Heavy, wet snow in upper Wisconsin, Minnesota raises risk of serious Mississippi River flooding
Heavy, wet snow that pounded upper Wisconsin and Minnesota is expected to cause flooding this spring along the upper Mississippi River, and it could be considerable.
The National Weather Service released a spring flooding outlook Thursday, saying the risk is "much above normal."
The prediction is driven almost entirely by the heavy, wet snow that fell across Minnesota and Wisconsin during the winter. The upper Midwest was walloped with such storm systems, especially over the last six weeks.
That means an abnormally high 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.
Snowpack covering the basins of the St. Croix, Chippewa and Wisconsin rivers — which drain into the Mississippi — are all "near their climatological max" of the amount of water that will melt from the snow and eventually end up in the Mississippi River, Wendt said, some having up to eight inches of water trapped in the snow.
"If it all melted at once, it would be the equivalent of getting eight inches of rain at once," he said.
Forecasters also look at how high the river level is currently and how much frost is on the ground, both of which Wendt said aren't playing much of a role in this year's flood forecast.
Though the Midwest and Great Plains experienced drought last summer, slowing down commerce on the Mississippi, the rain and snowfall from the winter have brought the river and its tributaries back to normal levels, Wendt said. And frost depth, which affects how much water can soak into the ground before it starts running off of it, is shallower than normal because of snow earlier in the winter.
Along the river as it flows out of the Twin Cities, some sites currently have more than a 50% chance of seeing major flooding over the next 90 days, according to the Weather Service flood outlook. As it flows south and forms the border with Wisconsin, some sites have more than a 50% chance of seeing moderate flooding.
The last time the river's mainstem saw a similar flood outlook was in 2019, when long-lasting floodwaters caused an estimated $20 billion in damage to public and private property and crop losses, according to the National Oceanic and Atmospheric Administration. Many spots along the river were at or above flood stage for the majority of that year, an unusual occurrence.
To decrease the likelihood of bad flooding, the next few weeks would have to experience slow and steady snowmelt and not much additional precipitation, Wendt said. For that to happen, snow would melt during the warmth of the day and run into the river, then stop melting as nighttime temperatures dropped below freezing, giving the river time to move the extra water.
On the flip side, the worst-case scenario would be a rapid warming event. If colder temperatures stick around and the snowpack stays until April, it’s more likely to be melted quickly by a string of 50- or 60-degree days, which could also be punctuated by extra precipitation from a thunderstorm, Wendt said.
It’s likely that the region will experience below-normal temperatures for the next few weeks, according to National Weather Service data.
More:Climate change imperils the upper Mississippi River backwaters. Now nature needs human help.
While significant Mississippi River flooding could occur, the flood risk is near normal for tributaries in Wisconsin and Minnesota that lead to the river, according to the outlook, and could be lower than normal in Iowa tributaries.
Why is the Mississippi River more likely to endure flooding? Its basin is vast, covering more than 1.2 million square miles of the contiguous U.S. More than 21,000 miles of other rivers join the Mississippi by the time it reaches Dubuque, according to the National Weather Service, dumping all of their water into it.
If all of that water makes its way south, flood control measures are activated along portions of the river to protect cities and towns. In Louisiana, the Army Corps will open two spillways to relieve the swollen river, which can have wide-ranging ecological impacts.
Flooding events on the Mississippi River typically unfold slowly, Wendt said, and it's likely that the river won't reach flood stage in the La Crosse area until late April or early May.
That gives surrounding communities time to prepare, but everyone should be monitoring the situation, he said.
"It's almost not a question of whether or not we do flood, but how severe it'll be," Wendt said.
This story is a product of the Mississippi River Basin Ag & Water Desk, an editorially independent reporting network based at the University of Missouri School of Journalism in partnership with Report For America and funded by the Walton Family Foundation.
Madeline Heim is a Report for America corps reporter who writes about environmental issues in the Mississippi River watershed and across Wisconsin. Contact her at 920-996-7266 or mheim@gannett.com.
This article originally appeared on Milwaukee Journal Sentinel: Above normal spring flooding predicted for Mississippi River
