In brief: Extreme storms have increased in number, with the most extreme storms increasing more than less extreme storms and both more than average storms; frequency of storms has increased; and, the interval between extreme storms has also increased.
In more detail:
- Since 1961, the Midwest has had an increasing number of large storms. The largest of storms, those of three inches or more of precipitation in a single day, increased the most, with their annual frequency having increased by 103 percent over the roughly half century period through 2011. For storms of at least two inches but less than three inches in a day, the trend was a 81 percent increase; for storms of one to two inches, a 34 percent increase. Smaller storms did not have a significant increase.
- The rates of increase for all large storms accelerated over time, with the last analyzed decade, 2001-2010, showing the greatest jumps. For the largest storms, in 2001-2010 there were 52 percent more storms per year than in the baseline period.
- The frequency of extreme storms has increased so much in recent years that the first 12 years of this century included seven of the nine top years (since 1961) for the most extreme storms in the Midwest.
- With more frequent extreme storms, the average return period between two such storms has become shorter. In 1961-1970, extreme storms averaged once every 3.8 years at an individual location in the Midwest. That is two to four times more frequent than a major hurricane making landfall at a typical location along the U.S. coast from North Carolina to Texas. By 2001-2010, the average return period for Midwestern extreme storms at a single location was down to 2.2 years—or four to eight times more frequent than landfalling major hurricanes.
As for the connection to climate change, report lead author Stephen Saunders says, "A threshold may already have been crossed, so that major floods in the Midwest perhaps now should no longer be considered purely natural disasters but instead mixed natural/unnatural disasters. And if emissions keep going up, the forecast is for more extreme storms in the region."