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| Surface Water Information Flood Forecasting Reports |
Spring Flood Outlook for Southern Manitoba
February 21, 2008
The potential for flooding is relatively low in most portions of southern Manitoba this spring due to a below average snowpack and average to below average soil moisture. Much of the meltwater is expected to soak into the ground. However the flood potential is moderate in a portion of western Manitoba from the Porcupine Mountain to Carrot River and above average runoff is expected in portions of northern Manitoba.
The spring flood potential is still very dependent on weather conditions from now through May. The amount of additional precipitation and the timing and rapidity of the breakup will have a significant effect on the flood potential. An updated outlook will be prepared during the third week of March, 2008.
An aerial soil moisture survey conducted on October 31, 2007 showed that moisture in the top 20 cm of soil was below average in all portions of southern Manitoba with the exception of the southern portions of the Riding Mountain. Topsoil moisture is believed to be high in southeastern Manitoba including the Whiteshell due to heavy October rains; this area is not covered by the airborne survey. A map showing the results of the airborne soil moisture survey is attached (NOHRSC logo).
Above average precipitation in southern Manitoba during May, June and early July of 2007 gave way to generally dry weather during most of the summer and autumn. Precipitation this winter has been generally below average in the southern prairies. It was unusually mild until mid January and unusually cold after that in the eastern Prairies. The causes of these major changes in the weather are not well understood but appear to be in large part due to a fairly strong La Nina condition in the tropics. The National Oceanic and Atmospheric Administration (NOAA) in the United States predicts that the La Nina will continue at least until the early summer, 2008. This favours below average temperatures in Manitoba and vicinity for the next 3 months; however the effect on precipitation is not clear as there are other climatic forces at work which complicate matters.
An aerial soil moisture survey conducted on October 31, 2007 showed that moisture in the top 20 cm of soil was below average in all portions of southern Manitoba with the exception of the southern portions of the Riding Mountain. Topsoil moisture is believed to be high in southeastern Manitoba including the Whiteshell due to heavy October rains; this area is not covered by the airborne survey. A map showing the results of the airborne soil moisture survey is attached (NOHRSC logo).
Fall soil moisture in the deeper root zone (30 cm to 120 cm), was below average in virtually all agricultural areas of Manitoba based on a field survey conducted by Manitoba Agriculture, Food and Rural Initiatives in late October, 2007. The attached map shows that soil moisture in this zone was less than 50 % of capacity in most areas of southern Manitoba. The survey also showed below average soil moisture in the top zone.
An index of soil moisture called the antecedent precipitation index (API) is commonly computed by Manitoba’s Hydrologic Forecast Centre based on weighted precipitation from May to October expressed as a percent of normal. The attached API map suggests that soil moisture on October 31, 2007 was generally below average in southern Manitoba. However it suggests above average soil moisture in areas close to the Ontario boundary, the Duck and Porcupine mountain area and in portions of far northern Manitoba.
An aerial snow survey over the Red, Assiniboine and Souris River watersheds was conducted February 11-14, 2008 under contract with the U. S. National Weather Service (map with NOHRSC logo). The survey showed that snow water content ranged from 1-2 inches (25-50 mm) over most areas of southern Manitoba, which is significantly below the average since 1995 when these surveys began.
A conventional surface-based snow survey for selected areas of Manitoba was conducted February 11-14 by Manitoba Infrastructure and Transportation staff under the direction of Manitoba Water Stewardship. Maps showing the water content in millimetres and as a percent of the long term normal are attached. The water content of the snowpack was found to be below average in most areas of southern Manitoba with some areas having only half the normal snowpack. The snow water content was above average from the Porcupine Mountain to The Pas. Snow samples can produce highly variable results from one location to another due to re-distribution of snow by wind and differences in land use, topography and vegetation.
Snowcover is also available from a satellite survey provided by Environment Canada. A map for February 15, 2008 is attached (EC-Climate Research Division logo). This survey suggests that snowcover is below average in most of southern Manitoba but is average to above average in portions of western and northern Manitoba. It is well above average in north-central Saskatchewan. Satellite estimates of snow water content may be inaccurate at times due to sudden changes in weather and crystal structure of the snowpack.
Cumulative winter precipitation from Environment Canada climate stations is also used as an estimate of snowcover. A map showing percent of normal precipitation from November 1, 2007 to February 18, 2008 is attached. The number of sites reporting winter precipitation has diminished in recent years and snowfall is difficult to measure accurately in gauges due to wind effects; therefore these estimates must also be treated with caution.
A map showing the expected percent of normal runoff potential for the spring of 2008 is attached. This prediction is based on present basin conditions and average future conditions for precipitation, date of breakup and melt rate.
The 2007 spring runoff is expected to be below average in much of southern Manitoba. Dry soils resulting from below average precipitation from July to October, 2007 should absorb most of the meltwater from a below average snowpack. Runoff in the Whiteshell and Nopiming Park areas is likely to be near average even though snowcover is quite meager. This is due to high soil moisture remaining from heavy rain last October.
