The fog developed near a warm front that has lingered near Toledo since Saturday, and also was responsible for a third straight day Monday of unusual warmth in the region.
While not a record, the 63-degree high at Toledo Express Airport was 22 degrees higher than normal for Dec. 3. The record for the date was 68, set in 1982.
That followed two days of upper 50s highs, and another such high is expected today -- albeit early in the day, before a cold front approaching from the Upper Midwest and Great Plains arrives.
"We are looking at it coming through the area tomorrow [this] morning," Tom King, a meteorologist at the National Weather Service in Cleveland, said Monday evening.
Temperatures are expected to drop to about freezing by Wednesday morning, peak only around 40 on Wednesday, then fall into the mid-20s Wednesday night.
On the plus side: It is expected to be a lot sunnier in the Toledo area on Wednesday and Thursday than it has been since late last week.
Monday's dense fog forced many school districts to delay classes by two hours because of poor visibility on school-bus routes.
Although some local patches of fog could accompany the rain today, he said, "it shouldn't be as murky and dense" because the storm center's turbulence will stir up the air.
Saturday's mild, dry conditions helped the Christmas in Ida Festival set an attendance record, with about 49,000 people attending that day.
The Lights Before Christmas event at the Toledo Zoo also attracted huge crowds, with traffic and pedestrian lines the longest in recent memory, although zoo spokesman Andi Norman said it was not a record-breaker.
Mr. King said the soonest Toledo could get its first measurable snow of the season is early next week.
After the near-normal temperatures Wednesday and Thursday, he said, local weather is likely to get warmer and wetter again for the weekend. But another storm front in the outlook for early next week could produce "snow in the area" after it passes.
But while some might expect December's warm start to portend a second straight mild winter, Mr. King said the weather service's winter outlook, covering December through February, calls for near-normal temperatures and precipitation.