* Tornadoes in Alabama and Arkansas
* Blizzard warnings for US Midwest and central Plains
* Some 340,000 customers without power across U.S. Midwest, South (Adds new details on flight cancellations, updated outages, deaths)
CHICAGO, Dec 20 (Reuters) - The first major winter storm of the year hit the U.S. Midwest on Thursday, bringing a blizzard to the Plains and tornadoes to Alabama and Arkansas, and leaving some 133,000 customers without electricity.
Bad driving conditions led to a 25-car pileup on a highway near Clarion, Iowa, that left three people dead, authorities said. Blizzard warnings were in effect in eastern Iowa and parts of Wisconsin and Illinois Thursday afternoon, according to the National Weather Service.
"It's going to be very windy with considerable blowing and drifting of snow," said Bruce Terry, a senior National Weather Service forecaster at the HydroMeteorological Prediction Center in College Park, Maryland. He called the pre-Christmas storm "a major winter snowstorm" for the Midwest and western Great Lakes.
Accumulations of up to a foot (30 cm) of snow were expected in some areas, Terry said, adding there was a potential for severe weather on the so-called "warm side" of the storm in the U.S. Southeast.
Blowing snow led to school closures in parts of Iowa, Nebraska, Kansas and Missouri, plus the closure of all state government offices in Iowa.
"Thunder" snow was reported in Iowa Wednesday night, especially in southeastern Iowa, as thunder and lightning accompanied the storm as it surged across the state.
Travel was not advised on Iowa roads for the rest of the day, according to Annette Dunn with the Iowa Department of Transportation.
"We're going to have visibility and drifting problems through midnight," she said.
Late Thursday morning, troopers responded to a 25-car crash which killed three people on southbound Interstate 35 in northern Iowa. Iowa DOT closed I-35 at Highway 30 due to deteriorating conditions.
The Iowa National Guard has deployed about 80 soldiers from across the state to help highway assistance teams cope with the storm.
In Nebraska, portions of I-80 were closed Thursday due to snow-packed and icy road conditions. The entire road was expected to reopen before 4 p.m. local time.
In Chicago, rain was expected to change to snow Thursday night, with wind gusts of as much as 50 miles per hour, the NWS said.
Due to low visibility, airlines at Chicago's O'Hare International Airport were reporting delays of up to 90 minutes and have canceled more than 200 flights. At Midway Airport in Chicago, airlines canceled 150 flights and Southwest Airlines canceled all flights after 4:30 p.m.
A twister near downtown Mobile, Alabama, damaged buildings, snapped trees, downed power lines and flipped vehicles early on Thursday, but there were no reports of injuries, authorities said.
"The potential is there certainly for some isolated tornadoes," Terry said, referring to a broad swath of Gulf of Mexico coast and inland territory stretching from southeast Louisiana through the western Florida Panhandle.
The National Weather Service confirmed on Thursday that a tornado destroyed a mobile home southwest of Sheridan, Arkansas. There were no reports of injuries.
High winds of around 45 miles per hour in Tennessee knocked down trees and power lines.
While the heavy snow in the Upper Midwest will create potentially dangerous travel conditions, meteorologist Jeff Masters said it put an end to this year's "record-length snowless streaks in a number of U.S. cities."
Writing on his website weatherunderground.com, Masters said the storm would also provide "welcome moisture for drought-parched areas of the Midwest."
The winter storm, named Draco by the Weather Channel, began Tuesday in the Rocky Mountains and marked a dramatic change from the mild December so far in most of the nation.
High winds kicked up a dust storm in West Texas on Wednesday, leading to at least one death in a traffic accident near Lubbock.
Power companies reported electrical outages in Iowa, Nebraska, Arkansas, Louisiana, Texas, Kansas, Missouri, Alabama, Mississippi, Oklahoma, West Virginia, Virginia and Tennessee, with a peak of 400,000 customers without power Thursday morning. That fell fell to 133,000 by Thursday afternoon. (Writing by Tom Brown and Nick Carey; Reporting by Mary Wisniewski in Chicago, Eileen O'Grady in Houston, Kaija Wilkinson in Mobile, Alabama and Keith Coffman in Denver, Tim Ghianni in Nashville, Kay Henderson in Des Moines, Iowa, Kevin Murphy in Kansas City, Brendan O'Brien in Milwaukee, Matthew Waller in San Angelo, Texas and Suzi Parker in Little Rock, Arkansas.; Editing by Bernadette Baum, Greg McCune, Tim Dobbyn and Jim Marshall)