Above average runoff is expected in an area from the Porcupine Hills to just south of The Pas where soil moisture and snowcover are somewhat above average.
Above average runoff is also expected in the portion of northern Manitoba from Thompson northeast to Hudson Bay where snowcover and soil moisture are above average.
Spring runoff could deviate significantly from that shown on the attached runoff potential map if future precipitation and breakup conditions differ greatly from the average. Below average precipitation from now through the spring could result in little or no runoff in many areas. However above average precipitation, a late breakup and a quick melt could still produce minor flooding in many areas of southern Manitoba this spring.
The spring flood outlook is based on three future weather scenarios relating to additional snow, melt rates and spring rain. These scenarios are based on climate statistics for the past 40 years. The three scenarios are superimposed on the present basin conditions for input to river forecasting procedures. The three future weather scenarios are sometimes referred to as ‘favourable’, ‘average’ and ‘unfavourable’ and relate to the statistical lower decile, median and upper decile conditions respectively. There is a one-in-ten chance of the weather being ‘favourable’ or better and there is a one-in-ten chance of it being ‘unfavourable’ or worse. Additional precipitation for this February outlook ranges from about 15-20 mm for the lower decile, 40-50 mm for the median and 80-90 mm for the upper decile condition with some variation due to location. The lower decile and upper decile conditions also assume that breakup will be slow and relatively fast respectively while average conditions are assumed for the median. Reference to the lower decile condition is sometimes omitted in the forecasts if it is not relevant.
Forecasts do not include effects of possible ice jams which are generally unpredictable. Brief flooding can occur in locations where ice jams develop, even with below average river flows. However ice jams are likely to be of lesser significance when river flows are low as expected this year under the average weather scenario.
Flood outlooks for international and inter-provincial streams include information obtained from the U.S. National Weather Service and the Saskatchewan Water Authority.
Tables showing numerical estimates of peak river flows and stages are not available for this outlook but will be presented with the March, 2008 updated spring flood and runoff outlook.
The following sub-sections describe the forecast and flood potential for specific watersheds.
The potential for spring flooding is quite low due to below average snowcover and below average soil moisture.
With average weather from now through April the Red River is expected to remain within its banks and operation of the Red River Floodway would not be required.
With unfavourable weather (1:10 chance) minor overbank flows would occur in the portion from Emerson to just south of Morris. Operation of the Red River Floodway would result in a crest near 18 feet (745.6 feet asl) in downtown Winnipeg while the crest upstream of the Floodway Inlet would be near 758 feet. Construction on the floodway channel would not interfere with floodway operation should an operation become necessary.
Serious ice jamming from Selkirk to Breezy Point appears unlikely this spring with normal weather conditions, due to expected low river flows. However ice jamming is very difficult to predict and unusual spring weather can increase the probability of its occurrence. The threat of ice jams will be reduced by ice breaking to be undertaken by the North Red Community Water Maintenance Incorporated Amphibex and by a second ice breaker being leased by the Province. One will begin work on the Red River from Selkirk to Lake Winnipeg in late February and the second is scheduled to begin March 6. Manitoba Water Stewardship is also undertaking river ice cutting to complement the work of the two ice breakers in portions of the river with thicker ice.
Flooding is not expected on Red River tributaries with average weather from now on. The unfavourable weather scenario could produce minor localized flooding.
Some lakes such as Pelican Lake might not reach the summer target if the dry weather scenario unfolds.
The lower decile condition would result in relatively little runoff and no flooding. The drawdown of Shellmouth Reservoir is being conducted in a manner which will minimize the risk of water shortages while maintaining enough storage space for flood control in the event that the upper decile weather condition develops. The Shellmouth Reservoir Regulation Liaison Committee is providing advice regarding operations and their potential impacts.
Flooding is not expected along the Assiniboine River with average weather conditions from now on. Shellmouth Reservoir would successfully store spring inflows for this condition. Operation of the Portage Diversion might not be required.
An unfavourable weather scenario (1:10 chance) would result in minor overbank flows along the Assiniboine River from Shellmouth to Brandon with crests similar to those of 2007. Operation of Shellmouth Reservoir would endeavor to end spring flooding along the Assiniboine River as soon as possible to enable seeding of agricultural lands. This could include above bankfull releases during April and early May if necessary to prevent overtopping of the spillway.
Flooding would be unlikely from Spruce Woods to Winnipeg due to low expected flows on the Souris River and due to operation of the Portage Diversion under this weather scenario.
Little or no flooding is expected with average weather from now on.
An unfavourable weather scenario could produce minor overbank flows in low lying areas.
Flooding is not expected along the Souris River with average weather conditions from now on.
The unfavourable weather condition could produce minor flooding in the Coulter area where the bankfull capacity is very low.
Minor localized flooding of low areas on Souris River tributaries could develop under the unfavourable weather scenario.
Oak Lake levels might not reach the post spring target if the dry weather scenario unfolds.
Average weather conditions would not produce spring flooding.
