A large, powerful storm system threatens to bring a white Christmas to parts of the Great Plains and dangerous tornadoes to the Southeast, forecasters say.

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 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 said in its severe weather outlook.

A moderate chance of severe weather is forecast from southeast Texas to portions of the Florida Panhandle, according to forecasters. The threat of severe storms may reach as far east as parts of Alabama, southeast Tennessee and western Georgia by Tuesday evening and could reach coastal South Carolina by early Wednesday, the weather agency said.

Dangerous conditions in the South included nickel-sized hail, frequent lightning strikes and strong winds. There was a risk of tornadoes Tuesday for almost all of Louisiana, southeast Texas, Mississippi, Georgia and the western Florida Panhandle, CNN meteorologist Bonnie Schneider said.

Even without tornadoes, the Mississippi Emergency Management Agency said that state residents could see sustained winds between 70 and 80 mph, heavy rain and hail, and dangerous lightning. The system is forecast to move through the state on Christmas Day.

A section of Interstate 40 in Oklahoma near Del City was temporarily closed Tuesday morning after an accident involving 20 vehicles, police said. Freezing rain was probably the cause of the chain reaction of vehicles, including three big rigs, the Oklahoma Highway Patrol said. No life-threatening injuries were reported.

That freezing rain will turn into sleet and snow, Schneider said.

Passengers flying in and out of Oklahoma City’s Will Rogers Airport are being urged to check with their airlines before going to the airport, because of the weather there and elsewhere around the county. Delta Air Lines has already canceled four Christmas Day flights, and it and United Airlines are 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: 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.