homes were also damaged in Iowa, according NBC station WOWT.
Large hail and strong winds seen in the state were expected to head south into Kansas, and a tornado watch was issued for parts of Oklahoma.
The storm was expected to weaken as it heads northeast from the Plains, possibly bringing rain as it moves into the Great Lakes, the weather service said.
The Weather Channel's Roth said the tornado threat would "almost completely" be gone by Tuesday, but added: "There will be another storm system coming through the Tennessee and Ohio Valley areas. We can't rule out tornadoes but there doesn't look like a severe threat at this point."
In Missouri, where a tornado damaged up to 300 homes in the town of Orrick on Saturday, Gov. Jay Nixon issued an emergency declaration and urged people to stay alert and heed weather warnings.
Denver officials said they were deploying 70 snowplows overnight to prepare for Monday's commute.
"May snow certainly isn't unheard of here in Colorado, even down in the Denver metro area," said David Barjenbruch, a Boulder-based weather service meteorologist. "If we see the total accumulations that we are anticipating from this storm, we are certainly going to see a top 10 May snow event for the Denver metro area."
The weather service said mountainous areas in south-central Wyoming got up to 2 feet of snow, and the metro areas of Cheyenne and Laramie averaged 6 to 10 inches.
—By Henry Austin of NBC News. The Associated Press contributed to this report.