The unfavourable weather scenario (1:10 chance) could produce minor flooding on the Fisher River (including Peguis Townsite) and other streams in the eastern Interlake, but this would be less than the minor flooding of 2006.
Flooding of low-lying agricultural lands near the Shoal lakes would increase in extent under this scenario, but would remain less than in 2006.
Peak flows in the Fairford River and Dauphin River would be similar to those of 2007 when significant flooding was avoided. On February 4, 2008 outflows from Fairford Dam were restored to the settings prior to last autumn’s operations to reduce ice jamming on the Dauphin River at freeze-up. No further operations are likely to occur this year unless precipitation above the upper decile condition develops.
Flows on the Waterhen River would be much above average under this scenario but would be somewhat less than in 2007.
While the Waterhen River and the Dauphin River contain a lot of ice buildups which developed last autumn, ice jam related flooding is unlikely during the spring.
Under the dry weather scenario there would be some concern about low flows and lake levels in southeastern Manitoba and the Whiteshell, since snowcover is well below average at this time.
Flooding is not expected with average future weather conditions. Flows on the Winnipeg River would show little or no increase from present flows and levels will decline as winter ice moves out.
The unfavourable weather scenario could produce high lake levels and flooding due to the high soil moisture remaining from heavy rain last October. The latter would cause a steep increase in runoff with increased precipitation. Lakes along the Winnipeg River would rise under this scenario but likely not to excessive levels.
The lower decile (dry) weather scenario would result in minimal runoff and little rise in the level of Dauphin Lake, leaving it up to three quarters of a foot below the summer target level.
Flooding is not expected on the Whitemud River and its tributaries, and on streams flowing into Dauphin Lake with normal weather from now on. Dauphin Lake would rise to a suitable level.
The unfavorable weather scenario could produce minor flooding on streams in the region especially if the melt is rapid.
The favourable weather scenario would result in minor localized flooding of low lying areas on most streams from the Bell River north to the Carrot River.
The average weather scenario would result in some flooding of the Red Deer River, Overflowing River, Pasquia River and smaller streams in the area but it would be significantly less than in 2007. Flooding would be unlikely on the Swan River, Carrot River and the Saskatchewan River. The settlement of Red Deer Lake should not experience difficulties due to high water.
The unfavourable weather scenario would likely produce a repeat of the flood of 2007 on Red Deer Lake which caused difficulties but was much lesser than the 2006 flood. The Carrot River and Pasquia Polder area could experience flooding similar to that of 2007, while flooding of the Saskeram area should still be less than the serious flooding of 2007. The peak water levels at The Pas should be somewhat lower than in 2007 but some diking might be required at OCN First Nation. Minor flooding could develop on the Swan River system including Swan Lake. Streams flowing east and north off the Duck Mountain and smaller streams in the Porcupine Mountain area could experience some flooding, especially if a rapid melt were to develop. Lakes in the vicinity of The Pas would rise above desirable levels under this weather scenario.
Runoff in northern Manitoba (north of latitude 54) is likely to range from average to above average as shown on the runoff potential map. Flooding is not expected on the Churchill River or the Nelson River. Manitoba Hydro may have more specific information for certain areas in northern Manitoba such as the Churchill River near Churchill.
The present level of Lake Winnipeg is 714.1 feet which is somewhat above normal for this time of year. A smaller than usual rise is likely this spring due to below average inflows from the Winnipeg River and the Red River. With average precipitation from now on, the lake is unlikely to rise higher than 714.4 feet this spring which is a little lower than average but in the desirable range.
The level of Lake Manitoba is presently at 812.0 feet, which is near the upper end of the operating range of 810.5 to 812.5. With average precipitation the lake is likely to rise to about 812.25 by the end of spring runoff. With upper decile weather it would rise to the top of the range of regulation. Lake St. Martin is expected to be at levels similar to those of 2007, which were somewhat below flood stage.
Lake Winnipegosis is presently at 832.1 feet which is still two feet above average for this time of year. With average precipitation from now on the level after spring runoff should be well below the 833.7 feet reached last July, which was the highest level since the 1950’s. With upper decile precipitation, the high levels of 2007 could be repeated with associated flooding of low-lying agricultural lands adjacent to the lake.
Lake Of The Woods is at a level of 1058.5 feet which is very close to the average February level. The southern portion of the watershed is very dry and therefore it is possible that the lake may be a little below its summer target level following spring runoff unless precipitation from now through May is at least average.
The level of Reindeer Lake has been declining slowly this winter but remains at above average levels. The Saskatchewan Water Authority predicts that the lake will rise to near the top of its operating range next summer based on normal weather conditions. Most other lakes in far northern Manitoba are expected to be at average or above average levels after the 2008 spring runoff.
This spring flood outlook will be updated near March 20, 2008. Daily flood reports will be issued for streams with a high flood risk when spring runoff gets underway.
This report may also be viewed on the Water Stewardship web site at:http://www.gov.mb.ca/waterstewardship/ click on Water Information, then Flood Forecast Reports
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