Flooding in northwest Wisconsin and the Mississippi River
NWS Milwaukee, WI
A near record snowpack that melted rapidly resulted in widespread flooding across the northwest quadrant of Wisconsin and major flooding of the Wisconsin River. Well above normal temperatures during the second week in April resulted in several location seeing over 20 inches of snow melt in just a few days. Despite no rainfall during this time, the flooding was significant. Many roads were flooded, culverts washed out, and homes were flooded. The Mississippi River crested at one of the highest levels on record.
Map of ranked simulated snow water equivalent across the Midwest. Values ranked in the 90-100th percentile across northwest Wisconsin.
NWS North Central River Forecast Center Snow Water Equivalent Simulation based on observed values input into a forecast model.
Setting the Stage
Precipitation for the winter was above normal. An early December storm jump started a deep snowpack that stayed early April. Little melting occurred prior to the big snowmelt induced flooding. At the beginning of April the snow depth was 20 to 40 inches across northern Wisconsin. The amount of water in the snowpack, called snow water equivalent, was well above normal. Snow water equivalent data is estimated from satellites and measured from snow core observations taken by weather observers. The snow water equivalent values ranked in the 90-100th percentile, meaning the 10% highest of all years across northwest Wisconsin. The led to some concern about flood risk during the winter season as the snowpack continued to build but it was unknown how quickly or slowly the snow would melt, which is a big factor in flood potential.
Map of accumulated snowfall departure from normal from November 1, 2022 to March 31, 2023 for Wisconsin. Values were 20 to 40 inches above average in northwest Wisconsin.
Map of snow depth across Wisconsin on April 1, 2023. Values are 10 to 35 inches across the northern half of Wisconsin.
Left: Accumulated snowfall departure from normal for the cold season, November 1, 2022 to March 31, 2023. Right: Snow depth in inches on April 1, 2023 measured by Cooperative Weather Observes.
The Big Melt
On April 8 the snow water equivalent of the snowpack across northwest Wisconsin was 5 to 10 inches with values up to 15 inches in northern Bayfield county. These are some of the highest values that have occurred that late in the season. Then a warm spell hit during the second week of April. Highs were in the 70s and 80s, which is 20 to 30 degrees above normal. The majority of the snow melted by April 12. The ground was still frozen and many culverts and ditches were blocked by melting snow and ice leading to poor drainage, so there was nowhere for the water to go.
Map of modeled snow water equivalent on April 8, 2023. Values are 4 to 10 inches across northwest Wisconsin.
Map of modeled snow water equivalent on April 12, 2023. Values are 0 to 2 inches across northwest Wisconsin.
Snow Water Equivalent from National Operational Hydrologic Remote Sensing Center on April 8 and April 12, 2023 across north-central Wisconsin. Modeled snow water equivalent is from observations input into a snow model to interpolate gaps in coverage.
Snow Water Equivalent from National Operational Hydrologic Remote Sensing Center on April 8 and April 12, 2023 across far northwest Wisconsin. Modeled snow water equivalent is from observations input into a snow model to interpolate gaps in coverage.
There was widespread standing water affecting roads and homes, water flowing across roadways, lakes were overfilling and impacting homes, and washed out culverts across Douglas, Bayfield, Ashland, Iron, Burnett, Washburn, and Sawyer counties. Rivers rose rapidly exceeding flood stage and many creeks approached the bottom of bridge decks. Some rivers crested close to record stage. Several roads were closed or damaged, including highways. Numerous townships and counties declared a state of emergency.
Flooded road in Draper, Wisconsin in Sawyer County.
Timeseries of water level values for the Tyler Forks River near Mellon, Wisconsin from April 7 to May 5, 2023. The river crested 3 separate times above flood stage.
Picture of Denomie Creek near New Odanah, Wisconsin approaching bottom of bridge.
