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.
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.