A motorist is stranded by storm water that collected under the McPhillips Street underpass Saturday evening. (CBC)Homeowners, public works crews and emergency responders in southern Manitoba are beginning a major cleanup after severe thunderstorms pounded the region over the weekend.

Environment Canada said 53 millimetres of rain fell on Winnipeg on Saturday and several major streets remain closed because of flooding.

The towns of Portage la Prairie and Emerson each recorded record amounts of rainfall, a spokesperson for the weather agency said. Emerson has declared a state of emergency because of the damage done to property in the community.

A further five to 10 millimetres of rain is expected in Winnipeg on Sunday.

Northbound Lagimodiere Boulevard at Warman Road is closed and the Jubilee Street underpass at Pembina Highway is flooded.

Winnipeg police were also called to Bishop Grandin Boulevard at Pembina Highway due to high water levels at the underpass, and they reported that some streets around the Maginot Arena in St. Boniface were flooded out.

Officers also reported traffic problems at Bishop Grandin and Waverley Street.

Const. Natalie Aitken said traffic lights were out at a number of intersections and was cautioning motorists to stay off the roads if possible.

"It appears it is getting actually worse out there," Aitken said. "I realize the rain is not coming down as heavy as it was, but a lot of roads are completely impassable and people are still trying to go through them."

A police car was stuck in Transcona while responding to a call from a motorist stranded because of the water, Aitken said.

Sewer backup feared

Provincial flood officials are keeping a close eye on water levels on major rivers and will soon have to decide whether to open the gates of the Red River Floodway to protect Winnipeg homes from flooding.

A taxi was just one of many vehicles that got stuck under the Jubilee Street underpass, which remained closed on Sunday. (CBC)The city said there have been no widespread reports of sewers backing up into people's basements. Instead, water is making its way into homes through foundations.

High river levels in Winnipeg impair the city sewer system's ability to operate at a normal capacity. That means there is an increased risk of basement flooding as the overloaded system backs up through household sewer lines that aren't protected by sump pumps and backwater valves.

Provincial officials issued a flood watch Friday.

"We kind of have to do a benefit-cost analysis," said provincial flood forecaster Alf Warkentin. "We don't just operate the floodway just at a whim."

The province has decided to operate the Portage Diversion to reduce the flow in the Assiniboine River. The diversion redirects some of the water in the Assiniboine to a 29-kilometre-long diversion channel that empties into Lake Manitoba.

In southwest Manitoba, felled power lines and direct lightning strikes on power equipment left about 4,000 homes and businesses in the dark on Saturday.

Areas in and around Brandon, Portage la Prairie, Reston, Melita, Pipestone and Hartney were affected by the severe weather.