A major storm system is set to bring “potentially catastrophic” winter weather across the southern U.S. this weekend, bringing snow, freezing rain and ice pellets to a substantial swath of the country.

It could be the “storm that defines the entire winter,” according to some meteorologists.

Overall, nearly 30 states could feel the effects from New Mexico all the way to New York starting Friday.

The National Weather Service warns that not only will this be a significant storm event but that there will be dangerously cold temperatures both before and after it passes — and in places that aren’t accustomed to such frigid conditions.

But there’s still “a lot of uncertainty,” David Nadler, a warning co-ordination meteorologist with the NWS in Peachtree City, Ga., said in a special briefing Wednesday afternoon.