DETROIT (AP) — Public works crews and contractors in Detroit are preparing to spend several days making sure city streets and roads are clear of more than a foot of snow that’s expected.

On Tuesday, Mayor Mike Duggan activated Detroit’s snow emergency routes, which means people had to remove their parked vehicles from those streets by before midnight to allow unhindered access for police, firefighters and emergency medical service vehicles.

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“Police, fire and ambulances have to get through,” Duggan told reporters, adding that the city has about 75 snow emergency routes, generally along heavily traveled streets.

City crews in 50 trucks are expected to work 12-hour shifts salting and plowing Detroit’s 673 miles (1,083 kilometers) of major roads. Contractors will take care of the city’s 1,884 miles (3,032 kilometers) of residential streets.

The National Weather Service projects 11 to 15 inches (28 to 38 centimeters) of snow for Detroit between Wednesday morning and Thursday night.

“Travel could be very difficult to impossible,” the weather service said on its website.

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A winter storm warning was issued for several southeastern Michigan counties.

The snowfall is part of a major winter storm blowing through the Rockies and into the Midwest. Airlines have canceled more than 800 flights in the U.S. scheduled for Wednesday, the flight tracking service FlightAware.com showed.

Winter storm watches and warnings were issued from El Paso, Texas, through the Midwest and parts of the Northeast to Burlington, Vermont. The storm follows a vicious nor’easter that brought blizzard conditions to many parts of the East Coast.

“With the volatility of the climate cycle, we’ve got to pay attention,” Duggan said. “Last June, we had a report there was going to be two inches of rain and we got the biggest rainstorm in Detroit’s history.”

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