Regional flood risks continue to increase, particularly along Mississippi River

Background

In early February, the National Weather Service’s Quad Cities bureau’s first spring flood outlook — mapping flood risks from February to April — showed little risk of major flooding in Eastern Iowa. Slightly above-normal risks of flooding were projected for areas along the Mississippi River if northern snowpacks on frozen ground melted rapidly.

A few weeks later, an updated spring flood outlook estimated significantly higher threats of flooding, especially along the Mississippi River.

Warmer weather in late February caused large reductions in local snowpacks. The deeper snowpacks on frozen ground farther north — which received a foot of fresh snowfall — were projected to lead to flood effects in Mississippi River communities.

Still, there were no locations with high chances for flooding. Most locations along the Mississippi had at least a 50 percent chance of moderate or minor flooding. And many local rivers showed at least a 25 percent chance of minor flooding.

What’s new?

Spring flood risks remain well above normal for all points along the Mississippi River and are increasing at points along tributary rivers such as the Iowa, Cedar and Des Moines rivers that eventually dump into the Mississippi, the latest spring flood outlook showed Thursday.

Despite well-above-average risks of flooding on the Mississippi River, high-impact flooding is not guaranteed. The rates of future snowmelt, snowfall and rainfall will shape flood threats throughout spring, hydrologist Matt Wilson of the Weather Service’s Quad Cities bureau said during a webinar Thursday.

NWS Spring Flood Outlook 3 by Gazetteonline on Scribd

As in the previous spring flood outlooks, average temperatures and precipitation in the region have remained above normal. In the headwaters of the Mississippi River, average precipitation has reached up to 300 percent above normal this winter.

Locally, snowpacks have dwindled thanks to continued mild weather. Thursday’s fresh snow and any future snowfall could change this, although Wilson suspects the powder will disappear over the next week or so.

The real concern lies with the snowpack farther north, he said. Much of the Upper Mississippi River basin is blanketed with 4 to 8 inches of snow. Accumulation throughout Minnesota and parts of Wisconsin and North Dakota reach the top 10 percent of historical snow water equivalent records.

“That water is eventually going to melt and come down,” Wilson said. “That's going to be what really makes or breaks our spring flood season.”

Stream flows are above normal and much above normal in local tributaries, leaving them with less capacity to store potential runoff from snowmelt and rain without flooding.

Those rivers then funnel into the Mississippi River main stem and increase flood risks there. Almost every location between Burlington and Dubuque along the Mississippi has a 1 in 2 chance of major flooding. Some streams in the Quad Cities already have reached minor and moderate flood stages from recent rains, Wilson said.

From the previous outlook, there was a slight increase in the number of tributaries that could expect minor to major flooding throughout Eastern Iowa.

“The risk for our tributaries is still normal for most of the area, but it is increasing,” Wilson said. “That's just due to the fact that we have had a wetter than average spring, and our stream flows are up.”

While there are low chances for precipitation in the second half of March, below-average temperatures also are expected. So, although snowpacks shouldn’t grow, they likely won’t shrink much either. When they do melt, their water could be absorbed into the ground depending on soil moisture and frost levels.

Most of Iowa entered the winter under drought conditions. Now, soil moisture levels have returned to normal to slightly above average in the Quad Cities bureau’s service area.

Farther north, much of the ground is frozen between 10 and 30 inches deep, which could allow for more runoff that increases stream flows. Local soils exhibit shallow to no frost, though, allowing the ground to absorb more snowmelt and rain.

“We will continue to be thankful that we're not dealing with snow on top of frozen ground down here,” Wilson said. “It didn't manifest this year.”

While this outlook originally was slated to be the final one for the season, the bureau plans to release another in late March to map flood risks through June.

“We’ll have maybe a little bit better news there for that seasonal outlook,” Wilson said.

Brittney J. Miller is the Energy & Environment Reporter for The Gazette and a corps member with Report for America, a national service program that places journalists in local newsrooms to report on under-covered issues.

Comments: (319) 398-8370; brittney.miller@thegazette.com