A powerful post-Christmas storm began raking the middle of the country Wednesday, with heavy snow expected to blast the northern and central Plains and severe rain sweeping to the South.

More than 6 million people across 13 states were under winter weather advisories and warnings that are expected to last through the week, according to the National Weather Service.

Parts of Colorado, Nebraska, the Dakotas and Minnesota could see up to 9 inches of snow from Wednesday to Friday. Sections of South Dakota could get up to a foot.

The snow is expected to be accompanied by winds gusting up to 50 mph, which could lead to near-blizzard and whiteout conditions during one of the busiest travel periods of the year.

To the south, Oklahoma and Texas can also expect heavy winds, but those gusts will be joined by rain, hail and possible tornadoes, forecasters said. The downpours will shift to the Gulf Coast on Thursday night, bringing with them the risk of flash floods in Alabama and Mississippi.

The stormy weather comes at an inconvenient time as a record-breaking number of holiday travelers try to make it back home before the end of the week. AAA estimated that 112.5 million Americans — the most since they have been tracking holiday travel — would take to the air, roads and rails during the long holiday weekend. Dec. 26 is considered the heaviest after-Christmas traveling day.