A tornado warning was issued for Goderich around 3:45 p.m. Sunday.
Radar images showed a supercell with a large hook and strong rotation moving off Lake Huron toward the town.
Within minutes, damage reports began flowing out of the area.
Ontario Provincial Police reported two separate tornado touchdowns, one in Goderich and one in nearby Londesborough. Neither has been confirmed.
As many as eight people were reported injured at a conservation area near Goderich.
Numerous buildings throughout the area were damaged, with roofs and siding ripped off. An entire residential block had to be evacuated as crews worked to contain a large gas leak.
Downed trees and hydro wires blocked roads in Londesborough and Benmiller, stranding drivers in their cars. Fist-sized hail smashed car windows in Goderich.
Marylena Stey, a spokeswoman for Hydro One, said four of the utility's feeder lines were knocked out by fallen tree limbs. About 1,800 customers were left without power.
Did you see anything? Send your photos and videos to The Weather Network.
Earlier in the day, severe thunderstorms swept through southern and eastern Ontario.
The Weather Network received multiple reports of a funnel cloud and possible waterspout near Gananoque Sunday morning.
Another cell blasted the Greater Toronto Area. In East York, more than 43 mm of rain fell in just 10 minutes, flooding roads.
Heavy rain also flooded Highway 401 near Yorkdale, snarling traffic.
Strong winds knocked down large trees across the city.
Multiple waterspouts were reported on Lake Erie and Lake Ontario throughout the day.