Spring snowmelt has cities along the Mississippi bracing for flooding

ByJennifer Gray, CNN Meteorologist, CNNWire

Thursday, April 27, 2023

Extreme April weather in Midwest, South brings large hail, flooding worries

Nearly 30 river gauges are in major flood stage across the Midwest, with more potentially damaging flooding expected into the coming weeks.

Nearly 30 river gauges are in major flood stage across the Midwest, with more potentially damaging flooding expected into the coming weeks as the snow from this year's blockbuster season continues to melt.

This year's river rise could lead to some of the worst flooding in 20 years in some places, with most river gauges in major flood stage along the Mississippi River from St. Paul, Minnesota, south to Burlington, Iowa. It follows this winter's relentless parade of storms soon after the Mississippi River near Memphis had dropped to record low levels from drought.

The Mississippi River has crested in the Twin Cities, and also crested Wednesday in La Crosse, Wisconsin, at 15.89 feet, which is the third highest on record and just 2 feet shy of the record. At 16 feet, "water is within one foot of Rose Street near Interstate 90, and the eastbound I-90 exit may be closed," the National Weather Service said earlier this week.

Some homes and businesses along Davenport, Iowa's riverfront already have taken on water, and the worst is yet to come, Mayor Mike Matson told CNN earlier this week. Several roads are already closed as flooding threatens towns all along the Mississippi.

"We're expecting a pretty good crest in a week," Matson said Tuesday. "Whether it's a record crest or close to a record, there's not definitive answers yet. But it doesn't really matter for us because we fight the river."

Near Davenport, the Mississippi River is already at major flood stage, topping 19 feet with perhaps another 1 to 2 feet to go by early next week -- which would be just a foot shy of that all-time record. Several gauges to the north, near Dubuque, Iowa, also are forecast early next week to reach their third-highest crest ever, behind 1965 and the river floods in 2001, the local weather service office said.

"We've used the lessons learned and the forensic analysis that the (US Army) Corps of Engineers gave us to set the barriers and set the pumps, and we have public works people doing 24/7 shifts," Matson said. "We're fully engaged, so we'll see where Mother Nature and the Mississippi decides to go."

In Dubuque, Iowa, officials have closed all of its floodgates along the Mississippi "and are operating several pumping stations 24/7 to convey water over the floodwall and into the river," the city said in a video explainer released Wednesday. This is only the third time the gates have been closed since they were installed in 1973.

The wall was built to keep water from the Mississippi River out of the city during high water levels. It has gates to allow passing through normal river levels. The city closes the gates during flooding to essentially seal the wall and keep the water out.

Widely changing temperatures put runoff in flux

Flood warnings now are in place for rivers in the Upper Midwest from the US-Canadian border to north of St. Louis, and such warnings extend more than 400 miles along the Mississippi River alone.

As much of the snow has melted in the Upper Midwest, the resulting swell of water upstream will creep south, causing a slow rise and fall of gauges.

"It takes time for that water to make it to the river. It's not all going into the river all at once. It's just a longer process," said Caleb Grunzke, meteorologist with the Twin Cities weather service office.

Unlike flash flooding, which can happen in seconds, seasonal spring river flooding is gradual, though last week's relative heat wave across the Midwest -- with temperatures in the mid-80s to 90 degrees -- will "just accelerate the melting," he said.

"It's above freezing the whole day, so snow is continually melting over that whole week and you're getting lots of runoff into the rivers very quickly," Grunzke explained.

Much cooler temperatures -- with dips below freezing overnight -- have settled in this week, melting the snow more gradually and "slow(ing) down the amount of runoff that goes into the rivers," he said.