Experts say the storm has developed into a "bomb cyclone", when atmospheric pressure takes a big drop creating hurricane winds, tornados and severe blizzards.
"We're already seeing flooding in a number of communities and evacuations in several communities".
Numerous creeks and streams were out of their banks and caused widespread flooding, the National Weather Service office in Des Moines said.
The severe conditions forced the closure of interstates in six states.
Though the storm was expected to carry less snowfall as it moves into the Midwest and the Great Lakes region, the National Weather Service and local officials warned of high winds and possible flash flooding from heavy rains.
Thirteen tornados were reported Thursday in Kentucky, Indiana and MI, said Bob Oravec, a meteorologist with the National Weather Center's Weather Prediction Center in College Park, Maryland, but no immediate reports of serious damage. Almost 1,400 flights were cancelled at Denver airport, where all runways were shut down.
"Widespread and extremely risky flooding will continue today and tonight", the National Weather Service office in Omaha, Nebraska, said on Friday in a statement.
Some flights resumed on Thursday at Denver International Airport and flight cancellations were down to more than 600 on Thursday, from more than 1,300 in the region a day earlier, according to FlightAware.com. Heavy rain caused flooding in eastern parts of both states, as well as in Iowa.
On the east side of the low, spring-like conditions and thunderstorms with wind gusts of up to 160 km/h (100 miles per hour) - the equivalent of a category 2 hurricane - spanned across the southern states, including New Mexico and parts of Texas, Oklahoma and Kansas.
A policeman in Lone Tree, Colo., talks to a driver as snow clogs the towns roads, caught in a later winter storm known as a bomb cyclone.
In parts of Arkansas, Mississippi, Missouri and Tennessee, tornado watches were issued until Thursday afternoon.
Most power outages were cleared by early Friday, according to the tracking site PowerOutage.
In Wolf Creek Pass, near the New Mexico border, has recorded 45 inches of snow.
About 50 National Guard soldiers and airmen used specialized vehicles with tank-like treads to rescue 75 people stranded in their cars during the storm.
Some drivers have been waiting hours to be rescued from roadways in central Colorado.
The Red Cross reported Thursday that 620 people had stayed in shelters overnight in Colorado and in Wyoming, where the state Transportation Department reported snow drifts on Interstate 80 that were as high as 10 feet (3 meters).
"We've had a variety of calls". A large pileup was reported on I-25 southbound near the Palmer Divide south of Denver on Wednesday, involving more than 40 vehicles. Also contributing were Associated Press reporters Dan Elliott in Longmont, Colorado; David Pitt in Des Moines, Iowa; Bob Moen and Mead Gruver in Cheyenne, Wyoming; Gretchen Ehlke in Milwaukee; James Anderson and Thomas Peipert in Denver; Jeff Baenen in Minneapolis; Josh Funk in Omaha, Nebraska; Seth Borenstein in Washington; David Warren in Dallas; Kathleen Foody in Denver.