Morning commuters make their way through a thunderstorm that drenched Winnipeg on Friday. (CBC)Heavy rains dumped by rolling thunderstorms have prompted Manitoba Water Stewardship to issue a flood watch for most of southern Manitoba.

Flash flooding has already occurred in areas along the La Salle River and Seine River watersheds due to a storm system that brought 50 to 70 millimetres of rain to those areas Friday.

Many other areas of Manitoba received 20 to 30 millimetres of rain in that same time, but as much as 70 milimetres more could fall along the Red River Valley from Emerson to Winnipeg and across southeastern Manitoba through the weekend, according to a news release issued by the watershed authority (MWS).

In Winnipeg, 30 milimetres has already fallen and 40 to 50 milimetres more could come by Sunday, CBC News meterologist John Sauder said.

Soil saturated

The rain is falling on soil already saturated from heavy rain on the Victoria Day long weekend. As a result, overland flooding is likely around ditches and smaller streams that are unable to take in anymore water.

The level of the Red River is also rising quite rapidly due to downpours in the United States last weekend, the MWS said. It will remain within its banks unless additional heavy rain develops in early June.

High river levels in Winnipeg impact 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.

Floodway operation considered

"In accordance with the Red River Floodway's rules of operation, Manitoba Water Stewardship will complete an assessment of whether operation of the floodway is necessary to prevent widespread basement flooding in Winnipeg if the rain this weekend results in a river level forecast above 4.3 metres," states the MWS release.

A tree topped onto a car at Flora Avenue and Parr Street during a thunderstorm Friday morning in Winnipeg. (CBC)The river level in downtown Winnipeg Friday was at 3.4 metres and could crest at about 4.9 metres by June 5 with the rain forecasted over the next few days.

Flooding is not expected along the Assiniboine River, however a decision on whether to operate the Portage Diversion to lower river levels in Winnipeg will be made in the next few days.

The diversion redirects some of the flow of water in the Assiniboine to a 29-kilometre long diversion channel that empties into Lake Manitoba.

Most tributaries of the Red, Assiniboine and Souris rivers are expected to remain within their banks but some over-bank flooding of low-lying areas is possible, MWS said.

The latest information on water levels and outlook reports is available on the Manitoba government website at the top right of this page.