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The National Weather Service has issued its 2021 Spring Flood Outlook for the Mississippi River.
This updated Spring Flood and Water Resources Outlook is for the Quad Cities Hydrologic Service Area (HSA), which covers portions of eastern Iowa, northwest and west central Illinois, and extreme northeast Missouri. This includes the Mississippi River from above Dubuque, Iowa to below Gregory Landing, Missouri and its tributaries. It also includes the Des Moiner River and the Fox River.
According to the report, the risk of minor flooding for the Mississippi River downstream from the Quad Cities is slightly above normal. The flood risk for local rivers is near normal to above normal.
Precipitation this winter has averaged above normal for much of the NWS Quad Cities Hydrologic Service Area (HSA), however the larger-scale Upper Mississippi River watershed has averaged near to below normal precipitation.
During the month of January, local climate sites received anywhere from three tenths to near one inch above normal liquid precipitation amounts. This was due to several weather systems that produced a mix of heavy rain and high water content snow. Much of this precipitation is contained in the snowpack as of this outlook.
Watersheds that have incurred above normal precipitation this winter will have an increased risk of flooding this spring. Conversely, locations that have received near to below normal precipitation this winter, at the location or upstream, will not see the flood threat increased.
Snow Cover and Liquid Water Content
As of mid February, there is snow cover across the entire Upper Mississippi River watershed, which encompasses all of the local area and includes locations to the north across much of Minnesota and Wisconsin. When being compared to normal values, it varies across the region by a wide range, from below to well above normal.
The local area has some of the deepest snowpack in the region as of mid February, which is well above normal. From central Iowa through northern Illinois, the snowpack ranges from 8 to 20 inches in depth with lesser amounts going through the southern part of the forecast area. Much of this snowpack also has a thick layer of embedded ice due to rain and freezing rain that fell in January.
Looking to the north across northern Iowa, Minnesota, and the northern half of Wisconsin, which include areas that feed the Des Moines, Cedar, Iowa, Wapsipinicon, and Mississippi Rivers, the majority of the region is observing below normal snow depths and liquid water content in the snowpack.
Warm temperatures earlier this winter kept the ground from freezing. Before cold weather set in, the deep snowpack built up, which has acted as a blanket over the ground and has prevented deep frost from developing. Even with the cold conditions that have been in place through much of February, frost depths across eastern Iowa, northeast Missouri, and western to northwest Illinois are quite shallow, frozen at depths of 4 inches or less.
There are some locations with deeper frost, especially going north in the region, but overall frost depths are below normal and will not contribute to a higher flood risk.
Ice Jam Flooding
Ice jams have not been an issue for much of this winter due to a prolonged period of above normal temperatures in December and January, keeping ice formation minimal. However, temperatures have averaged well below normal to start February, which is forecast to continue heading through the remainder of the month as Arctic air outbreaks remain favorable.
River ice will build and thicken across the area through the remainder of February, which will raise the potential for break up ice jams at some point this spring.
According to the U.S. Drought Monitor, there are no local areas observing drought conditions, however portions of the Midwest are seeing Abnormally Dry to Severe Drought conditions.