Much of northern Illinois is under a tornado watch ahead of strong to severe thunderstorms coming to the region later Friday.

The National Weather Service issued the tornado watch about 12:45 p.m. Friday, and it is set to expire at 8 p.m. Friday.

Counties included in the watch include DeKalb, La Salle, Lee, McHenry, Ogle, Whiteside, Boone, Bureau, Carroll, Hancock, Henderson, Jo Daviess, Mercer, Putnam, Rock Island, Stephenson, McDonough, Warren and Winnebago.

Strong to severe thunderstorms are expected to sweep across northern Illinois Friday evening, with the greatest threat arriving after a warm, windy day.

According to the National Weather Service, dense fog developed early Friday morning, especially near Lake Michigan, reducing visibility to a half-mile or less in parts of the Chicago metro area. Conditions will improve quickly after sunrise as gusty southerly winds increase.

Those winds will help push temperatures into the lower 80s on Friday afternoon with gusts reaching 30 to 40 mph. That warmth and moisture will set the stage for potentially severe storms later in the day.

Forecasters say the main window for severe weather will be between about 5 p.m. and midnight, as a cold front moves west to east across the region. Storms are expected to organize into a fast-moving line, entering areas near Rockford and the Interstate 39 corridor early in the evening before pushing toward DeKalb, Joliet, and eventually Chicago later at night.

The primary threat will be damaging winds capable of downing tree limbs and causing power outages. Large hail also is possible, along with a limited risk of brief, embedded tornadoes, especially along and near the I-39 corridor.

In addition to severe weather, heavy rainfall could lead to flooding issues. Soils across the region are already saturated, and rivers including the Rock River, Fox River and Des Plaines River are running high. Rainfall totals of 1 to 2 inches are possible in some areas, and even a half-inch could trigger localized flooding.

Storms are expected to weaken as they move east later in the evening, though showers may linger into the overnight hours.

Behind the cold front, a dramatic temperature drop is expected. Highs this weekend will fall into the 40s and 50s, more than 30 degrees cooler than Friday, with frost or freeze conditions possible Sunday and especially Monday morning.

Residents are encouraged to have multiple ways to receive weather warnings and to stay alert for rapidly changing conditions on Friday evening.