Extreme rainfall events don’t occur very often – they are events considered out of the ordinary. Different weather conditions can result in different kinds of extreme rainfall. Intense thunderstorms can produce a lot of rain in a short period of time in a relatively small area. Hurricanes or tropical storm systems can result in extreme rainfalls, over small areas or sometimes over potentially very large areas.
Climatologists use three measurements to categorize extreme rainfall events: rainfall duration, rainfall intensity, and accumulation of rainfall. Rainfall duration indicates how long the rainfall event lasted. Rainfall intensity describes how much rain fell during a single storm’s given time period (duration). Rainfall accumulation is how much rain has fallen in an area over time, including multiple rainfall events.
The Average Return Interval (ARI) is a way to describe how 'rare' a rainfall event is based on historical data. It is the average period, in years, between exceedances of an event at a location. While we use terms like '100 year rainfall event', that doesn't mean it will happen only once every one-hundred years. More accurately, it means there is a 1% chance that an event of that size will happen in any given year. The percent chance that an event will happen in a given year is called the Annual Exceedance Probability (AEP). It is simply the inverse of ARI (1/ARI). See the table below for a breakdown of ARI values, and this glossary from NOAA for a more detailed definition of ARI.
| Recurrence Interval, in years |
Probability of occurrence in any given year |
Percent chance of occurrence in any given year |
|---|---|---|
| 100 | 1 in 100 | 1 |
| 50 | 1 in 50 | 2 |
| 25 | 1 in 25 | 4 |
| 10 | 1 in 10 | 10 |
Engineers and hydrologists use return periods in planning and designing the built environment. These data help determine what kind of rainfall events are likely to occur and design their projects accordingly. Urban stormwater systems, city planning departments, water supply systems, wastewater management systems, and natural resources conservation planning are all groups who use this information.
This map will change based on the drop-downs selected. When a DMA is selected, it will highlight on the map. When a station is selected, it will show up on the map with a red "+".
This chart changes with DMA and Duration selection. Station names are on the Y-axis of this graph. The x-axis is time. Black lines show the period of record at the individual station. Most stations have at least 50 years of data, though some stations have recorded data from the 1890s. Where there is no line, there was no data from that time period. The diamonds indicate an occurrence of a extreme rainfall event. The color of the diamond represents the ARI/AEP of the event, seen in the key to the right of the graph.
The Y-axis on this graph represents precipitation amount in inches while the X-axis represents the year. Black dots represent rainfall events for that year, indicating the rainfall amount. Lines across the graph show the ARI rainfall value in inches, for comparison.
The maps show precipitation amounts in inches for the selected duration and return interval.
The maps and graphs shown on this website are based on the data found at NOAA Atlas 14 Point Precipitation Frequency Estimates website.
Use the drop-downs to select a drought management area, event duration, and station to view the corresponding graphs. Click on an image to enlarge. Click out of an image or on the X to go back to the main screen.
Use the drop-downs below to select a storm duration and return interval to view the corresponding map. Click on the image to enlarge. Click out of the image or on the X to go back to the main screen.
Multiple factors other than rain determine the occurrence of flooding, such as:
A 500-year rainfall event will not necessarily produce a 500-year flood.
A 500-year rainfall event can occur in consecutive years, or it might only happen once in 100 years.