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February 12, 2013

Drought expected to persist

State climatologist discusses Iowa’s roller coaster wet and dry seasons over past century

OTTUMWA — The drought Iowa has endured since 2011 is extreme, but compare it to temperature and precipitation over the last 100 years, and it’s actually not abnormal.

State Climatologist Harry Hillaker spoke to the Ottumwa Rotary Club Monday afternoon at Hotel Ottumwa.

“It’s extreme, but so far the duration is not at all in the ballpark of years past,” Hillaker said of last year’s drought. “The question now is how long is it going to last? It does look like it’s going to persist this year based on years past.”

In the last 50-60 years, wet years have actually outnumbered dry years almost 2:1, he said.

Southeast Iowa was not hit as hard in this drought thanks to the Des Moines River, watershed, and lakes and aquifers upstream, he said.

“You were less drought-impacted,” he said.

Western Iowa really took the brunt of the drought, “since they rely on shallow aquifers and watersheds,” he said.

“Therefore they respond very quickly to dry conditions, as well as wet conditions.”

While last summer’s drought meant no significant rainfall or thunderstorms, that brought with it the possible “silver lining” of the drought: no tornadoes.

“There have been no tornadoes in Iowa since May 24,” Hillaker said. “This was the earliest end to a tornado season since 1914.”

Typically, Iowa sees the bulk of its tornadoes in June and July. Last year, Iowa saw 16 tornadoes in total, the lowest annual total since 1963.

Statewide, July 2012 was the hottest July since 1936, he said, which coincided with Iowa’s fifth-lowest precipitation in more than 100 years.

That’s just two years after Iowa’s second-wettest summer in 2010, when floods again plagued the state.

Last summer was also the hottest summer since 1988, which has been the go-to comparison for last year’s drought.

This up-and-down of hot vs. cool temperatures and high vs. low rainfall is typical, he said. If you look at a longer time span going back to the 1800s, the severe weather Iowa has endured in the last several years is not abnormal.

While Iowa has seen chunks of time with floods and chunks of time with drought, that’s nothing compared to the 1920s and the Dust Bowl in the 1930s, when “drought was almost a permanent fixture,” Hillaker said.

The National Weather Service’s forecast for the upcoming week shows highs in the 40s, with Friday and Saturday dipping back down into the 20s.

NWS’s spring and summer outlooks show warmer temperatures than usual. By July, NWS has predicted very dry weather will be concentrated over much of the Midwest.

Iowa’s current drought began in July 2011, he said, and for 19 months the state has seen below-normal precipitation levels.

Unfortunately for Ottumwa, the majority of the drought lies along the Des Moines River, he said, which flows from the northwest corner of the state to the southeast.

In contrast, right before this drought struck, Wapello County was sitting 28 inches above normal in terms of rainfall.

From September 2012 to soil freeze-up (which generally occurs from Dec. 15-20), the county has been close to normal moisture levels.

“Our areas of concern are in the way northwest corner of the state,” Hillaker said. “It’s going to take a wet spring to get them back in good shape.”

Hillaker compared average statewide precipitation starting in the 1800s until today and found that even the flood the state shouldered in 1993 wasn’t as wet as what Iowa experienced in the early 1800s.

“We’ve had a lot of wet years since the 1950s,” he said. “But from 2007 to 2010, those were four very wet years in a row.”

In the past two weeks, southern Wisconsin, the state of Illinois and parts of Missouri have actually seen higher-than-normal rainfall for this time of the year, at around 3-4 inches.

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