Flood risk rises with snow, rain on horizon
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Flood forecasters are keeping close watch on a “potentially significant” amount of snow and rain headed towards southern and central Manitoba, with the Red River Valley set to record its second wettest April in more than 70 years.
Fisaha Unduche, executive director of the province’s hydrologic forecasting centre, said moderate to major flooding is possible in the Red River basin, following last week’s snowstorm and an incoming Colorado low.
“We are currently monitoring another significant weather event that could bring 40 to 80 millimetres of precipitation,” Unduche said Wednesday, noting the rain and snow mix is expected to begin Friday and last through Sunday.
“The weather models do not fully agree on the amount, location and intensity of this precipitation event, but there is a very high confidence that most southern and central Manitoba basins… could receive up to 40 to 50 mm precipitation.”
Impacted basins are forecast to include the Red, Souris, Pembina and Roseau.
The spring flood risk was previously downgraded, but that was before last week’s Colorado low which dumped roughly 30 centimetres of snow on Winnipeg, and as much as 82 cm on Onanole, south of Riding Mountain National Park.
If the forecast is accurate, April will be the second wettest month in the Red River Valley since 1950, only eclipsed by April 1986, Unduche told reporters at a news conference on the southern grounds of the Manitoba Legislative Building, adjacent to the Assiniboine River.
There is plenty of uncertainty, but the latest forecast suggests the precipitation could fall as up to 50 mm of rain and 30 cm of snow. Localized areas could see up to 80 mm of precipitation by April 24.
Precipitation, surface run off and subsequent snowmelt as temperatures rise could lead to overland flooding occurring in some parts of the southern basins, depending on the intensity of rainfall, Unduche explained.
The southwest part of the province is most likely to see overland flooding based on the forecast.
The Assiniboine and Red rivers are likely to experience peak flows in late April or early May, said Unduche. For the Red River, peak flows will likely occur between April 30 and May 7.
Depending on where rain and snow falls, peak flows of 55,000 to 75,000 cubic feet per second are possible between Emerson and the floodway inlet in south Winnipeg.
Graphic: Today's James Avenue water levels
Water levels in Winnipeg are measured in “feet James,” the level above the normal winter ice level as measured at a gauge on the Red River east of James Avenue in the Exchange District, just downstream of The Forks where the Red and Assiniboine rivers meet.
Zero feet James is the baseline winter ice level on the Red River at James Avenue. This is the baseline for measuring river levels in Winnipeg.
In the summer, the St. Andrews Lock and Dam regulates water levels to try to maintain 6.5 feet James.
The walkway elevation at The Forks is 8.5 feet James. It was placed at this elevation on the expectation the walkway would sit above the river most of the summer.
This would be on par with water levels in the springs of 2011 and 2020, said Unduche.
“In other words, levels could rise from what they are today within the valley by 13 ft. to 17 ft.,” he said. “These flows, if they occur, could create moderate to major flooding within the Red River Valley.”
Within Winnipeg, Unduche said the Red River could peak at between 17 and 19 ft., with the Red River Floodway and Portage Diversion in operation. On April 9, the river crested at 18.55 ft. at James Avenue.
As of Wednesday, the Red River was just above 14 ft. at James Avenue.
The floodway could begin operating again as early as May 1, Unduche said.
For the Assiniboine River, Unduche expects moderate flooding between the Shellmouth Dam and Portage la Prairie, where the diversion is likely to be used again.
The same is expected for the Souris River between the U.S. border and Wawanesa, and eastern regions including Roseau River, Rat River and Whiteshell Lake areas.
There is a low risk of spring flooding in northern Manitoba and for the province’s lakes. In the Interlake, where ice jam-related issues remain a concern, there is a moderate flood risk, dependent on rainfall.
Most communities are protected from a one-in-200-year flood or a flood of record, said Unduche, noting the province is not near that level.
“The chance of having property damage or those kinds of issues in Manitoba will remain very rare. But if we see heavy intensity rainfall, that will create overland flooding, and it could happen anywhere,” Unduche said.
“Those kind of high intensity rainfalls you only know after the fact, unfortunately,” he said. “But we prepare for all those events.”
Johanu Botha, head of the province’s Emergency Management Organization, said his team will be watching for municipal road closures following the weather system and will be prepared to communicate changes to the public.
“We’ll be in response phase this weekend, related to the weather system, and then the flood response, we will have to monitor and see,” Botha said.
Updated forecast flows and levels across the basins will be available as the system continues to develop in the coming days, according to the province.
As a general assignment reporter, Chris covers a little bit of everything for the Free Press.
Danielle Da Silva
Danielle Da Silva is a general assignment reporter.
Updated on Wednesday, April 20, 2022 6:40 PM CDT: Adds video
Updated on Wednesday, April 20, 2022 10:07 PM CDT: Fixes typo.