A funnel cloud was spotted over Thorold this morning, and more could be seen through the day, a Special weather statement said on Saturday morning.
These types of funnel clouds are generated by weak rotation under rapidly growing clouds or weak thunderstorms. This weak rotation is normally not a danger near the ground. However, there is a chance that this rotation could intensify and become a weak landspout tornado, the statement said.
Environment Canada says local rainfall of 50 to 100 mm are possible today and warns of risk for localized flooding.
The warning was issued for Niagara Falls, Welland, St Catharines, and Grimsby.