A polar vortex is slated to sweep most of the continental US bringing winter storm warnings and a hazardous freeze to millions.

A combination of frigid air with a low pressure system over the Gulf are behind the storm, which could bring heavy snow just south of the Interstate 20 corridor across northern Louisiana and into Mississippi and a mix of snow, sleet, and freezing rain near the Interstate 10 corridor from Houston to Mobile, Alabama.

The updated forecast on timing and track of the storm calls for the snow to end from southwest to northeast as midnight approaches. “The heaviest precipitation looks to occur between roughly 4 and 9 p.m. during which time snowfall rates could reach or exceed 1 inch per hour,” the National Weather Service said.