Extreme Precipitation

  • The frequency and intensity of severe storms has increased. This trend will likely continue as the effects of climate change become more pronounced.
  • The amount of precipitation falling in the heaviest 1% of storms increased by 37% in the Midwest and 71% in the Northeast from 1958 to 2012.
  • Heavier storms are projected to increase in frequency at a faster rate than storms that are less intense.
  • The amount of precipitation falling during intense multi-day events has increased dramatically.

Observed Extreme Precipitation

Percent increases in the amount falling in the heaviest 1% of daily precipitation events from 1958 to 2012.Percent increases in the amount falling in the heaviest 1% of daily precipitation events from 1958 to 2012. Trends toward more intense precipitation are particularly clear in the Midwestern and Northeastern US.


Changes in the Heaviest 1% of
Daily Precipitation Events from 1951-1980 to 1981-2010 in the
Great Lakes Region
Change in Intensity of
1% Heaviest Storm (%)
5.1
Change in Number of
1% Heaviest Storm Days (%)
23.6
Change in Amount Falling in 1% Heaviest Storms (%) 24.5
Values are averages based on observations from 230 U.S. and Canadian stations across the Great Lakes region.

The increase in extreme precipitation is one of the clearest climate changes observed in the Great Lakes region. In the Midwest and Northeast United States, as defined by the USGCRP1, the most intense precipitation events have become stronger and more frequent. The amount of precipitation falling in the most intense 1% of precipitation events increased by 37% in the Midwest and 71% in the Northeast from 1958 through 2012.2 In the extreme western extent of the Great Lakes region, as much as 50% of annual total precipitation falls during 10 days of the year. Accumulated precipitation during these 10 days has increased dramatically over that same area of the country, with increases of 20-30% observed from 1971-2000 in many locations.3

More Total Precipitation from More Heavy Precipitation

Heavier, more frequent storms have been responsible for most of the observed increase in total precipitation during the last 60 years. From 1951-1980 to 1981-2010, the average of 230 observational stations in the Great Lakes region recorded that the heaviest 1% of daily precipitation events have become 5.1% more intense.4  Similarly, the frequency of these same heavy storms increased by 23.6% and the amount of precipitation falling in those storms increased by 20.2%.5

In many areas, heavy precipitation has increased while changes in the frequency and intensity of moderate precipitation events have been less significant. Total average precipitation over the United States saw a moderate increase of about 7% during the past century. Almost 20% more precipitation fell in the heaviest 1% of storms.6

More Multi-day Heavy Precipitation

Precipitation falling during heavy, multi-day wet periods has also increased dramatically. From 1931 through 1996, the amount of precipitation falling during week-long, once a year precipitation events7 increased by 25% to 100% in a broad region from the central Great Plains through the southern Great Lakes basin.6 Two-day precipitation events that typically occurred once in 5, 10, and 20 years8 have all become substantially more frequent since the early 1960s.9 

Projected Extreme Precipitation

Model projections of future precipitation vary greatly, but overall, the current observed trends of increasing frequency and intensity of extreme precipitation is anticipated to continue. The Great Lakes region, which is projected to experience a greater increase in total precipitation than most other parts of North America, is also expected to experience large increases in heavy precipitation events.

The number of days per year exceeding one inch of precipitation is projected to increase by 15 to 23% on average in the Midwest and Northeast regions of the United States through mid-century. The number of days exceeding 2 inches of precipitation is projected to increase at a faster rate, by 37 to 46%, on average.10 11

Change in intensity of the heaviest 1% of daily precipitation events at GLISA climatology stations in the Great Lakes region.Change in intensity of the heaviest 1% of daily precipitation events at GLISA climatology stations in the Great Lakes region.

Climate Change, Extreme Precipitation and Drought

It's counterintuitive, but as temperatures warm, the potential for both wetter and drier conditions can increase. While annual precipitation totals have generally increased the seasonal and regional distribution of precipitation can also change. In the Great Lakes region, precipitation totals during the fall, winter, and spring have increased in most locations, while summer precipitation has remained relatively stable or even declined. On top of that, the distribution of the intensity of precipitation events has also changed, so that more precipitation is falling during heavier storms. These effects, the clustering of precipitation into heavier storms and the polarization of wet and dry seasons, can allow for a greater chance of both extreme precipitation and of prolonged dry periods by extending the time between rainfalls.

Impacts of Extreme Precipitation

In general, climate change will likely amplify existing stresses across a range of sectors in the Great Lakes region.12 As disruptive, potentially damaging storms become more frequent and more intense, many agricultural, economic, and environmental regimes will be impacted. Most directly, changing extreme precipitation events will affect stormwater management, water quality, and public health.

Stormwater Management

The observed increased in heavy rain events across the region is already contributing to more storm water management problems.13 14 15 Intense precipitation overloads sewage systems and water treatment facilities, increasing the risk for waterborne disease outbreaks when untreated sewage is discharged into surface water.1 Increased urbanization further amplifies flooding risks, because more impervious surfaces channel storm water into fast-moving flows that are more difficult to control.16 17 In areas with poor storm water management, more frequent heavy rains, impervious surface, and degrading infrastructure will likely result in even greater damage.

Water Quality and Public Health

Heavy rainfall increases contaminated runoff, and warmer lake waters mobilize pollutants stored in sediment, contaminating fish.1 15 18 16 Floods could increase the likelihood of waterborne diseases and lake contamination.15 1 12 Warmer temperatures may conspire with stronger storms and greater nutrient loading to increase the occurrence of toxic algal blooms and oxygen depleted dead zones.15

Potential Causes of Increasing Extreme Precipitation

There are several possible causes for the observed increase in extreme precipitation events over the last century. Perhaps the most prominent hypothesis is that rising global surface temperatures have increased evaporation and added water vapor to the atmosphere. With more heat and more moisture, two of the key ingredients for storm development are now more abundant, leading to an increase in extreme precipitation.19 20 21 22 23