A massive winter storm blew across the center of the US on Monday, threatening millions of people with heavy snow, freezing rain and flooding.

The National Weather Service warned that there would be “numerous, widespread, and impactful weather hazards in the heart of the country this week.” People from Idaho spanning into Wisconsin and as far south as Louisiana were warned to be ready for blizzard-like conditions in the north and flash flooding in the south.

By Tuesday, Texas and northern Louisiana could be pummeled by severe hail, winds and tornadoes, AP reported. The storm will continue southeast into Florida later in the week, forecasters said.

“It will be a busy week while this system moves across the country,” said Marc Chenard, a meteorologist at the National Weather Service at the national center in College Park, Maryland.

Officials in western South Dakota on Monday told residents to brace for 15 centimeters or more of snow: “Get your shovels handy, get your groceries, and check other needed supplies. The roads will be hard to travel.”

Regions stretching along the front range of the Rockies from Montana south into Colorado were under blizzard warnings Monday, and the National Weather Service said that as much as 61 centimeters of snow was possible in some parts of western South Dakota and northwestern Nebraska. Meanwhile, ice and sleet were expected in the eastern Great Plains.