Above-normal flood risk this spring

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Marquette, McGregor and other nearby towns along the Mississippi River should brace for the potential of above- or well-above-normal flooding this spring, according to the preliminary hydrologic outlook issued by the National Weather Service (NWS) office in La Crosse on Feb. 21. 

In addition to the main-stem Mississippi River, the flooding risk is similar for many tributaries in Wisconsin that feed into it. Western tributaries in southeast Minnesota and northeast Iowa, on the other hand, have a near- to below-normal risk of spring flooding. 

The NWS predicts a greater than 95 percent chance the Mississippi will reach the minor flood stage of 16 feet at McGregor this year. Normally, the river has a 52 percent chance of reaching that level in the spring. 

There’s a 90 percent chance it will top 19 feet, which is considered moderate flood stage. The probability of major flooding—22 feet or more—is currently at 59 percent . Other years, the likelihood of a major event is just 9 percent. 

Breaking it down further, the NWS gives a 25 percent chance the river will exceed 24.8 feet at McGregor, just shy of the local record of 25.38 feet, set on April 24, 1965. There’s a 5 percent chance it will top 28.9 feet. At 27.9 feet, water would go over the levee. 

Per NWS records, the Mississippi has only exceeded 19 feet five times at McGregor in the past three decades. It hit 21.32 feet on April 18, 2011 and 23.75 feet on April 20, 2001. 

Several factors play into the enhanced flood risk, including expansive snow cover across the region, as well as thick river ice and saturated soils due to a wet fall. 

“Temperatures throughout meteorological winter, which began Dec. 1, have averaged below normal since mid-January,” noted the NWS report. “Meanwhile, precipitation during the same period has been above normal.” 

Area-wide, precipitation surpluses range from 1 to 3 inches. The NWS said that’s paired with a wet fall, when precipitation averaged 150 percent of normal. 

Snowpack in the region presently ranges from 10 to 25 inches, which the hydrologic outlook explained is well above normal for this time of year. 

“Within that snow, water equivalent of 2 to 3 inches is present,” the report continued, “highest across central Wisconsin into west-central and northern Minnesota.” 

The long stretch of cold weather has driven frost 18 to 30 inches into the ground. Most rivers are partially to completely ice-covered. 

“That ice, both in the rivers and in the ground, could play a significant role in flooding potential this spring, with any heavy rain that occurs likely to run off quickly due to frozen ground,” stated the NWS. “Any big warm-up or rises in river levels could also lead to ice jam formation through March and early April.” 

In March, the Climate Prediction Center indicates temperatures will remain below normal. 

“Should that occur, cold weather could help ease the concern of rapid snow melt, with snow melting slowly during the day and re-freezing at night rather than melting all at once,” the NWS said. 

As of now, the NWS indicated there is not a clear signal of the amount of precipitation the region will receive this month. El Niño conditions could make an impact by April. 

“Historically, the month of April during El Niño years has featured below-normal precipitation, though many recent Aprils since the year 2000 have featured wet conditions,” the report noted. “Weekly trends will need to be closely monitored, as only one significant rainfall event could rapidly produce flooding.” 

The NWS’s long-range outlooks contain forecast values determined using several season scenarios from 30 or more years of climatological data. That includes current river conditions, soil moisture, snow cover and 30- to 90-day temperature and precipitation predictions. 

In addition to the NWS, the United States Geological Survey, U.S. Army Corps of Engineers, Midwest Regional Climate Center, High Plains Regional Climate Center, U.S. Drought Monitor and National Operational Remote Sensing Center contribute information for the report. 

The NWS will release an updated spring flood outlook later this week. 

The North Iowa Times will continue to provide updates as we receive them.

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