Flood potential analyses are based on current soil and snow conditions combined with a broad spectrum of potential spring weather conditions reflected in the climate record from 1948 to 2013. The analyses contain herein are very general. A quantified risk of flooding with respect to climatology is available through web graphics and tables can be found at NWS Long_Range Flood Risk, along with links to NWS Forecast Office Home Pages
2018 Late Winter Basin Conditions as of February 28, 2018
Since our last outlook, colder than normal conditions have continued from the Dakotas and Minnesota, into Iowa and Wisconsin. But warmer weather has dominated from Missouri and Illinois into lower Michigan. February precipitation has been generally been above normal, especially from Missouri up into lower Michigan, where 2 to 3 times the normal has been seen.
Last week very warm temperatures and heavy rainfall of 2 to 6 inches impacted northern portions of Illinois and Indiana, as well as southern Michigan. This rain fell in conjunction with temperatures in the 50s and 60s, which caused a rapid snowmelt. Snow water amounts on the ground of 1 to 1.5 inches was quickly liberated as the snow melted.
The resulting rapid runoff on top of saturated and frozen soil caused major and record flooding in these areas. Water levels were mainly receding at this time, but creeks and rivers continue to run higher than normal.
Farther north, a couple of storm systems brought more snow in bands from Minnesota, across Wisconsin, and into the Upper Peninsula. This added a bit more snow cover and water equivalent in these areas, with water content increasing to between 1 and 4 inches across much of the Upper Mississippi basin. The highest snow water remains from the Minnesota Arrowhead region, across the U.P. where greater than 6 inches of snow water is present. But this is fairly common for these locales.
With the warmer temperatures recently, the snow line extends across far northern Iowa, through central Wisconsin, and a small portion of northwest lower Michigan. South of that, little snow remains.
In areas where the snow has melted, soil conditions are very wet. There is also still frost in the ground, although it is not very deep. Frost depth is 3 inches or less from Missouri and Illinois into southern lower Michigan. The rest of lower Michigan only has frost depth of 12 inches or less. Meanwhile, further north and west, where there was very little snow cover for much of the first half of winter, the frost has gone pretty deep. Frost depth of 3 to 4 feet is reported from the Dakotas into the northern half of Wisconsin. Frost of greater than 5 feet was reported in western North Dakota.
Current stream flow is well above normal for areas that experienced the rain and recent flooding. In these areas, they will remain susceptible to additional flooding due to future precipitation. Looking ahead this week and into the weekend, there are a couple of storm systems that will bring precipitation to the region. Depending on where and how much falls, flooding is a concern. In addition to the areas that had recent high water, we will monitor potential from Iowa and southern Minnesota, and into Wisconsin. If this precipitation falls as rain, we could see enhanced snowmelt. That melt, combined with rainfall, and running off over deeply frozen ground, could be a recipe for flooding in a similar fashion to what was observed last week farther south.
The outlook for March suggests temperatures near to below normal across the region, which suggests that areas that have snow, will maintain it for a few more weeks at least. But the outlook also calls for near or below normal precipitation.
GREAT LAKES DRAINAGE
Overall, the flood potential for this area this spring is near normal.
Overall, the flood potential for this area this spring remains near normal. However portions of lower Michigan, Indiana, and Illinois will have an enhanced susceptibility to flooding for the next few weeks due to the current high water.
After a cool winter, especially early February, temperatures were trending above normal in recent weeks. This has eliminated the snow for parts of the region, south of a line generally from central Wisconsin to northern lower Michigan. But we expect to see more snow fall from systems this week and into the weekend.
A significant snow pack remains across the Minnesota Arrowhead and the Upper Peninsula, where amounts over 2 feet are still on the ground. These areas have snow water equivalent of over 6 inches. Snow water amounts taper off quickly southward. A good deal of ice cover remains on the Great Lakes, especially Lake Superior.
River ice had been thicker than normal, but the recent melt and high water allowed most of it to break up and move out. Any ice jam risk from break up appears minimal.
UPPER MISSISSIPPI RIVER DRAINAGE
Overall, the flood risk for this drainage area is near normal.
February temperatures continued the overall below normal trend, but we have seen some moderation in the past week. This is especially true for Iowa and Missouri, into Illinois and Indiana, where we saw temperatures surging into the 50s and 60s. Precipitation has also trended a bit higher in recent weeks, after a relatively dry winter overall.
Recent storm systems have finally enhanced the snow cover across Minnesota, but the warmer weather this week is already eroding it. Snow water content range from less than an inch in western Minnesota, to around 2 to 3 inches over parts of southern Minnesota into northern Wisconsin. Little or no snow currently exists south of a line from northern Iowa to Milwaukee.
Frost depth continues to be quite deep over Minnesota and the northern two thirds of Wisconsin. Depth of 2 to 4 feet is common. Farther south, frost depth is less than 3 to 6 inches from Iowa and Illinois into Missouri.
In general, the flood potential for this area this spring is near normal. However, there are caveats. Portions of Illinois and Missouri have seen high water and flooding from recent events. If additional heavy precipitation occurs in these areas, flooding will become more likely. In addition, farther north where some snow still remains, the expected storm systems this week and into the weekend could bring rainfall on top of snow cover. Depending on where and how much falls, flooding is a concern for northern Iowa, the southern half of Minnesota and Wisconsin, and also northern Illinois. If this precipitation falls as rain, we could see enhanced snowmelt. That melt, combined with rainfall, and running off over deeply frozen ground, could be a recipe for flooding.
HUDSON BAY DRAINAGE
Red River of the North River Basin
Overall, the risk of flooding for the Red River basin is below normal this spring.
February temperatures continued the overall below normal trend for this winter. There have also been a few storm systems that brought a little snow to the region. Snow water equivalent around the basin has increased to a 2 to 4 inch range, especially in the middle portion of the Red valley, and the Minnesota tributaries.
Ice thickness in the water courses is above normal, and the most recent water levels are also slightly above normal. If the spring melt and future rainfall generate some enhanced runoff, break-up ice jams could form. The effects of ice jams are not included in any of the long range probabilistic outlooks.
Frost is very deep, with several reports of 3 to 4 feet.
With deep frost, it will be a while before the soil is able to absorb much of the spring melt. So we could see enhanced runoff especially if the melt is sudden and accompanied by rainfall. So although the risk is below normal, we do have a chance if these factors combine.
Souris River Basin and Devils Lake Basins
Overall the risk for flooding in these basins is below normal.
Cold temperatures have prevailed, allowing frost to go quite deep across the region. Frost depth of 3 to 5 feet has been reported. But, the soils were pretty dry going into the freeze up period, so the soil is not frozen solid, and should allow for some infiltration.
Despite a few storm systems moving through the region, snow continues to be sparse. Water equivalent in the snow cover is generally an inch or less west of a line from Bismarck to Rugby. Snow water of 1 to 2 inches is seen in the Devils Lake basin.