Flood 2016

Recent rains considered 'extreme'

Weekend rain not expected drastically to change flood projections

The Cedar River is expected to crest at a little over 24 feet Tuesday. Photographed from the Eighth Avenue bridge in southeast Cedar Rapids, Iowa, on Thursday, Sept. 22, 2016. (Jim Slosiarek/The Gazette)
The Cedar River is expected to crest at a little over 24 feet Tuesday. Photographed from the Eighth Avenue bridge in southeast Cedar Rapids, Iowa, on Thursday, Sept. 22, 2016. (Jim Slosiarek/The Gazette)

CEDAR RAPIDS — Iowa’s rainfall totals this week are not only unusual for this time of year, they’re more than what’s usually seen all month.

Daryl Herzmann, a systems analyst with Iowa Environmental Mesonet, which collects environmental data across the state, said that while spring months are often Iowa’s wettest, it’s not unheard of to see heavy rainfall in August and September.

However, the severe weather over northern portions of the state in the past few days — Greene in central Iowa was hit with more than 10 inches of rain earlier this week — would be considered extreme any time in the year.

“Having a couple of events like this somewhere in the state during this period isn’t necessarily exceptional, it’s something we see pretty much every other year or so. But to have three or four or more events over this northeast Iowa area is getting into the very extreme, unprecedented area as far as having this many events back to back to back,” Herzmann said Friday.

Greene received 10.4 inches of rainfall this week, which fell in less than 24 hours. The average normal rainfall in that area for the entire month is about three inches, Herzmann said.

“Events like this, they are more or less extreme for any time in the year. Getting 10 inches in a 24-hour period — or even less — is more or less off the charts for anywhere in the state,” Herzmann said.

For this event, the large majority of rainfall landed within the Cedar River basin, which means all that water is heading downstream.

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The Cedar River projected to crest at around 25.3 feet Monday night. That level — about nine feet above major flood stage — would be the second highest since a record 31.52 feet in June 2008. Levels at about 26 feet would make for a 500-year flood event.

A 500-year flood event is a measurement of probability, explained Witold Krajewski, director of the Iowa Flood Center. A 500-year flood event has a .02 percent chance of occurring in any given year.

“It all has to do with chances, and the chances in a given year maybe are not that high, but they do increase over time,” Krajewski said. “If you wait long enough, that probability increases.”

Krajewski said the biggest variable now will be any future rain events, which could once again push the Cedar River into that 500-year flood range if it occurs in the right location.

“The key point there would be that location and timing and severity,” he said.

Zack Uttech, meteorologist with the National Weather Service in the Quad Cities, said on Friday it doesn’t appear weekend storms will be much of a factor.

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