DENVER (AP) — A powerful spring storm dropped more than a foot of sloppy, wet snow in parts of Colorado and Wyoming on Mother's Day, causing crashes and leading to road closures, and forecasters are warning that conditions could get worse as temperatures plummet overnight.

The National Weather Service issued a winter storm warning for most of northern Colorado and parts of southern Wyoming for all of Sunday and for Monday morning.

Forecasters were also warning that strong thunderstorms and tornadoes could develop in Nebraska and Iowa on Sunday. A tornado was reported in south-central Nebraska, the weather service said, but no significant damage was seen immediately. There was a moderate risk of severe weather in the area into Sunday night, the weather service said.

In Colorado, Department of Transportation officials said plunging temperatures and heavy, wet snow have created icy conditions and forced several closures along Interstate 70 west of Denver on Sunday afternoon. Multiple accidents were reported on the mountain corridor, frustrating skiers and snowboarders eager to get a few more runs in before the season ends. Authorities also closed parts of Interstate 25 because of several accidents Sunday afternoon.

Snow amounts could vary greatly, but up to 15 inches could fall at higher elevations and 4 to 9 inches could fall at lower elevations, including Denver and other cities along Colorado's Front Range.

"May snow certainly isn't unheard of here in Colorado, even down in the Denver metro area," said David Barjenbruch, a weather service meteorologist in Boulder. "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."

Barjenbruch said a foot of snow had already fallen in the foothills of Larimer County northwest of Denver by Sunday morning, and workers along much of the Front Range can expect a "slushy, sloppy morning commute" Monday.

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The weather service also warned that snow could be heavy and wet enough to snap tree limbs and power lines, causing power failures. Winds gusting up to 30 mph could reduce visibility, and slushy roads could be treacherous to drive.

Julie Smith, a spokeswoman for Denver International Airport, said crews have treated runways in anticipation of dropping temperatures Sunday night.

"At this point we are seeing some delays with our airlines while they are getting their deicing operations up and running, and we do expect the airlines to be fully deicing in the morning," she said.

Meanwhile, Denver officials plan to deploy up to 70 snowplows overnight to prepare for Monday's commute.

In southern Wyoming, the storm forced transportation officials to close a 150-mile stretch of Interstate 80 from Cheyenne to Rawlins on Sunday morning.

The weather service said mountainous areas in Wyoming could get a foot or more of snow, and 5 to 10 inches are forecast for Cheyenne and Laramie.

The storm is the result of a low-pressure system moving east colliding with a cold air mass from the north. It's expected to deliver rain in the northern Plains once it leaves Colorado.

Spring-like weather was expected to return to the Rockies by Tuesday, with mostly sunny skies and temperatures in the mid-50s in Denver, the high 40s in Cheyenne and the low 40s in Laramie.