Heavy Precipitation

2023-03-21

Table of contents

Heavy Precipitation
Key points
Background
About the Indicator
About the Data
- Indicator Notes

Heavy Precipitation

This indicator tracks the frequency of heavy rainfall events in the United States.

Figure 1. Extreme One-Day Precipitation Events in the Contiguous 48 States, 1910–2020

Year	Index value	9-year moving average
1910	0.09	0.10466
1911	0.151	0.110684
1912	0.103	0.113895
1913	0.077	0.117645
1914	0.167	0.120289
1915	0.135	0.114098
1916	0.093	0.099379
1917	0.045	0.088074
1918	0.086	0.088289
1919	0.12	0.095527
1920	0.088	0.102152
1921	0.116	0.105922
1922	0.099	0.106277
1923	0.132	0.100938
1924	0.086	0.091
1925	0.047	0.082883
1926	0.095	0.080094
1927	0.092	0.07868
1928	0.068	0.074512
1929	0.068	0.069586
1930	0.059	0.069273
1931	0.062	0.076297
1932	0.085	0.086453
1933	0.143	0.091508
1934	0.065	0.089707
1935	0.064	0.088332
1936	0.106	0.091758
1937	0.1	0.094406
1938	0.12	0.090004
1939	0.051	0.081387
1940	0.069	0.076289
1941	0.077	0.076965
1942	0.092	0.078742
1943	0.077	0.077352
1944	0.074	0.073336
1945	0.061	0.0695
1946	0.074	0.067375
1947	0.057	0.067242
1948	0.077	0.069988
1949	0.053	0.076586
1950	0.1	0.085488
1951	0.107	0.093453
1952	0.077	0.099285
1953	0.123	0.102855
1954	0.099	0.101262
1955	0.129	0.091711
1956	0.042	0.077965
1957	0.068	0.068512
1958	0.049	0.06809
1959	0.089	0.074625
1960	0.07	0.083059
1961	0.104	0.088992
1962	0.11	0.091367
1963	0.049	0.093348
1964	0.125	0.097055
1965	0.098	0.098836
1966	0.117	0.094313
1967	0.066	0.085348
1968	0.078	0.077969
1969	0.062	0.075543
1970	0.088	0.076789
1971	0.076	0.079398
1972	0.075	0.082453
1973	0.093	0.084676
1974	0.097	0.084348
1975	0.072	0.082664
1976	0.064	0.083031
1977	0.113	0.086875
1978	0.071	0.094156
1979	0.094	0.104762
1980	0.141	0.115156
1981	0.125	0.118602
1982	0.129	0.112484
1983	0.087	0.102344
1984	0.082	0.096457
1985	0.087	0.096969
1986	0.131	0.098492
1987	0.088	0.097234
1988	0.084	0.097285
1989	0.077	0.102551
1990	0.155	0.107727
1991	0.117	0.106172
1992	0.063	0.102625
1993	0.1	0.108594
1994	0.113	0.127316
1995	0.162	0.149797
1996	0.21	0.164551
1997	0.135	0.166516
1998	0.196	0.156293
1999	0.151	0.137891
2000	0.07	0.11982
2001	0.114	0.110184
2002	0.114	0.109574
2003	0.108	0.11477
2004	0.1	0.124727
2005	0.168	0.138055
2006	0.131	0.150023
2007	0.182	0.155523
2008	0.176	0.154145
2009	0.092	0.15052
2010	0.194	0.147637
2011	0.142	0.144246
2012	0.117	0.141531
2013	0.149	0.144477
2014	0.128	0.154082
2015	0.209	0.165031
2016	0.146	0.171324
2017	0.196	0.168871
2018	0.175	0.15475
2019	0.136	0.130621
2020	0.078	0.10525

Figure 1. Extreme One-Day Precipitation Events in the Contiguous 48 States, 1910-2020
Data source: NOAA, 2021
Web update: April 2021
Units: percent of land area

This figure shows the percentage of the land area of the 48 contiguous states where much more than the average total annual precipitation comes from extreme single-day precipitation events. The bars represent individual years, while the line is a weighted average of nine years.

Figure 2. Unusually High Annual Precipitations in the Contiguous 48 States, 1895–2020

This figure shows the percentage of the land area of the 48 contiguous states that receive much higher than average precipitation in any given year; This means it scores 2.0 or higher on the annual Standardized Precipitation Index.

The thicker line shows the nine-year weighted average and smoothes out some year-to-year fluctuations.

