A winter storm warning was issued overnight for nearly the entire Chicago area including southeastern Wisconsin, with heavy snow expected “all day long Saturday,” with as much as 12 inches of snow or more for some parts and major travel impacts possible, with experts warning of snow-covered roads and reduced visibility.

The warning comes as more than 41 million people from the northern and central plains to the Midwest were under winter storm alerts, with the National Weather Service advising people in northeast Illinois to “consider altering any travel plans originally scheduled for Saturday.”

The winter storm warning goes into effect at 3 a.m. and lasts through 6 a.m. Sunday. The NWS originally issued a winter storm watch for the area, but it was upgraded overnight.

When will the snow start?

The snow was expected to move into the Chicago area late Friday night and after midnight, starting in the far western counties. During the daytime hours, Friday would remain dry and sunny, NBC 5 Storm Team Meteorologist Kristen Kircchaine said, with high temperatures in the 30s but breezy conditions making it feel even colder.

By 6 a.m. Saturday, widespread snow will “already be going,” Kircchaine said, and covering the entire Chicago area. It’s expected to pick up in intensity at times in the afternoon and evening.

“Highest rates around one inch per hour,” the NWS warned. “Greatest travel impacts expected Saturday afternoon.”

During that time, forecasters with the NWS warned people to consider delaying travel unless its “absolutely necessary,” with driving expected to be very difficult to impossible. Roads and bridges were likely be slick and dangerous, and strong winds along with the weight of snow on tree limbs could cause sporadic power outages.

“Drive with extreme caution,” the NWS said. “Consider taking a winter storm kit along with you,” including tire chains, blankets, flashlights, shovels and more.

“Also take water, a first aid kit, and anything else that would help you survive in case you become stranded,” the NWS said.

How much snow is expected in Chicago?

By nighttime, the snow will be lighter, Kircchaine said, tapering off by Sunday morning. When all is said and done, the Chicago area is looking at “big time snow totals.”

Most of the Chicago area was expected to get between eight and 12 inches of snow, with locally higher amounts possible northwest of Ottawa, to Aurora and near the Waukegan line.

Further south and southeast, where a wintry mix was possible totals were likely stay between six and eight inches, Kircchaine said.