By Kevin Murphy
KANSAS CITY, Missouri (Reuters) - A spring snowstorm hit Colorado on Tuesday, canceling flights and closing schools as forecasters warned residents in the middle of the United States to brace for sleet, gusty winds, flash floods and tornadoes.
"A dynamic storm system evolving over the central United States will impact much of the nation with a variety of hazards," the National Weather Service said.
Rain turned to snow overnight in the Denver area, which was forecast to get up to 11 inches of snow, much of it by noon. Temperatures that had been in the low 70s (Fahrenheit) on Monday dropped into the teens on Tuesday.
Airline tracker service FlightAware.com said 175 flights in and out of Denver International Airport had been canceled. Most schools in the Denver area were closed, the Denver Post said.
Moderate to heavy snow and gusty winds were forecast for Colorado, Nebraska, South Dakota and Wyoming. Just a day earlier in northeast Colorado, four tornado sightings were reported.
"There will be heavy rains through Thursday of 1.5 inches up to 3 inches in the Plains and central Midwest with 3 to 12 inches of snow in Nebraska, northeast Colorado, South Dakota, Minnesota and northern Iowa," said Don Keeney, a meteorologist for MDA Weather Services, a private forecaster.
Large parts of western South Dakota including the Black Hills could get up to 20 inches of snow through Wednesday morning, the National Weather Service said.
Heavy snow was falling in western Nebraska, where a winter storm warning was in effect and Interstate 80 was closed in both directions due to blowing snow and poor visibility, according to the Nebraska Roads Department.
Areas south and east of the snowstorm will see sleet and freezing rain and potential flash flooding as the storm moves east, the Weather Service said.
Strong to severe thunderstorms were expected to move through north Texas, Oklahoma and Kansas through Tuesday evening, the weather service said.
"This is what we call transitional weather, from winter to spring, when we get a little bit of everything from snow to severe storms," said Fred Stasser, a National Weather Service meteorologist in Goodland, Kansas, near the Colorado border.
Arctic air sweeping east will mix with warm, moist air surging north from the Gulf of Mexico, bringing heavy downpours, damaging winds, large hail and possibly tornadoes to the central part of the nation.
Tornado season in the United States typically starts in the Gulf Coast states in the late winter, and then moves north with the warming weather, peaking around May and trailing off by July.
(Additional reporting by Jane Sutton in Miami, Katie Schubert in Omaha and Carey Gillam in Kansas City; Editing by Eric Beech, David Bailey and Maureen Bavdek)