(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 snow storm of historic proportions expected in Chicago, Illinois, and large sections of the country's midsection, some airlines are already allowing passengers to change plans.
The storm will impact 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 over 12 inches in parts of Missouri and Illinois.
Monday will be the best day to travel over the next four or five, said HLN meteorologist Bob Van Dillen.
If you have plans to fly in the region this week, several carriers will let you make changes to your itinerary:
United Airlines has issued a travel waiver for passengers traveling to, from or through Chicago from Tuesday to Thursday. Travelers will be able to alter their travel 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, Michigan; and Milwaukee, Wisconsin -- to change their plans without penalty. For the full list, click here.
Delta is planning to issue a travel waiver for the Midwest later Monday morning, and another for the Northeast on Monday afternoon, said spokesman Anthony Black.
AirTran passengers traveling through two dozen cities in the region -- including Chicago; St. Louis, Missouri; and Boston, Massachusetts -- 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. CNN
