National Wildlife Federation Report: Heavy Rainfall and Increased Flooding Risk

Global warming is bringing more heavy rainfall to the central United States.

In the upper Midwest, the frequency of the most intense rainfall events has increased by 20 percent since the late 1960s.
The number of days each year with precipitation greater than 4 inches has increased by 50 percent over the last century.

These increases are correlated with a corresponding increase in days with heavy streamflow in the medium and large river basins of the eastern United States.

Major flooding—like the Great Floods of 1993 and 2008—requires prolonged periods of higher-thannormal precipitation. Across North America, we are having more 90-day intervals with precipitation totals in the top 5 percent of the historical average. The last 25 years have seen 20 percent more of these episodes than any other 25-year window over the 20th century.

As the climate continues to warm, the atmosphere will be able to hold more water. With more moisture in the air, the trend towards increasingly intense precipitation events will continue. In the Midwest, big storms that historically would only be seen once every 20 years are projected to happen as much as every 4 to 6 years by the end of the 21st century.