CHICAGO (CBS) – A storm system that is expected to drop anywhere from three to six inches of snow, possibly followed by a freezing drizzle that may leave a glazing of ice on some surfaces, is already making area roads treacherous, state police say.

The Chicago area is under a winter weather advisory Monday night, as ice and as much as three to six inches of snow are expected to make travel difficult Monday night into Tuesday, according to the National Weather Service. That is up from the two to four inches the weather service had earlier forecast.

The advisory is in effect until 10 a.m. Tuesday for much of northern Illinois and northwest Indiana, including all Chicago-area counties, according to the weather service. Monday night’s snow was expected to be heavy at times, according to the weather service. In some parts of the Chicago area Monday night’s snow was expected to result in treacherous driving conditions, with bursts of heavy snow, making it “near impossible” for road crews to keep up with snowfall rates, according to the weather service.

Snow plows and salt trucks from the Illinois Tollway and Chicago’s Department of Streets and Sanitation have been out on the roads since mid-afternoon.

Snow in the area was expected to continue Monday night, then turn to freezing drizzle or mix with freezing drizzle after 3 a.m. Tuesday, according to the weather service. Two to four inches of snow are forecast to fall Monday night before snow changes to freezing drizzle then light rain as temperatures creep back above freezing by Tuesday morning, according to the weather service. The drizzle could leave a minor glazing of ice, mainly on elevated and untreated surfaces.

Area roadways were seeing dozens of crashes Monday night. As of about 9:40 p.m., an Illinois State Police District Chicago trooper said there were about 30 crashes on area highways, with more calls coming in as he spoke. The trooper was unable to provide details on any of the crashes.

The city’s Department of Streets and Sanitation at 2:30 p.m. notified 174 Snow Fighting trucks to prepare for the evening rush hour and possible snow duty on Chicago’s Main Route System, which includes Lake Shore Drive and main streets. Trucks were in place by 4 p.m. and soon thereafter were out patrolling their routes, a release from Streets and San. said.

Chicago’s Snow Command will be monitoring this incoming weather system and will deploy personnel and equipment as necessary, the release said.

The Illinois Tollway mobilized its full fleet of 183 snowplows for the overnight snow event.

The Illinois Tollway’s Snow Operations Center was activated at about 2 p.m. Monday and staffed with traffic center operators and snow and ice control supervisors ready to manage this winter weather event, according to a release from the Tollway.

“We are asking our customers to slow down during winter conditions and give our snowplow drivers the room they need to clear the way,” Illinois Tollway Executive Director Kristi Lafleur said. “We also are reminding our customers to keep a safe distance from other vehicles and reduce their speed to match roadway and traffic conditions. Driving defensively during wintry conditions is more important than ever.”

A Tollway District trooper was not immediately available for comment Monday night. An Elgin District State Police trooper said as of about 9:45 p.m. there had been “a couple” crashes that could be weather-related on the roads northwest of the city, but there had not been an extraordinarily high number of weather-related incidents.

To help with snow removal efforts, the Tollway canceled all temporary lane closures between 2 p.m. Monday and 9 a.m. on Tuesday.

On Tuesday morning — the first official day of Winter — area residents should prepare for freezing drizzle, and snow and motorists should be cautious while driving, and be prepared for slippery roads and limited visibility, the weather service said.

Watch & Listen LIVE