The havoc was caused by a single low pressure system that intensified over the northern United States Saturday night.
Severe thunderstorms hammered Iowa, generating two F3 tornadoes and three F2 tornadoes between the towns of Mapleton and Early, both of which sustained heavy damage but reported no fatalities.
Another storm over Michigan became so organized and intense, it had enough energy to cross the Great Lakes overnight on Saturday and dump golf ball-sized hail on some Huron shore communities Sunday morning.
That cell fizzled out over Hamilton mid-morning on Sunday, and temperatures quickly heated up with an influx of warm air ahead of a cold front. Several communities in southern Ontario enjoyed record-breaking warmth, with about 20 weather stations reporting highs over 20°C in the heat of the afternoon.
However, as evening set in, so did the cold front, bringing with it a double whammy of large hail for Thunder Bay. One cell hit around 4:30 p.m., followed by another at 9 p.m.
One resident said the hail was so big, “it sounded like kids were throwing rocks at the house.”
The front also fired up severe storms that spawned several tornadoes in Wisconsin.
Another organized cluster of storms that crossed Lake Huron around 9 p.m. prompted Environment Canada to issue its first tornado watch of the year for the Mount Forest, Barrie and Collingwood areas. That watch remained in effect for two hours.
As the storms pushed eastward through the night, heavy rains washed out roads in the west end of Ottawa, while lightning struck a home in the community of Navan, setting the roof on fire.
Thousands of people across Ontario were left without power through the night.
The severe weather threat had diminished significantly by Monday morning, with only scattered showers and gusty winds reported throughout southern Ontario.
With files from CBC Ottawa