A look at weather extremes and the big-picture climate connections.

Heavy Midwestern Rains Lead to Mississippi Floods

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Heavy rains have left the ground saturated, rivers swollen, and has caused widespread flooding in Missouri, Illinois, Kentucky and Arkansas. Credit: Scott Olson/Getty Images

Last week, after a record number of tornadoes swept across the South and left a path of death and destruction the likes of which haven’t been seen in US since at least 1925, it seemed like this year’s spring season had doled out its last big strike. And yet, here we are, less than a week later and still the weather conditions are wreaking havoc, this time in the Central States, with record flooding along the Mississippi River.

According to Jeff Masters at Weather Underground, the Mississippi River flooding through Missouri, Kentucky and Tennessee has been caused by a combination of high waters on both the Mississippi and the Ohio Rivers upstream from where the two join:

Snowmelt from this winter's record snow pack across the Upper Mississippi River has formed a pulse of flood waters that is moving downstream on the Mississippi. This pulse of floodwaters passed St. Louis on Saturday, where the river is now falling. This floodwater pulse is headed south to Cairo, Illinois, and will join with the record water flow coming out of the Ohio River to create the highest flood heights ever recorded on a long stretch of the Mississippi, according to the latest forecasts from the National Weather Service.”

The Ohio River swelled following two weeks of intense rainfall across Kentucky, Illinois and Indiana. Although scientists have not yet found evidence that global and regional climate changes are changing tornado risks, that isn’t the case with extremely heavy rainfall events in the Midwest. According to several studies, bursts of extreme precipitation have been steadily increasing in the Upper Midwest for several decades.   

In a 2008 report released by the U.S. Climate Change Science Program, called Weather and Climate Extremes in a Changing Climate, researchers describe how the average amount of precipitation falling each year in the U.S. has increased, and most of this extra precipitation comes from more intense bursts of heavy rainfall, rather than a greater number of light rain events. In other words, though the average number of rainy days isn’t changing dramatically, the amount of rain pouring down in short periods of time is on the rise. This is consistent with the effect that global warming is known to have on precipitation patterns and amounts: as average temperatures rise, the atmosphere is able to hold more moisture. So, during a rainstorm, there is more water vapor available to be turned into precipitation.

According to the 2008 report,

“The amount of precipitation falling in the heaviest one percent of rain events increased by 20 percent during the 20th century, while total precipitation increased by seven percent.”

Furthermore, research highlighted in the same report shows that during the last century, the frequency of days in the Upper Midwest with more than four inches of precipitation had increased by 50 percent (check out page 47 of the report’s second chapter for more details on how precipitation extremes have been changing in the U.S. over the past few decades).

On the Mississippi, the high waters are being attributed to rapid melting of the snowpack upriver. Of course, the intensity of this melt depends on how much snow accumulated over the winter. But the timing of this meltwater surge is influenced by how early and quickly spring arrives, and since the 1950’s, spring has been starting earlier in the U.S. (although this effect is more pronounced in the West).

An Ohio River water guage at Cairo, Ill., recorded record high water levels on May 2nd, but after the U.S. Army Corps of Engineers blasted a whole through a nearby levee, water levels began to drop. Credit: U.S. Army Corps of Engineers.

The combination of early snowmelt and a repeated rains from intense storm systems across the Midwest culminated in the devastating flood situation over the past few days. Yesterday, just south of Cairo, Ill., where the Mississippi and Ohio Rivers meet, the U.S. Army Corps of Engineers blasted a hole in the Birds Point levee, helping to relieve some of the flooding that threatened nearby communities. Within hours of breaking through the levee, the river level had dropped several inches, but floodwaters had since poured over 100,000 acres of farmland.

And as the New York Times reported late yesterday, communities farther south on the Mississippi River, throughout Missouri, Tennessee, Arkansas and Mississippi, are now bracing for floods as the high waters continue their journey downriver.

This year’s severe floods may be a case of bad timing, with the Upper Mississippi meltwater mixing with heavy spring rains elsewhere in the Midwest and South Central states. In the future, the circumstances may not always be so perfectly timed to cause such devastating flood levels. But with the trend of early spring onset predicted to continue, along with the expected increase in intense rainfall events in the coming decades, this year’s floods may be a preview of what could happen more often along the Mississippi.  

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