The scene of a deadly 25-vehicle pileup on Dec. 20 north of Des Moines. Authorities said drivers were blinded by snow and didn't see vehicles that had slowed or stopped on Interstate 80. / Iowa State Patrol
A major snowstorm that was sweeping across the Midwest and beyond on Thursday left at least seven people dead, clogged highways and forced hundreds of flight delays and cancellations at airports already bracing for the holiday rush.

The deaths included a woman in Utah who died trying to walk for help after her car became stuck in the blizzard. Storm-related traffic deaths also were reported in Kansas, Iowa, Wisconsin and Nebraska.

Blizzard warnings were in effect in Iowa, Wisconsin, Minnesota and Illinois, according to the National Weather Service. The Weather Channel reported that parts of 17 states are under winter weather advisories -- as far west as Washington state and as far east as Maine.

The southern edge of the storm system brought winds and damaged homes in Arkansas, the National Weather Service said. Alabama has confirmed that a tornado hit Mobile on Thursday morning, but the tornado's length and intensity were still being assessed. Tornado watches remained in place in Georgia and Florida.

The storm brought much-needed precipitation for drought-plagued regions but was bad news for holiday travelers. Most of the nation's big airlines responded to the storm's threat by enacting flexible rebooking policies for passengers scheduled to fly into the storm's path.

As of 5 p.m. ET Thursday, more than 1,000 flights had been canceled at airports across the Midwest, according to flight-tracking service

More than 600 flights were canceled at Chicago's two airports, O'Hare and Midway. Other flights at O'Hare were delayed between 30 and 90 minutes, the city's Department of Aviation reported at 3:30 p.m. CT. FlightAware reported inbound delays averaging 2½ hours because of high winds.

American Airlines announced that it was canceling flights scheduled to depart after 8 p.m. CT and that flights delayed earlier might leave after 8 p.m.

O'Hare is a major hub for both United and American, meaning the disruption could ripple out and affect fliers at other airports.

Southwest, which flies more domestic passengers than any other airline, said it is suspending all of its flights at Midway as of 4 p.m. CT and at Milwaukee as of 5 p.m. CT, Southwest spokeswoman Olga Romero told USA TODAY's Today in the Sky blog.

The airline already had suspended operations Thursday morning in Kansas City.

Storms knocked out power to an estimated 400,000 customers in 13 states -- Iowa, Nebraska, Arkansas, Louisiana, Texas, Kansas, Missouri, Alabama, Mississippi, Oklahoma, West Virginia, Virginia and Tennessee.

Overnight, about 65,000 customers in East Texas were without electricity. By midmorning, 7,000 still did not have power.

More than 30,000 Kansas City Power & Light customers in Kansas and Missouri lost power in the morning, but by midafternoon the number was down to 6,380, mostly in Missouri.

The snowstorm was walloping Iowa, and about a foot of snow had fallen in Des Moines. State police said a 25-vehicle pileup killed one person, and across Iowa 30,900 customers were without power, including more than 25,000 in the Des Moines area.

More than 36,000 Omaha Public Power District customers lost power in eastern Nebraska, including 25,000 in Omaha. Efforts to restore electricity were hampered by 50-mph winds and heavy snow.

Blizzard conditions remained across portions of Iowa, Wisconsin, Illinois and Minnesota late Thursday.

The heaviest snow is falling at a rate of up to an inch per hour in this zone, according to AccuWeather. Gusts up to 50 mph are causing blowing snow, lowering the visibility to less than a quarter of a mile.

Some of the heaviest snow fell in Iowa and Wisconsin, where amounts of 12-18 inches were common, the National Weather Service reported.

Heavy snow and howling winds were rapidly closing in on Chicago, which had seen rain all day. Snow was forecast to start falling overnight in Chicago, with 1-3 inches expected.

In southeastern Wisconsin, where a blizzard warning was in effect, sheriff's officials said slick conditions led to at least two fatalities late Wednesday when a driver lost control of his car in Rock County, about 90 miles northwest of Chicago.

The weather service was forecasting "thundersnow" in Milwaukee and the surrounding areas, where snow could fall at the rate of 2-3 inches an hour.

Wisconsin Gov. Scott Walker issued an executive order declaring a state of emergency, which put the state emergency government, National Guard, State Patrol and other agencies on high alert.

Travel Trouble:New winter storm could snarl Christmas flights

At least two people in Nebraska died in crashes blamed on poor driving conditions, the State Patrol said. In many areas, drivers were being told to stay off the roads because of whiteout conditions. Most of I-80 in the state was closed. Nebraska's biggest school districts canceled classes, as did many districts across the region.

In Utah, Washington County sheriff's detective Nate Abbott says the woman and a man were driving when their car got stuck in rural Washington County on Tuesday night during the storm that hit the Rocky Mountain region.

He says the pair started walking for help, but the woman couldn't go any farther and sought shelter while the man continued on. Search-and-rescue teams later found her body.

Although the Plains and Midwest could do without the travel headaches, precipitation should be welcome.

More than 93% of the high Plains region and 54% of the Midwest are enduring drought conditions, according to Thursday's U.S. Drought Monitor, a federal website that tracks drought.

Out west, the leading edge of a powerful storm system arrived in Washington, Oregon and Northern California. Rain is expected across Northern California from late Thursday through the weekend, with heavy snow possible in the Sierra Nevada range as the system moves east.

Four to seven feet of snow is forecast to fall above 7,000 feet, prompting the U.S. Forest Service to issue an avalanche warning for Mount Shasta.

Source: USA TODAY ; Contributing: Ben Mutzabaugh, Michael Winter; the Associated Press; The Des Moines Register