Above normal Mississippi flood risk

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The National Weather Service (NWS) on March 9 released its third and final outlook for flooding on the Upper Mississippi River this spring, and concludes that the overall risk for flooding is well above normal for the Mississippi River. 

The Mississippi tributaries across Northeast Iowa have near to below normal chances for flooding this spring, notes the report.  The greatest chances of flooding this spring are on the mainstem of the Mississippi River.

The information for the outlook was collected from a number of partners of the National Weather Service including the United States Geological Survey, US Army Corps of Engineers, Midwestern Regional Climate Center, US Drought Monitor, Climate Prediction Center, and NOAA’s Office of Water Prediction.

At least one significant storm system a week has moved across the Upper Mississippi River basin in the last month, adding to the large snowpack to the north. Rains brought minor flooding across parts of Northeast Iowa along the Turkey River.

According to the NWS, some areas to the north of Guttenberg recorded their wettest winters: La Crescent Dam 7, Minn., (7.83 inches); Prairie du Chien, Wis. (7.16 inches); Rochester, Minn. (6.57 inches); and Steuben, WI (8.12 inches).

Precipitation from the start of the "water year" (Oct. 1) through March 8 was near to above normal. Overall, the amount of water currently moving through area rivers is near the long-term average, according to NWS. Due to drought, rivers were flowing well-below normal this past fall prior to winter; however, recent rains and runoff have brought rivers higher, closer to normal flows.

Thanks to an early snowpack as well as above-normal temperatures, frost depths are much more shallow than normal. There was enough snowpack across the area during the colder periods (around Christmas through the New Year) to limit deeper frosts from developing. With the loss of snow in February, there was a slight increase in frost depths along and south of Interstate 90.

Below-normal precipitation in the summer and fall of 2022 across the region allowed for soil moisture content to continue to fall below their long-term averages. Recent rains, however, have brought increased moisture in the regions soils. This will allow for more runoff than what was expected a month ago, but soils are not abnormally moist. Frost depths will initially hinder infiltration if there is a sudden thaw, which will allow more runoff.

Snow Water Equivalent (SWE) values across the upper reaches of the Mississippi River Basin are one of the biggest factors leading towards an above-normal chance of spring melt flooding. As of March 6 SWE amounts across Minnesota and Wisconsin have between 5 and 8 inches of SWE. These values are in the top 10 to 20% of the historical record and are the primary driver of the above-normal chance of flooding along the Mississippi River.

This winter has experienced periods of warmer than normal temperatures and shots of arctic air across the region. While this has allowed river ice to develop, there is no indication that the amount of ice is abnormal. The above conditions can and often change. The biggest factor affecting spring flood risks are the weather conditions leading into and during the sensitive period of melting snow. A slow and steady melt, with little to no additional precipitation will lower the flood risk significantly. Conversely, a fast warm up, coupled with moderate to heavy amounts of rainfall would significantly increase the flood risk.

The flood risk numbers are indicating that flooding could rival what was seen in the spring of 2019. Any mentionable precipitation between now and the end of spring thaw within the Upper Mississippi River basin will only increase probabilities for flooding - especially the chances for moderate to major flooding.

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