What is civil danger warning?

Civil Danger Warning: A warning of an event that presents a danger to a significant civilian population. The CDW, which usually warns of a specific hazard and gives specific protective action, has a higher priority than the Local Area Emergency (LAE).

They are usually planned to be issued in the event of a terrorist or military attack, but they also are issued for things like contaminated water or large scale road closings.

The current one being issued is because I-94 and I-43 in Wisconsin are completely impassable because of the Blizzard and that if you attempt to use them you will get trapped and they might not be able to save you.