The first snow of the season in the Chicagoland area could lead to “intense snow rates” depending on lake effect snow on Sunday into Monday, the weather service said.
The National Weather Service (NWS) said rain will spread across the northern Illinois and Indiana area from west to east on Saturday afternoon.
It will transition from rain to snow from northwest to southeast after sunset.
The warm ground will likely limit the snow from sticking but there is a 20-30% chance that a few inches of slushy snow sticks to grass and untreated roads, NWS said.
Lake effect snow may start as early as Sunday morning along the lakeshore but could quickly move eastward toward Michigan City by Sunday afternoon.
The NWS said they are “growing concerned” in a period of intense lake effect snow in the vicinity of southern Lake Michigan Sunday night into Monday.
“The deep magnitude of the upcoming cold airmass and continued warmth of Lake Michigan waters will lead to unusually high instability levels capable of intense snow rates wherever the lake effect snow goes,” the weather service said.
A winter storm watch is in effect for Cook County, as well as numerous counties in Indiana, for the lake effect snow.
“If you’re planning to travel around the southern bowl of Lake Michigan Sunday into Monday, PLEASE stay up to date on the forecast! Lake effect snow can lead to dramatic differences in conditions across mere miles,” the NWS said.
Forecasters say there are two scenarios possible for lake effect snow in northeastern Illinois.
The first scenario involves little to no lake effect snow in Illinois, with the majority being in northwestern Indiana.
The second scenario involves intense lake effect snow with dangerous travel conditions in the Chicago area, including Lake County and part of McHenry County.
Lake effect bands could produce snow rates of more than two inches per hour and over 35 mph wind gusts that could make travel conditions dangerous.
“Uncertainty remains on exactly where the lake effect snow bands will go. Northeastern Illinois through northwestern Indiana are primary threat areas,” the weather service said.