Manitoba Water Stewardship

A stationary upper low pressure system over the western Dakotas will produce unsettled weather over southern Manitoba from today until Thursday. Occasional showers or thundershowers are expected for the next three days, mainly over the Red River watershed. Environment Canada has indicated that some thunderstorms could be severe with heavy rainfall. Areas on southern Manitoba closer to the U.S. boundary received another 25-35 mm of rain on Saturday, in addition to previous rains last week.

Rainfall of 10-15 mm is likely in the upper Assiniboine River watershed today and/or tomorrow. It appears that heavier rain of 20-40 mm will pass north of the Assiniboine watershed, falling on the Red Deer and Carrot River watersheds. Winds over Shellmouth Reservoir are expected to shift from southerly to northerly late tonight or early tomorrow.

A FLOOD WARNING CONTINUES FOR THE ASSINIBOINE RIVER FROM SHELLMOUTH TO MILLWOOD.

The Shellmouth Reservoir water level has declined a tenth of a foot to 1410.15 feet due to southerly winds.

The reservoir inflow had risen to 1900 cfs this morning and is expected to crest at 2100 cfs later this week based on little additional rainfall.

The total reservoir outflow remains near 1750 cfs. The conduit flow was increased from 400 to 500 cfs this morning to compensate for a 100 cfs decline in the spillway flow due to southerly winds. Conduit flows will be adjusted as required to prevent total outflows from exceeding 1750 cfs as long as possible.

The amount of rainfall during the next few days will determine whether it will be possible to keep peak reservoir outflows below 1750 cfs. The present forecast on the attached spreadsheet is based on rainfall of 10 mm or less upstream of Shellmouth Dam from the present storm system.

Minor flooding of the lowest lands in the Assiniboine Valley from Shellmouth Dam to Millwood is taking place at this time, mostly due to lack of good drainage due to the high river levels. A visual inspection this morning showed that the river is not overflowing its banks.

Portions of the Assiniboine River from St. Lazare to Brandon have been experiencing some rises due to recent rainfall. The crest from rain downstream of the Reservoir is presently at Miniota. Additional rises from Virden to Grand Valley should be less than half a foot unless heavy additional rain develops.

The level of Red Deer Lake rose 0.3 feet since last Friday and is within half a foot of the crest in early May, 2007. Little further rise is expected unless additional heavy rain develops in the next few days. The weather forecast is not favourable.

Levels if the Carrot River rose somewhat last week due to heavy rain. Little further rise is expected unless heavy more heavy rain develops.

Levels of the Saskatchewan River at the Pas have been declining since last week and should continue to fall slowly.

Heavy rain in the Red River Valley in both southern Manitoba and the areas north of Grand Forks in the United States has resulted in significant rises of the Red River since the middle of last week. The river level at Emerson has risen 5.5 feet and may rise another foot before it crests in the next few days even without additional rain. The river level in the City of Winnipeg rose above the Forks walkways this morning and is expected to rise an additional two feet before it crests late this week. The Red River could experience significant additional rises beyond those presently predicted if more heavy rain develops in the next few days. The amount and spatial distribution of precipitation for the next three days is particularly difficult to predict since it will be primarily generated by atmospheric instability.

Some overland flooding could develop in the Red River Valley if more than 50 mm of rain develops during the next few days, especially of the rain comes in the form of thundershowers. Soil moisture levels are unusually high due to recent rains.