Heavy Precipitation

Weather and Climate

Precipitation

This indicator tracks how the frequency and intensity of heavy precipitation events have changed across the United States.

Figures

Figure 1. Extreme One-Day Precipitation Events in the Contiguous 48 States, 1910-2023

Data source: NOAA, 2024
Web update: June 2024

Figure data (CSV) EPA source data (CSV)

Figure 1

This figure shows the percentage of the land area of the contiguous 48 states where a much greater than normal portion of total annual precipitation has come from extreme single-day precipitation events. The blue bars show the annual value for each year from 1910 to 2023. The orange line is a nine-year weighted moving average, which EPA’s source file provides directly rather than something this site calculates.

Show the data behind this figure

year	Index value	9-year moving average
1910	0.092	0.104
1911	0.140	0.109
1912	0.106	0.113
1913	0.075	0.117
1914	0.180	0.118
1915	0.113	0.108
1916	0.090	0.091
1917	0.030	0.081
1918	0.083	0.083
1919	0.119	0.094
1920	0.089	0.103
1921	0.121	0.108
1922	0.099	0.110
1923	0.143	0.105
1924	0.085	0.095
1925	0.052	0.088
1926	0.098	0.086
1927	0.107	0.085
1928	0.074	0.079
1929	0.060	0.073
1930	0.064	0.073
1931	0.072	0.082
1932	0.089	0.093
1933	0.154	0.097
1934	0.066	0.094
1935	0.065	0.093
1936	0.114	0.097
1937	0.106	0.099
1938	0.127	0.093
1939	0.045	0.083
1940	0.072	0.077
1941	0.075	0.079
1942	0.095	0.082
1943	0.088	0.082
1944	0.077	0.077
1945	0.063	0.072
1946	0.075	0.069
1947	0.054	0.069
1948	0.083	0.073
1949	0.053	0.080
1950	0.117	0.088
1951	0.093	0.095
1952	0.082	0.101
1953	0.121	0.105
1954	0.118	0.103
1955	0.120	0.091
1956	0.028	0.077
1957	0.077	0.068
1958	0.058	0.068
1959	0.082	0.072
1960	0.063	0.077
1961	0.095	0.081
1962	0.098	0.084
1963	0.045	0.088
1964	0.123	0.094
1965	0.099	0.098
1966	0.118	0.096
1967	0.061	0.091
1968	0.092	0.088
1969	0.089	0.089
1970	0.099	0.090
1971	0.088	0.088
1972	0.077	0.086
1973	0.082	0.085
1974	0.101	0.085
1975	0.080	0.083
1976	0.060	0.082
1977	0.104	0.083
1978	0.072	0.089
1979	0.081	0.099
1980	0.135	0.110
1981	0.122	0.115
1982	0.130	0.111
1983	0.086	0.102
1984	0.082	0.097
1985	0.091	0.097
1986	0.132	0.097
1987	0.083	0.095
1988	0.073	0.095
1989	0.080	0.103
1990	0.162	0.111
1991	0.129	0.111
1992	0.059	0.107
1993	0.101	0.113
1994	0.124	0.134
1995	0.169	0.159
1996	0.231	0.173
1997	0.135	0.172
1998	0.200	0.158
1999	0.148	0.135
2000	0.061	0.114
2001	0.096	0.104
2002	0.117	0.106
2003	0.113	0.113
2004	0.096	0.123
2005	0.162	0.135
2006	0.128	0.146
2007	0.183	0.151
2008	0.165	0.147
2009	0.081	0.142
2010	0.187	0.139
2011	0.131	0.137
2012	0.108	0.136
2013	0.154	0.142
2014	0.121	0.153
2015	0.219	0.165
2016	0.140	0.169
2017	0.194	0.164
2018	0.164	0.149
2019	0.121	0.132
2020	0.066	0.123
2021	0.161	0.121
2022	0.138	0.119
2023	0.095	0.111

Figure 2

Figure 2. Unusually High Annual Precipitation in the Contiguous 48 States, 1895-2023

Data source: NOAA, 2024
Web update: June 2024

Figure data (CSV) EPA source data (CSV)

Figure 3

This figure shows the percentage of the land area of the contiguous 48 states that experienced much greater than normal precipitation in any given year, which means it scored 2.0 or above on the annual Standardized Precipitation Index. The thin blue line shows the annual value for each year from 1895 to 2023. The thicker orange line is a nine-year weighted moving average that smooths out some of the year-to-year fluctuations.

