DENVER — The call went up across America: Mother Nature, please have mercy.

A punishing winter that has already produced record snowfalls and crippling temperatures in many areas sent another wave of severe weather blasting across the country over the weekend. Fresh snow buried cars and snarled travel from Denver to St. Louis, and behind the snow came a mass of air so frigid that the National Weather Service declared a wide area of the north central United States to be entering another deep freeze.

The intense cold was expected to spread into the Mid-Atlantic states and the Northeast through Monday, and to stretch into parts of the Deep South by midweek.

Ryan Maue, a meteorologist with weathermodels.com, said much of the country should prepare for an “all-week cold snap.”

On Sunday, the National Weather Service also warned of severe thunderstorms moving across the South and Southeast. Tornadoes were reported in at least three states, with severe damage and at least 23 fatalities reported in Alabama.