(CNN) -- A large, powerful storm system threatened to bring a white Christmas to parts of the Great Plains and dangerous tornadoes to the Southeast, forecasters said.
A winter storm warning is in effect through at least midnight Tuesday for much of Oklahoma and Arkansas and parts of southern Missouri. By the time the storm is through, these areas could have between 4 to 6 inches of sleet and snow.
But precipitation is just part of the concern. The same system could produce relatively long-lasting tornadoes not normally seen this time of year, the National Weather Service notes in its severe weather outlook.
"A severe threat is expected to spread (eastward and northeastward) across parts of" Alabama, southern Tennessee and western Georgia on Tuesday night "as the system continues to strengthen," the weather agency said.
Even without tornadoes, the Mississippi Emergency Management Agency said that its residents could see sustained winds between 70 and 80 mph, heavy rain and hail, and dangerous lightning. The system is supposed to move through the state on Christmas Day.
Passengers flying in and out of Oklahoma City's Will Rogers Airport are being urged to check with their airlines before coming to the airport, because of the weather there and elsewhere around the county. Delta has already canceled four Christmas Day flights, while it and United are both waiving fees for passengers who want to change their scheduled tickets, the airport said in a news release.
One reason for widespread delays this week is the fact the weather is expected to be frightful in much of the United States, not just the Plains and Southeast.
A winter storm watch is in effect from Missouri through Vermont, a huge swath of territory where significant snows are expected to fall through Thursday.