Show the data behind this figure

year	Fraction of Area >=2.0	9-yr Moving Average
1895	0.000	0.004
1896	0.010	0.006
1897	0.015	0.007
1898	0.000	0.006
1899	0.002	0.005
1900	0.005	0.005
1901	0.010	0.006
1902	0.006	0.007
1903	0.002	0.012
1904	0.000	0.022
1905	0.069	0.031
1906	0.043	0.032
1907	0.016	0.026
1908	0.010	0.019
1909	0.024	0.013
1910	0.000	0.009
1911	0.005	0.006
1912	0.006	0.006
1913	0.000	0.011
1914	0.006	0.019
1915	0.073	0.022
1916	0.003	0.019
1917	0.000	0.013
1918	0.000	0.011
1919	0.039	0.012
1920	0.001	0.011
1921	0.000	0.012
1922	0.004	0.015
1923	0.058	0.017
1924	0.000	0.015
1925	0.000	0.015
1926	0.000	0.021
1927	0.077	0.027
1928	0.010	0.027
1929	0.031	0.019
1930	0.000	0.011
1931	0.000	0.005
1932	0.002	0.003
1933	0.007	0.003
1934	0.000	0.004
1935	0.005	0.004
1936	0.003	0.004
1937	0.005	0.006
1938	0.009	0.012
1939	0.000	0.030
1940	0.005	0.055
1941	0.233	0.068
1942	0.012	0.056
1943	0.000	0.032
1944	0.003	0.015
1945	0.022	0.012
1946	0.001	0.015
1947	0.030	0.018
1948	0.026	0.018
1949	0.000	0.018
1950	0.023	0.017
1951	0.032	0.016
1952	0.000	0.012
1953	0.007	0.009
1954	0.011	0.008
1955	0.000	0.011
1956	0.000	0.018
1957	0.073	0.022
1958	0.000	0.020
1959	0.015	0.015
1960	0.003	0.010
1961	0.017	0.009
1962	0.003	0.011
1963	0.000	0.015
1964	0.044	0.018
1965	0.022	0.016
1966	0.000	0.011
1967	0.001	0.007
1968	0.013	0.005
1969	0.000	0.004
1970	0.002	0.006
1971	0.000	0.013
1972	0.016	0.024
1973	0.082	0.030
1974	0.000	0.029
1975	0.042	0.025
1976	0.000	0.025
1977	0.015	0.029
1978	0.080	0.031
1979	0.022	0.027
1980	0.000	0.023
1981	0.000	0.032
1982	0.031	0.051
1983	0.189	0.060
1984	0.000	0.050
1985	0.003	0.032
1986	0.042	0.018
1987	0.000	0.012
1988	0.000	0.011
1989	0.005	0.015
1990	0.039	0.022
1991	0.029	0.028
1992	0.011	0.031
1993	0.074	0.033
1994	0.000	0.035
1995	0.028	0.038
1996	0.087	0.042
1997	0.006	0.041
1998	0.075	0.033
1999	0.006	0.022
2000	0.000	0.011
2001	0.000	0.008
2002	0.000	0.013
2003	0.034	0.022
2004	0.047	0.025
2005	0.014	0.024
2006	0.011	0.022
2007	0.022	0.025
2008	0.038	0.030
2009	0.032	0.037
2010	0.029	0.042
2011	0.095	0.042
2012	0.004	0.037
2013	0.034	0.032
2014	0.000	0.034
2015	0.093	0.039
2016	0.016	0.046
2017	0.016	0.059
2018	0.115	0.073
2019	0.126	0.074
2020	0.049	0.055
2021	0.006	0.030
2022	0.000	0.012
2023	0.001	0.004

Figure 4

Key Points

In recent years, a larger percentage of precipitation has come in the form of intense single-day events. Nine of the top 10 years for extreme one-day precipitation events have occurred since 1995 (see Figure 1).
The prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s, but has risen substantially since then. Over the entire period from 1910 to 2023, the portion of the country experiencing extreme single-day precipitation events increased at a rate of about half a percentage point per decade (see Figure 1).
The percentage of land area experiencing much greater than normal yearly precipitation totals increased between 1895 and 2023. There has been much year-to-year variability, however. In some years there were no abnormally wet areas, while a few others had abnormally high precipitation totals over 10 percent or more of the contiguous 48 states’ land area (see Figure 2). For example, 1941 was extremely wet in the West, while 1983 was very wet nationwide.³
Figures 1 and 2 are both consistent with other studies that have found an increase in heavy precipitation over timeframes ranging from single days to seasons to years.¹ For more information on trends in overall precipitation levels, see the U.S. and Global Precipitation indicator.

Background

“Heavy precipitation” refers to instances during which the amount of rain or snow experienced in a location substantially exceeds what is normal. What constitutes a period of heavy precipitation varies according to location and season.