Left: Flooded road in Draper, WI (Sawyer County), Image Credit: Town of Draper. Middle: Tyler Forks River near Mellon, Image Credit: USGS. Blue line shows observed river levels with 3 separate crests above flood stage. Right: Denomie Creek near New Odanah, WI approaching bottom of bridge, Image Credit: Nathan Kilger.
Round Two
In mid April some flood waters had receded but there were still some areas with standing water and river and lakes continued to be high. The ground was saturated, so conditions were very sensitive to any additional rainfall or snow and remaining snow to melt. Temperatures cooled back down to normal levels, then a second round of melting along with rain occurred April 18 to 22, 2023. It resulted in a second but lower crest on many rivers. By April 22, most of the snow had melted. During the second and third weeks of April the volume of water in the rivers across much of northwest Wisconsin exceeded the 90th percentile. Several rivers had 3 or 4 separate crests through early May before flood waters finally receded.
Map of streamflow percentile for April 2023 across Wisconsin. Values were greater than 90 for the northern two thirds of the state and in the 76-90th percentile across much of southern Wisconsin.
Timeseries of water level values compared to normal from October 2022 to September 2023 for the Bois Brule River in Brule, Wisconsin. Values were in the normal range, 25-75th percentile for much of 2023 then over 90th percentile in April 2023.
Left: Streamflow percentile for April, Image Credit: USGS. Right: Timeseries of daily average discharge for the past water year, Image Credit: USGS.
Mississippi River Flooding
Observed river level of the Mississippi River in La Crosse shows a crest exceeding major flood on April 26.
Observed river level of the Mississippi River in La Crosse shows a crest exceeding major flood stage on April 26, 2023. The blue line shows the forecast river level as of May 1, 2023
The Mississippi River reached the highest level in over 20 years (since the spring of 2001). Many locations along the river had a top 5 highest crest on record. The river started rising after the majority of the snow melted April 10 to 12, 2023. The river basin extends across central Minnesota and northwest Wisconsin. The snowpack was near record across the entire basin with a snow water equivalent of 4 to 8 inches. Most of the snow melted rapidly during the second week in April, which contributed to a large amount of the water running off into the river. April was also very wet month across the basin. From April 1 to 22, precipitation was up to 4 inches above normal in the river basins that flow into the Mississippi River. This additional rainfall likely helped increase the height of the crest. Being a large river basin, it takes some time for all the water to make its way to the river and it remains elevated for a longer time. The river rose above flood stage during the second week in April and crested from April 25 to 28. Water levels didn't fall below flood stage until the first to second week of May.
Picture of Mississippi River flooding in Trempealeau, Wisconsin showing homes flooded along the river.
Picture of Mississippi River flooding in Prairie du Chien, Wisconsin showing structures along the river are flooded.
Left: Mississippi River flooding in Trempealeau, Photo Credit: WI DNR. Right: Prairie du Chien April 26, Photo Credit: WI DNR
Widespread low land flooding was experienced along the Mississippi, impacting parks, campsites, roads, boat houses and near shore homes, docks etc. Some roads and parks were closed due to the flooding.
Flooding - Be Prepared
Flooding can happen anytime of year in Wisconsin, whether its from rain on frozen ground in winter, melting snow in spring, or heavy rain in summer and fall. Take the time now to prepare yourself and your property.
Get a NOAA Weather Radio, make sure you have wireless emergency alerts active on your phone, and monitor NWS river forecasts online at water.weather.gov/ahps
Be aware of roads that are susceptible to flooding in your area and plan alternate routes
Make sure your sump pump is working in your home
Elevate critical utilities and valuables in your basement
Observed river level of the Mississippi River in La Crosse shows a crest exceeding major flood stage on April 26, 2023. The blue line shows the forecast river level as of May 1, 2023
NWS North Central River Forecast Center Snow Water Equivalent Simulation based on observed values input into a forecast model.
Snow Water Equivalent from National Operational Hydrologic Remote Sensing Center on April 8 and April 12, 2023 across far northwest Wisconsin. Modeled snow water equivalent is from observations input into a snow model to interpolate gaps in coverage.
