(Adds canceled flights, school closures, details on preparations, expected weather in Chicago, Delta also offering travel vouchers)

By Brendan O'Brien and Suzannah Gonzales

MILWAUKEE, Wis./CHICAGO, Feb 8 (Reuters) - A significant storm is expected to descend on Chicago, Detroit and Milwaukee Thursday evening and Friday, with heavy snow and freezing temperatures expected to slow travel for millions of commuters.

The storm system stretches from western Montana across the Dakotas and parts of Minnesota, Wisconsin and Illinois, and reaches as far east as southern Michigan. It could drop up to 14 inches (36 cm) of snow in some areas, the National Weather Service said.

Chicago was anticipating eight to 14 inches of snow over the next 24 hours, with the heaviest snowfall on Thursday night and early Friday morning, Gino Izzi, a National Weather Service meteorologist in Romeoville, Illinois, said in a phone interview on Thursday.

Wind chill temperatures were expected to drop below 0 Fahrenheit (-18 C) in many areas across the region, and officials warned of limited visibility on roads.

Chicago's O'Hare and Midway international airports canceled more than 170 flights on Thursday before the storm hit, and several airlines were also anticipating delays or cancellations.

United Airlines said on Twitter that travel waivers were in effect for snow-hit areas this week, and Delta Air Lines offered to rebook flights on Friday for 18 Midwest cities.

City officials announced school closures on Friday in Chicago, Detroit and Ann Arbor, Michigan because of the weather.

Winter weather across the United States over the last several days has killed several people in accidents in the Midwest since Monday, including six in Iowa, two in Missouri and one in Montana, local media in those states reported. (Reporting by Suzannah Gonzales; Additional reporting by Jonathan Allen in New York; Editing by Matthew Mpoke Bigg, Patrick Enright and Diane Craft)