DENVER - A powerful spring storm dropped more than a foot of sloppy, wet snow in parts of Colorado and Wyoming on Mother's Day, and forecasters warned that conditions could get worse as temperatures plummeted 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 Monday morning.

Forecasters were also warning that strong thunderstorms and possibly tornadoes could develop in eastern Nebraska and Iowa Sunday afternoon. There was a moderate risk of severe weather in the area starting Sunday afternoon and continuing into Sunday night, the Weather Service said.

In Colorado, 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.

"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," said David Barjenbruch, a Weather Service meteorologist in Boulder.

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 could expect a "slushy, sloppy morning commute" Monday.

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 m.p.h. could reduce visibility, and slushy roads could be treacherous to drive on.

Colorado Department of Transportation officials said plunging temperatures and heavy, wet snow 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.