Climate change can affect the intensity and frequency of precipitation. Warmer oceans increase the amount of water that evaporates into the air. When more moisture-laden air moves over land or converges into a storm system, it can produce more intense precipitation—for example, heavier rain and snow storms.¹ The potential impacts of heavy precipitation include crop damage, soil erosion, and an increase in flood risk due to heavy rains (see the River Flooding indicator)—which in turn can lead to injuries, drownings, respiratory health impacts from exposure to mold, and other flooding-related effects on health.² In addition, runoff from precipitation can impair water quality as pollutants deposited on land wash into water bodies.

Heavy precipitation does not necessarily mean the total amount of precipitation at a location has increased—just that precipitation is occurring in more intense events. However, changes in the intensity of precipitation, when combined with changes in the interval between precipitation events, can also lead to changes in overall precipitation totals.

About the Indicator

Heavy precipitation events can be measured by tracking their frequency, examining their return period (the chance that the event will be equaled or exceeded in a given year), or directly measuring the amount of precipitation in a certain period (for example, inches of rain falling in a 24-hour period).

One way to track heavy precipitation is by calculating what percentage of a particular location’s total precipitation in a given year has come in the form of extreme one-day events—or, in other words, what percentage of precipitation is arriving in more concentrated periods of time. Figure 1 of this indicator looks at the prevalence of extreme single-day precipitation events over time.

For added insight, this indicator also tracks the occurrence of unusually high total yearly precipitation. It does so by looking at the Standardized Precipitation Index (SPI), which compares actual yearly precipitation totals with the range of precipitation totals that one would typically expect at a specific location, based on historical data. If a location experiences less precipitation than normal during a particular period, it will receive a negative SPI score, while a period with more precipitation than normal will receive a positive score. The more precipitation (compared with normal), the higher the SPI score. The SPI is a useful way to look at precipitation totals because it allows comparison of different locations and different seasons on a standard scale. Figure 2 shows what percentage of the total area of the contiguous 48 states had an annual SPI score of 2.0 or above (well above normal) in any given year.

About the Data

Indicator Notes

Weather monitoring stations tend to be closer together in the eastern and central states than in the western states. In areas with fewer monitoring stations, heavy precipitation indicators are less likely to reflect local conditions accurately.

Data Sources

The data used for this indicator come from a large national network of weather stations and were provided by the National Oceanic and Atmospheric Administration’s (NOAA’s) National Centers for Environmental Information. Figure 1 is based on Step #4 of NOAA’s U.S. Climate Extremes Index; for data and a description of the index, see: www.ncei.noaa.gov/access/monitoring/cei. Figure 2 is based on the U.S. SPI, which is shown in a variety of maps available online at: www.ncei.noaa.gov/access/monitoring/monthly-report/drought. The data used to construct these maps are available at: www.ncei.noaa.gov/pub/data/cirs/climdiv.

Technical Documentation

Download related technical information (PDF)

References

Marvel, K., Su, W., Delgado, R., Aarons, S., Chatterjee, A., Garcia, M. E., Hausfather, Z., Hayhoe, K., Hence, D. A., Jewett, E. B., Robel, A., Singh, D., Tripati, A., & Vose, R. S. (2023). Chapter 2: Climate trends. In USGCRP (U.S. Global Change Research Program), Fifth National Climate Assessment. https://doi.org/10.7930/NCA5.2023.CH2
Bell, J. E., Herring, S. C., Jantarasami, L., Adrianopoli, C., Benedict, K., Conlon, K., Escobar, V., Hess, J., Luvall, J., Garcia-Pando, C. P., Quattrochi, D., Runkle, J., & Schreck, C. J., III. (2016). Chapter 4: Impacts of extreme events on human health. In USGCRP (U.S. Global Change Research Program), The impacts of climate change on human health in the United States: A scientific assessment (pp. 99–128). https://health2016.globalchange.gov/extreme-events
NOAA (National Oceanic and Atmospheric Administration). (2012). Personal communication: Analysis by Derek Arndt, April 2012. National Climatic Data Center.
NOAA (National Oceanic and Atmospheric Administration). (2024). U.S. Climate Extremes Index. Retrieved April 18, 2024, from www.ncei.noaa.gov/access/monitoring/cei
NOAA (National Oceanic and Atmospheric Administration). (2024). Standardized Precipitation Index data files [Data set]. Retrieved April 18, 2024, from www.ncei.noaa.gov/pub/data/cirs/climdiv

Text on this page is EPA’s own published wording (a U.S. Government work, not subject to domestic copyright), extracted from EPA’s source document and verified paragraph by paragraph against the published page. Data, chart code, and the full extraction pipeline are maintained in the heavy-precipitation repository, which this page reads its data from directly.