This index, from NOAA's National Weather Service, attempts to scale the impacts from winter weather, taking into account factors such as snowfall, accumulated snow on rooftops, ice accumulation, the potential for a flash freeze, and blowing snow. Not all factors are in play in a given winter storm.

Mixed precipitation -- snow and ice -- are likely to begin Saturday. We will see winter weather, but it's too early to be confident about the precise location and amounts.

PHOENIX (AZFamily) — Big weather changes are on the way for Arizona during the second half of this week, with First Alert Weather Days issued for Friday and Saturday.

A massive winter storm is set to impact tens of millions of Americans from the South to the Northeast, with some areas expecting up to a foot of snow. Over a dozen states have declared emergencies with a Louisiana official telling NBC News that the governor's office is urging people to stay off the roads to avoid dangerous icy conditions.