Above: Floodwater covers Highway 2 on March 23, 2019, near Sidney, Iowa. In March 2019, Midwest states battled some of the worst flooding in decades as rain and snowmelt from a "bomb cyclone" inundated rivers and streams. (Scott Olson/Getty Images)
Spring flooding may not be as widespread and severe in 2020 as it was in 2019, but large areas of the U.S. are still at risk, according to the spring flood outlook released by NOAA on Thursday. Areas stretching from the eastern Great Plains to the Appalachians and the Southeast U.S. coast—spanning parts of 23 states—have a better-than-even chance of moderate to major flooding this spring, according to NOAA.
The corridor from the eastern Dakotas to western Minnesota, still dealing with the hangover of heavy rains and snows since 2018, has the highest odds of serious flooding. The outlook called for a greater-than-50% chance of major flooding in the Red River Valley of the North, which separates North Dakota and Minnesota, and in the James River Valley of eastern South Dakota.
Major flooding is also likely along parts of the Mississippi Valley in far northwest Illinois and southeast Iowa.
Persistently high soil moisture is a key driver of the enhanced flood threat. Large parts of the Upper Midwest and Northern Plains have slogged their way through almost two years of above-average precipitation, including long spans of record- or near-record moisture.
NOAA also noted ongoing rainfall as well as elevated odds of above-average precipitation this spring in many areas.
Waterlogged and snowbound
In a presentation at the American Meteorological Society’s annual meeting in Boston this past January (see the online recording), South Dakota state climatologist Laura Edwards and Natalie Umphlett (High Plains Regional Climate Center) summarized the Missouri River basin’s phenomenal extremes of the last couple of years.
For starters, unusually wet conditions in autumn 2018 segued into a cold winter with near-record snowfall in 2018–19.
“We had water in the ditches all winter long, so we kind of knew we were getting into a tough situation,” said Edwards.
Next up was the spectacular “bomb cyclone” of March 2019, which dumped heavy rain (on the order of 3”) across areas with still-frozen soils as well as deep snowpack holding roughly 3” of liquid equivalent. The result was a massive sequence of flooding and ice jams that produced more than 40 record-high crests on the Missouri and its tributaries. Damages totaled at least $1.6 billion in Nebraska and $1.3 billion in Iowa, and some 350 miles of levees had significant damage.
The floods of spring 2019 grabbed national headlines, but the continued spells of sogginess from summer into autumn 2019 got less attention. On September 11, parts of eastern South Dakota picked up 10” of rain in just 24 hours, an especially extreme amount for this time of year. Runoff from the James, Vermillion, and Big Sioux Rivers during September was more than twice the previous record and some 16 times higher than average, said Edwards.
By the end of 2019, a large chunk of the north central United States, including parts of eastern Montana, Nebraska, and North and South Dakota, had seen close to 200 percent of average annual precipitation. According to Edwards, parts of South Dakota had already recorded their wettest year on record by October: “The last two months were just the whipped cream on top.”
Canton, Sioux Falls, and Worthington all broke yearly precipitation records in 2019 that had been set just a year earlier. Several other locations in South Dakota saw two-year precipitation totals in 2018–19 of more than 90 inches. The town of Sibley picked up 98.83”, which is about as much as Houston would receive in an average two-year period!
Eastern South Dakota: The flooding that’s never stopped
No region has seen flooding as relentless since 2018 as the James River Valley. The river’s broad flood plain across eastern North and South Dakota is slow to send water downstream, with only a 700-foot elevation change over 700 miles.
Large parts of the James River never dropped below flood stage during the past winter, a time of year when cold weather and reduced precipitation typically allow high water to recede. As of mid-March, parts of the river valley had literally been in flood stage for more than a year.
“We are really struggling with water, and I’m willing to re-gift it to anyone who wants it,” Edwards said in her AMS talk.
Some rays of hope
The outlook isn’t quite so dire this spring as it was a year ago. Precipitation has finally dropped below average during recent weeks across much of the Northern Plains and Upper Midwest.
Moreover, relatively mild temperatures have thawed out the soil and eroded much of the winter snowpack south of eastern North Dakota and northern Minnesota and Wisconsin without major incident. Ice jams are no longer a big concern, according to the Missouri Basin River Forecast Center.
The Missouri’s headwaters across the Northern Rockies have seen slightly above average snowfall, but runoff volume later this spring is projected to be near or just above average. “Significant flooding due to mountain snow runoff alone is not likely,” said the MBRFC.
Taken as a whole, the relentlessly wet soils and high streamflows are ample cause for flood concern if the coming spring happens to be unusually wet across the Northern Plains and Midwest. If the weather trends warm and dry, the risk will be far less.
