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Floodwaters in Eastern Iowa

Rare Fall Flooding Swamps Iowa

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Normally, late August and early September are fairly dry in Iowa. Just how rare is the heavy rainfall that’s fallen across Iowa over the past 10 days?

To answer that question, Professor Gabriele Villarini, a faculty affiliate of the Iowa Flood Center at the University of Iowa, worked with Assistant Research Scientist Wei Zhang to provide some context for the current rainfall situation. Zhang and Villarini are both part of IIHR—Hydroscience & Engineering, where Villarini serves as director. He says that the extremely heavy precipitation is indeed unusual, as seen in the graphic (above) that covers the period between August 24 (when the rain started) and Sept. 5. It is based on a gridded rain gauge product.

Four maps of the upper Midwest showing recent rainfall using colors from blue to red.Here is Villarini’s interpretation of the graphics and the soggy story they tell:

  • Top-left panel: This panel displays historic average precipitation for August 24–Sept. 5 during the 1981–2010 period. This 30-year period is generally used to frame recent events in a longer-term climatological period. Take home: We would expect on average about 2 inches of rainfall or less in Iowa during this time period.
  • Bottom-left panel: This graphic displays the current observed precipitation in 2018. Take home: The current rainfall is much more than we would expect for this time of the year; some areas of eastern Iowa have values on the order of 8–10 inches.
  • Top-right panel: This panel shows the ranking of 2018 precipitation during this period with respect to the 1948–2018 period. The redder areas represent rainfall extremes, with values of “1” indicating that this year was the highest since 1948. Take home: The current rainfall is the largest on record (since 1948) in large areas of central and eastern Iowa.
  • Bottom-right panel: For the locations where precipitation this year was the greatest on record, we show how much greater the rainfall has been this year than the next largest year. For instance, a value of 80% indicates that this year was 80% larger than the second largest year on record. Take home: The rainfall this year is extreme, in some areas up to 80% larger than the second largest year.

For more information, visit the Iowa Flood Information System.

Changing Frequency of Heavy Rainfall over the Central United States

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Gabriele Villarini photo

Gabriele Villarini

Villarini et al. used daily rainfall measurements from 447 rain gauges with a record of least 50 years throughout the central United States to examine the presence of changes in the frequency of heavy rainfall, which they defined as days exceeding the 95th percentile of the at-site rainfall distribution. The observational records covered at least the second half of the 20th century and the first decade of the 21st century, providing information about the most recent changes in heavy rainfall events over this area. They found a generally increasing trend in the northern part of the study region (roughly the Upper Mississippi River basin).

“We tried to explain these results and the differences between the northern and southern parts of the study region in light of changes in temperature,” Villarini says. “We found that the northern region is experiencing large increasing trends in temperature, resulting in an increase in atmospheric water vapor. Therefore, there is more water vapor available for precipitation.” In addition to increasing temperatures, they also indicated the increased irrigation over the Ogallala Aquifer, which likely resulted in an increase in water vapor in the area.

Villarini, G., J.A. Smith, and G.A. Vecchi, Changing frequency of heavy rainfall over the central United States, Journal of Climate, 26(1), 343-350, 2013.