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. 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. Figure 1 of this indicator looks at the prevalence of extreme single-day precipitation events over time.įor added insight, this indicator also tracks the occurrence of unusually high total yearly precipitation. 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 short, intense bursts. 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). 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. 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. 2 In addition, runoff from precipitation can impair water quality as pollutants deposited on land wash into water bodies. 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, and other flooding-related effects on health. 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. Warmer oceans increase the amount of water that evaporates into the air. What constitutes a period of heavy precipitation varies according to location and season.Ĭlimate change can affect the intensity and frequency of precipitation. 'Heavy precipitation' refers to instances during which the amount of rain or snow experienced in a location substantially exceeds what is normal. 4 For more information on trends in overall precipitation levels, see the U.S. 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.For example, 1941 was extremely wet in the West, while 1982 was very wet nationwide. 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). There has been much year-to-year variability, however. The percentage of land area experiencing much greater than normal yearly precipitation totals increased between 18.Over the entire period from 1910 to 2020, 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 prevalence of extreme single-day precipitation events remained fairly steady between 1910 and the 1980s, but has risen substantially since then.

Nine of the top 10 years for extreme one-day precipitation events have occurred since 1996 (see Figure 1). In recent years, a larger percentage of precipitation has come in the form of intense single-day events.

Weather precipitation totals