Large parts of the eastern and central United States are expected to see heavy snow. But south of the snow line, forecasters are warning that the storm could deliver dangerously icy conditions. Exactly where that snow vs. ice line will fall was still uncertain on Wednesday.

Millions of Americans across the South are forecast to be slammed by ice storms in the coming days as crippling ice accretion threatens widespread power outages, major transportation disruptions, falling trees and downed power lines.