Feb. 12 (Bloomberg) -- Snow, freezing rain and sleet threatened parts of Illinois, Iowa, Indiana and Ohio today, the first of a series of storms expected to tangle travel across much of the northern U.S. from Iowa to Maine.

``There is a huge region of the Midwest and Northeast that is going to see travel conditions deteriorate over the next 24 hours,'' said Brian Korty, a meteorologist with the National Weather Service's Hydrometeorological Prediction Center in Camp Springs, Maryland.

Chicago's O'Hare International Airport reported arrivals delayed almost an hour today because of low visibility, according to the U.S. Federal Aviation Administration.

The National Weather Service issued a revised warning for ``a potentially dangerous winter storm'' expected to hit Chicago and suburbs in northern Illinois and northwestern Indiana tonight and tomorrow.

Heavy snow and winds of 35 mph or more will cause dangerous travel conditions in Illinois, especially in the central part of the state, the service said. Areas of Missouri are expected to receive as much as 8 inches of snow, the agency said.

Travel Trouble Anticipated

AMR Corp.'s American Airlines, which operates 300 departures a day out of O'Hare, ``will certainly be canceling some flights in and out of the Chicago hub, as well as into and out of the Northeast, especially the Washington and New York areas,'' spokesman John Hotard said in an interview. O'Hare is the airline's second largest hub, after Dallas-Fort Worth International Airport.

The airline will decide on cancellations tonight, and travelers with flights late tonight or tomorrow should check with their travel agents or the airline's Web site, he said.

American is offering travelers to and from O'Hare, Philadelphia, all three New York-area airports and all three Washington-area airports one free itinerary change because of the storm, spokeswoman Mary Frances Fagan said in an e-mail. The offer is for tickets booked by today for travel through Feb. 18.

UAL Corp.'s United Airlines, which operates its biggest hub at O'Hare, said in an e-mailed statement that it has made a similar offer to customers traveling in 15 states stretching from Colorado to Connecticut.

Chicago Airports

Southwest Airlines Co., the largest carrier operating at Chicago's Midway International Airport, is letting customers with reservations through Feb. 14 rebook to avoid possible weather delays, spokeswoman Beth Harbin said in an e-mail. She said she wasn't aware of any planned cancellations.

The policy applies to airports in Indianapolis; Kansas City, Missouri; St. Louis; Oklahoma City; Tulsa, Oklahoma; Omaha, Nebraska; Cleveland; Detroit; Pittsburgh; Philadelphia; Washington Dulles; Baltimore; Albany, Islip and Buffalo, New York; Providence, Rhode Island, and Manchester, New Hampshire, she said.

As the storm over the Midwest moves east, it's forecast to combine with another winter storm moving in from the Atlantic Ocean and drop even more snow and ice over the U.S. Northeast. As much as 2 feet of snow is likely by midweek over parts of upstate New York that have received as much as 146 inches (371 centimeters) from lake-effect storms since Feb. 3.

While snow is forecast for parts of northern Indiana, northern Ohio, northern Illinois, Missouri, the interior of New England and upstate New York, areas in the southern portion of the band of winter weather are likely to receive a combination of snow, sleet and freezing rain. That line stretches from Ohio to include most of West Virginia, Virginia, Maryland and the District of Columbia.

Snowy Valentine's Day

``The D.C. area will probably see what's defined as a wintry mix,'' Korty said. ``They'll probably see very light snow starting late tonight and into tomorrow morning. As tomorrow goes on, that will change into a winter mix and freezing rain. They'll get freezing rain tomorrow afternoon, evening and early tomorrow night before it changes into a regular rain.''

The heaviest snow on the East Coast isn't expected until Feb. 14, Casey Crosbie, a meteorologist at the National Storm Prediction Center in Norman, Oklahoma, said yesterday. Travel in Washington and Philadelphia is likely to be impaired because of slippery conditions from freezing rain and sleet, he said.

New York City, Connecticut and eastern Massachusetts probably will receive 6 inches of snow and cold rain showers, Crosbie said.

New York and much of the Northeast have experienced an unusually warm start to winter. Snow flurries that dusted New York's Central Park on Jan. 10 were the latest to arrive since recordkeeping started in 1869, according to the National Weather Service. December's average temperature of 43.6 degrees Fahrenheit (6.4 Celsius) was the third-warmest on record.

To contact the reporters on this story: Margot Habiby in Dallas at mhabiby@bloomberg.net; Adam Satariano in San Francisco at Asatariano1@bloomberg.net.

Last Updated: February 12, 2007 19:56 EST