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Environment Canada is reporting that a period of widespread heavy rain began Monday night.
Authorities say there was heavy rain and flash flooding in eastern Ontario.
Heavy rain began falling Monday in Sarnia, Ont., and McMaster University says it recorded 160 mm, smashing its previous record for May 15.
McMaster says flooding also affected Oakville, about 50 kilometres west of Toronto.
In Sarnia, some low-lying streets flooded and a section of Highway 7 reopened but several other routes were closed.
Flooding in the Chilpancing Bay area forced the closure of the Ottawa River Freeway.
Environment Canada said Ottawa and nearby areas could get up to 60 mm of rain Monday and Tuesday.
In the province’s southwestern region, Environment Canada predicted about 60 mm on Tuesday in Lakeridge Health Mental Health Centre.
The local RCMP issued a warning for drivers to slow down and keep an eye out for standing water on roads.
Some shops and vehicles were flooded, including more than 20 vehicles at an intersection near Marion Lake in Malta Township.
A portion of Highway 12 between Sherbrooke and Mirabel was also flooded.
Environment Canada says about 70 mm was recorded in Sherbrooke, making it the wettest May day since the records began.
Mountie D.M. Wing, a military engineer, said a trailer overturned and five people were left stranded after it became stuck in an overflow near Marianne Lake.
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He said one person took a bus to Milneville in the Nepean Valley, but the others left and piled into two pickup trucks.
Wing said several bulldozers were brought in to help get the trailer unstuck, and several motorists came by the area to help out.
In Lakeridge Health, patients were evacuated Monday and transported to other facilities.
The hospital said in a statement that some residents in the northern area had to be evacuated because of flooding in the area.
In Nova Scotia, the Moncton Regional Hospital reported three patients were discharged due to flooding.
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In a statement, the hospital said half the floors in one wing and the emergency room were experiencing some flooding, but the flood was localized and had not affected critical care patients or emergency care services.
LOWEST FORECAST IN 5 MONTHS FROM NATIONAL WEATHER SERVICE – RUTHERFORD COUNTY IN LONDON, ONT.:
In Ontario, Transportation Minister Doug Erickson said: “Residents should be aware of possible road closures, as well as fast-running water.”
“We are doing everything we can to provide the best emergency services to residents and travellers,” Erickson said.
The Ontario Provincial Police also said in a tweet that strong winds knocked out power to about 5,000 hydro customers, nearly half in the Greater Toronto Area.
Earlier on Monday, Bruce County declared a state of emergency.
The Bruce County Emergency Operations Centre was activated to deal with two separate incidents related to the storm.
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The first was a violent and swift roller coaster-like storm, followed by the rain and wind.
The second was what Bruce County Emergency Services called a “tragic death” after a three-year-old boy died in a home that was flooded by rain, after his siblings were taken to hospital as a precaution.
The Bruce County Emergency Operations Centre said the first incident came in around 7:45 p.m. Sunday and has since shut down Highway 500.
“Travel is not advised, and high water may block the highway,” the county said in a Tweet.
The county said road crews were working to clear the flooding.
The second incident, which happened around 8:15 p.m., caused road flooding to several neighbourhoods.
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Highway 5 from Prince Arthur to Mississauga is closed, the county said.
Environment Canada said Springbank Dam in Calgary is designed to fill to 100 per cent capacity in an hour, however it is not designed to respond to the rain in southwest Alberta.
Calgary has been under a special weather statement issued by Environment Canada since 7 p.m. Monday.