The Chicago area is under an Ice Storm Warning, prompting slick, dangerous conditions on roadways and possible power outages as widespread icing occurs.
This marks the area's first Ice Storm Warning in nearly a decade. So, what does it mean?
First things first: Ice will not fall from the sky over Illinois.
Here's what happens during an icing weather event, according to NBC 5 meteorologist Andy Avalos: "The moisture aloft is frozen, but it's falling to a deeper layer of warm air, so it turns into just rain, and then rain comes all the way to the ground. So the rain itself isn't freezing, it's actually freezing on contact with surfaces that are at or below 32 degrees."
That could produce one- to two-tenths of an inch of ice Tuesday night in far northern Illinois counties. Some southern areas could get three- or four-tenths by dawn.
Significant icing is expected for areas mainly along and north of I-80.
The National Weather Service says the term ice storm "is used to describe occasions when damaging accumulations of ice are expected during freezing rain situations."
"Significant accumulations of ice pull down trees and utility lines resulting in loss of power and communication. These accumulations of ice make walking and driving extremely dangerous," the NWS says.
Any outdoor surface that isn't treated, such as roads, parking lots and sidewalks, will be covered in ice. Even trees could become covered in ice, bringing a risk of limbs breaking under the weight of ice.
The Ice Storm Warning runs through 6 a.m. Wednesday for most of the metro area.