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Heavy, wet snow hitting upper Wisconsin and Minnesota is expected to cause flooding along the upper Mississippi this spring, and it could be significant.
The National Weather Service released an outlook for spring flooding on Thursday and said the risk was “much above normal”.
The forecast is almost entirely driven by heavy snowfall. The upper Midwest has been hit with such storm systems, especially in the past six weeks.
Snow in the region has been wetter than usual this winter, meaning an unusually large amount of water is trapped in the snowpack and ready to melt, said Jordan Wendt, service hydrologist for the National Weather Service in La Crosse.
Many inches of water are stored in the snowpack covering the basins of the St. Croix, Chippewa, and Wisconsin rivers, which empties into the Mississippi. “If everything melted at once, that would be equivalent to eight inches of rain at once,” he said.
Although the Midwest and Great Plains experienced a drought last summer that slowed trade on the Mississippi, rain and snowfall have brought the river and its tributaries back to normal levels, Wendt said. And the frost depth, which affects how much water can penetrate the ground before it runs off, is shallower than normal.
Along the river flowing out of the Twin Cities, some locations currently have a more than 50% chance of experiencing major flooding in the next 90 days, according to the Weather Service’s flood forecast. Because it flows south and forms the Wisconsin border, there is a greater than 50% chance of moderate flooding in some locations.
The last time the river’s main stem experienced similar flood prospects was in 2019, when prolonged flooding caused an estimated $20 billion in public and private property and crop losses, according to the National Oceanic and Atmospheric Administration. Many places along the river were at or above high water level for most of the year, an unusual occurrence.
If the snow steadily melts and the region doesn’t get more snow or rain, the risk of flooding will decrease, Wendt said. But the snow needs to melt slowly to give the river time to move the extra water.
On the other hand, the worst-case scenario would be rapid warming. If colder temperatures persist and snowpack lasts through April, it’s more likely to melt quickly through a series of 50- or 60-degree days, which could also be punctuated by additional precipitation from a thunderstorm, Wendt said.
According to data from the National Weather Service, the region is likely to experience below-normal temperatures for the next few weeks.
While there could be significant flooding of the Mississippi River, flood risk for Wisconsin and Minnesota tributaries leading to the river is forecast to be near normal, and could be less than normal in Iowa tributaries.
The Mississippi is more likely to flood because it drains such a vast region, with more than 21,000 miles of other rivers joining it until it reaches Dubuque, Iowa. If all of that water finds its way south, flood defenses could be activated along parts of the river to protect cities and communities. In Louisiana, the Army Corps could open two spillways to relieve the swollen river, which could have far-reaching ecological impacts.
Flooding on the Mississippi usually develops slowly, Wendt said, and it’s likely the La Crosse-area river won’t reach flood stage until late April or early May.
That gives surrounding communities time to prepare, but everyone should monitor the situation, he said.
“It’s almost not a question of whether we’ll get floods or not, but how severe it will be,” Wendt said.