The greatest risk of tornadoes is from southeastern Michigan into Ohio.
The National Weather Service's (NWS) Storm Prediction Center (SPC) said that more than 41 million people could see severe weather on Wednesday, with 8 million under a moderate risk. That includes Detroit and Toledo, Ohio.
The storm system that merged with Cristobal will intensify Wednesday and lift into Canada by Thursday.
Cristobal on Tuesday became the fourth tropical system in recorded history to reach Wisconsin, but all prior storms only clipped the far southeastern part of the state.
The fact that this tropical system lifted so far north is remarkable.
As Cristobal moved north, heavy rain and gusty winds were reported across the region.
Chicago's Midway International Airport reported a wind gust of 62 mph and trees were downed in the city.
Electricity provider Commonwealth Edison reported about 19,000 without power in northeastern Illinois late Tuesday. Thousands also lost power in southeast Wisconsin, The Milwaukee Journal Sentinel reported.
As the remnants passed through Milwaukee, many of the outages were caused by trees and branches hitting power lines.
Boaters also were warned of gale-force winds on Lake Michigan through Wednesday.
Elevated fire threat across Plains
High winds from the storm system brought breezy conditions from Nebraska to Wisconsin.
In parts of Iowa, Minnesota and South Dakota, the gusty winds and low humidity on Wednesday will bring the threat of wildfires in areas with dry grass.
Temperatures also will warm up the rest of the week, adding to the fire threat.
Fox News' Brandon Noriega contributed to this report.