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(CNN) -- Here we go again.
More of the bad weather that has delayed, bumped and stranded passengers this winter is on the way, forecasters said Monday.
With a snowstorm of historic proportions expected in Chicago and large sections of the country's midsection, some airlines are already allowing passengers to change plans.
The storm will affect about three-quarters of the United States, from New England all the way to New Mexico, said CNN meteorologist Rob Marciano. Travelers should brace for a "three, three-and-a-half day event," he warned.
"Travel will likely be difficult to impossible in many areas," the National Weather Service said.
Snowfall, primarily on Tuesday, could be more than 12 inches in parts of Missouri and Illinois. Chicago-area airports are monitoring the forecasts and preparing accordingly, officials said.
"We certainly are concerned any time there are inclement weather conditions," said Karen Pride, a spokeswoman for the Chicago Department of Aviation. "We add additional staff as the conditions warrant."
O'Hare International and Midway International are also ready to implement a passenger assistance program in the event the storm leaves travelers stuck at the airports. Hundreds of cots will be available and some concessions will remain open to accommodate any stranded passengers, Pride said.
Nearly all flights from Oklahoma City's Will Rogers World Airport have been canceled for Tuesday morning, airport spokeswoman Karen Carney said. Late in the afternoon, Frontier still had a departure scheduled, but it will likely be canceled later Monday. No arrivals are expected before 10:30 a.m., Carney said.
If you have plans to fly in the region this week, several carriers will let you make changes to your itinerary:
Continental Airlines will allow changes for travel to, through and from nearly three dozen airports, including the airline's hubs in Newark, New Jersey, and Cleveland, Ohio, for travelers scheduled to fly between Monday and Thursday.
US Airways has also relaxed its change-fee policies passengers scheduled to fly Monday through Thursday to or from 27 cities from Portland, Maine, to Akron, Ohio.
Travelers on JetBlue flights to and from six cities and metropolitan areas in the Northeast on Tuesday and Wednesday will also be able to change their plans with no fees.
United Airlines has issued travel waivers for passengers traveling to, from or through Chicago and 17 states in the Midwest and Northeast as well as five Canadian cities. Travelers will be able to alter their plans without fees or get a full refund if their flight is canceled.
American Airlines is allowing travelers who are scheduled to fly on Monday through Thursday from more than 30 airports in the Midwest -- including those in Chicago, Detroit and Milwaukee, Wisconsin -- to change their plans without penalty. For the full list, click here.
Delta has issued a winter weather advisory for the northern U.S.Passengers traveling in 20 states on Monday through Friday can reschedule their itineraries without a fee or get a refund if their flights are canceled or significantly delayed.
AirTran passengers traveling through two dozen cities in the region -- including Chicago, St. Louis and Boston -- can change their reservation without penalty.
Southwest Airlines is "keeping a close eye on the conditions," the carrier says on its website. Passengers with reservations for travel through Chicago, St. Louis, Detroit and other cities are eligible to reschedule their flights. Check the airline's winter weather page for details.