Precipitation Frequency

Precipitation Frequency in Southeastern Wisconsin

Commission staff compiled information to aid in stream and stormwater analyses, including recommended precipitation depths and rainfall distributions, as well as recurrence interval determinations for recent storms in the Region. This information can be used for developing peak flows for use in floodplain mapping and stormwater analyses and design.

Precipitation Frequency

Intensity-Duration-Frequency Equations

In response to considerable interest expressed by counties and municipalities in the Region, Commission staff developed county-specific intensity-duration-frequency (IDF) equations utilizing NOAA Atlas 14 precipitation depth-duration-frequency information. The IDF equations developed are empirical formulas representing the relationship between maximum precipitation intensity, precipitation duration, and frequency.

IDF Equations

Heavy Rain Events

Commission staff has analyzed the probabilities of several heavy rain events occurring in the Region over recent years. These probabilities use information from the NOAA Atlas 14, Volume 8, Version 2.0 Precipitation Frequency Data Server.

• September 2022 probabilities of heavy rain for various durations from one-hour through four days
• May 2020 probabilities of heavy rain for various durations from one-hour through 10 days
• July 2017 probabilities of heavy rain for various durations from one-hour through 10 days (note: this event resulted in record flooding in the Fox River and Des Plaines River Watersheds)
• June 2008 and July 2010 probabilities of heavy rains for various durations from one-hour through 10 days and rainfall patterns for June 2008 and July 2010 storms

Rainfall Pattern Maps

Commission staff prepared the following rainfall pattern maps for the storms of August 1986, June 1997, and August 1998:

• Rainfall pattern on August 6, 1986 (24-hour period)
• Rainfall pattern on June 20-21, 1997 (26-hour storm)
• Rainfall pattern on August 6, 1998 (24-hour period)