Will a snowstorm hit Sunday into Monday? Forecasts are uncertain.

Seeing widely varying predictions for snow Sunday into Monday?

Don't blame it on post-Thanksgiving turkey brain. Even meteorologists on Friday said they were uncertain about an approaching storm, which promises moisture, but could dump mainly rain or switch to snow, depending on the temperature and track of the system.

As of Friday afternoon, the National Weather Service's Chicago office predicted a roughly 50 percent likelihood that towns across the region would see 4 inches or more of heavy, wet snow, starting sometime Sunday and concluding Monday morning.

Part of the problem with forecasting this storm, National Weather Service Meteorologist Kevin Donofrio said, is it's still off the Pacific Coast and hasn't yet been measured by a network of U.S. weather balloons, which provide information that improves the accuracy of computerized forecast models.

So far, the exact path of the system - and where it will bring more rain, versus more snow - is difficult to foresee.

"It could fall as a bunch of rain and a little snow, or it could fall as a bunch of snow," Donorfio said. "A minor shift in the storm track, say 30 miles, means a big difference for some people."

A 30-mile difference could bring rain in Naperville, with snow in Arlington Heights, or rain in Naperville and Arlington Heights, with snow in Antioch and Gurnee.

Yet another forecasting challenge is the temperature before the storm arrives.

When systems like this build in December or January, Donofrio said meteorologists can say with more certainty that they'll bring only snow. But with temperatures near 50 Friday and Saturday, the snowfall potential of the storm hinges on the speed of the cool-down Sunday to readings below freezing.

"Surface temperatures are warm initially," Donofrio said. "It just depends on how much of this cold air comes in."

The National Weather Service predicts chances for heavy snow accumulation are greatest north and west of I-55. Donofrio warns that, when and where snow does strike, it will come with strong winds and reduced visibilities during "a pretty important travel period for people returning from holiday travel."

Forecasters advise residents and travelers to stay alert for the latest forecasts and plan ahead for potential travel hazards.