Year	Fraction of Area >=2.0	9-yr Moving Average
1895	0	0.00359
1896	0.010258	0.005517
1897	0.012304	0.005812
1898	0	0.004728
1899	0	0.004129
1900	0.005236	0.004832
1901	0.01014	0.005826
1902	0.006461	0.007227
1903	0.001634	0.012037
1904	0	0.021615
1905	0.068688	0.030629
1906	0.042507	0.031934
1907	0.016181	0.025686
1908	0.008524	0.017846
1909	0.02358	0.011851
1910	0	0.007389
1911	0	0.004595
1912	0.005511	0.005247
1913	0	0.010665
1914	0.00626	0.018311
1915	0.073127	0.02169
1916	0	0.017932
1917	0	0.01252
1918	0	0.011038
1919	0.039175	0.011626
1920	0.001323	0.011133
1921	0	0.011686
1922	0.003625	0.014998
1923	0.057772	0.017086
1924	0	0.015496
1925	0	0.015334
1926	0	0.020842
1927	0.077311	0.02705
1928	0.010377	0.026587
1929	0.031216	0.019367
1930	0	0.010879
1931	0	0.005406
1932	0.002453	0.003478
1933	0.007398	0.003367
1934	0	0.003572
1935	0.005118	0.003864
1936	0.003408	0.004401
1937	0.005035	0.005744
1938	0.009048	0.011936
1939	0	0.029512
1940	0.004605	0.0547
1941	0.232839	0.067836
1942	0.0122	0.05586
1943	0	0.031548
1944	0.003378	0.015498
1945	0.021692	0.012439
1946	0.001132	0.015245
1947	0.031163	0.017745
1948	0.026306	0.017927
1949	0	0.016847
1950	0.020227	0.015861
1951	0.028465	0.014216
1952	0	0.011198
1953	0.00651	0.008274
1954	0.011262	0.007751
1955	0	0.011324
1956	0	0.017878
1957	0.072896	0.02213
1958	0.000191	0.020134
1959	0.014896	0.01462
1960	0.002604	0.010484
1961	0.017222	0.00922
1962	0.002506	0.010751
1963	0	0.014717
1964	0.044332	0.017902
1965	0.022344	0.016364
1966	0	0.011028
1967	0.001017	0.006275
1968	0.009609	0.004016
1969	0	0.003584
1970	0.002096	0.005905
1971	0	0.013051
1972	0.016736	0.023089
1973	0.077385	0.029616
1974	0	0.029055
1975	0.041739	0.025391
1976	0.00335	0.025307
1977	0.015327	0.029674
1978	0.079869	0.03179
1979	0.022044	0.027015
1980	0	0.022855
1981	0	0.031395
1982	0.026827	0.049607
1983	0.187861	0.058304
1984	0	0.046966
1985	0.003026	0.026781
1986	0.020797	0.012626
1987	0	0.007472
1988	0	0.008638
1989	0.004776	0.014306
1990	0.039153	0.021581
1991	0.028722	0.027318
1992	0.011331	0.030583
1993	0.071261	0.032075
1994	0	0.033938
1995	0.026657	0.038041
1996	0.089272	0.042082
1997	0.005546	0.041234
1998	0.074667	0.03355
1999	0.006048	0.021613
2000	0	0.0112
2001	0	0.00806
2002	0	0.013155
2003	0.032114	0.021105
2004	0.047239	0.025043
2005	0.014068	0.023734
2006	0.010911	0.022062
2007	0.022141	0.02412
2008	0.035476	0.029645
2009	0.03182	0.036865
2010	0.028971	0.042994
2011	0.100413	0.043494
2012	0.004228	0.037815
2013	0.034228	0.032757
2014	0	0.033563
2015	0.091272	0.037919
2016	0.013336	0.044622
2017	0.01561	0.057096
2018	0.115255	0.07159
2019	0.119515	0.075971
2020	0.042939	0.06663

Figure 2. Unusually High Annual Precipitation in the Contiguous 48 States, 1895-2020
Data source: NOAA, 2021
Web update: April 2021
Units: percent of land area

Key points

In recent years, a more significant percentage of precipitation has come from extreme single-day events. Nine of the first ten years for extreme one-day precipitation events have occurred since 1996 (see Figure 1).
The prevalence of extreme single-day precipitation events remained relatively constant between 1910 and the 1980s but has increased significantly since then.

During the entire period from 1910 to 2020, the portion of the country that experienced extreme single-day precipitation events increased at a rate of about half a percentage point per decade (see Figure 1).
Between 1895 and 2020, the percentage of land areas receiving much more than the typical annual total precipitation increased. However, there has been a lot of variability from year to year.

Some years had no abnormally wet areas, while other several years had abnormally high precipitation totals of more than 10 percent or more of the land area of the 48 contiguous states (see Figure 2). For example, 1941 was extremely rainy in the West, while 1982 was very wet nationwide.
Figures 1 and 2 are consistent with other studies that found increased heavy precipitation over periods ranging from odd days to seasons and years.

For more information on trends in overall precipitation levels, see the USA and Look at the Global Precipitation indicator.

Background

“Heavy precipitation” refers to situations where the amount of rain or snow experienced in a place is significantly higher than usual. What constitutes a period of heavy rainfall varies by location and season.

Climate change can affect precipitation intensity and frequency. 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 result in heavier precipitation (for example, more heavy rain and snowstorms).

The potential effect of heavy rainfall includes crop damage, soil erosion, and increased air pollution. The risk of flooding from heavy rains can lead to injuries, drowning, and other health-related effects. In addition, runoff from rainfall can degrade water quality as land-washed pollutants accumulate in water bodies.

Heavy precipitation does not mean that the total amount of rainfall in a location has increased; it only means that precipitation occurs during more intense events.

However, changes in precipitation intensity, 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 return times, or measuring the amount of precipitation over a given period.

One way to track heavy precipitation is to calculate the percent of the total rainfall for a given location in a given year as extreme one-day events. In other words, what percentage of the precipitation comes in short, intense bursts.

Figure 1 of this indicator looks at the prevalence of extreme single-day precipitation events over time.

For more information, this indicator also tracks an unusually high total annual precipitation occurrence. It does this by looking at the Standardized Precipitation Index, which compares yearly actual precipitation totals with the range of precipitation totals that can typically be expected at a given location based on historical data.

Suppose a site receives less than usual rainfall during a given period. In that case, it will receive a negative SPI score, while a location that receives more rain than usual will receive a positive score. The more precipitation, the higher the SPI score.

SPI is a valuable way to look at precipitation totals as it allows different locations and seasons to be compared on a standard scale. Figure 2 shows what percentage of the total area of the 48 contiguous states has an annual SPI score of 2.

About the Data

Indicator Notes

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

The data for this indicator comes from an extensive network of national weather stations and is provided by the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information. Figure 1 is based on Step 4 of NOAA’s US Extreme Climate Index; for the description of data and array. Figure 2 is based on US SPI.