Estimating the maximum amount of precipitation that could fall in a given area, like a watershed where all precipitation drains into a common place like a river, is essential for safeguarding critical American infrastructure.

Understanding this probable maximum precipitation (PMP) is essential when engineers design and build high-risk facilities such as dams and nuclear power plants so that they can withstand a “worst case” extreme precipitation event. It also helps water resource managers better operate these facilities to reduce the risk of a catastrophic failure that can significantly endanger lives and property.

The PMP estimates currently in use throughout the country are decades old and need to be updated to reflect the observed increase in the frequency and severity of extreme precipitation events. A NOAA team led by the Physical Sciences Laboratory is working to do just that.

The team recently passed an important milestone by creating the first iteration of a historic extreme precipitation event data library.

Background

• In 2024, at the direction of NOAA, the National Academies of Science, Engineering, and Medicine released a Probable Maximum Precipitation Estimation Modernization report that outlined a multi-year roadmap for NOAA to update PMP estimates.
• A research team led by the NOAA Physical Sciences Laboratory (PSL) with partners at the NOAA Global Systems Laboratory, the University of Colorado Boulder’s CIRES, Colorado State University’s CIRA, and other select contributors are spearheading the PMP modernization initiative.
• As a first step, the National Academies’ report recommended consolidating measurements and observations from historic extreme precipitation events into one central location for PMP research.
• As such, the NOAA PMP team has launched an interactive historical precipitation event data library, hosted by PSL. This library currently features ten historical storm cases from 2017 to 2024. The team plans to continuously expand this resource, with a second set of cases that includes storms from before 2015 expected to be added by the end of March 2026.

More about the data library

• The Historical Extreme Precipitation Event data library contains a vast array of meteorological variables including precipitation amount, precipitable water (how much water the atmosphere holds from the surface, upwards), temperature, soil moisture, and more from specific extreme precipitation events. It also includes information on existing precipitation estimates, as well as useful diagnostics for determining the rarity of the event.
• The interactive library allows users to select a region, an historical extreme precipitation event, and a variable to see the selected data visualized in an image or animation.
• The first iteration includes data from Hurricanes Harvey (2017), Ida (2021), and Helene (2024), as well as other significant events such as the extreme rainfall that almost led to the failure of the Oroville Dam in California.
• Data for these events was gathered from multiple sources, including ERA5 for all of the meteorological variables except precipitation, AORC for precipitation amounts and quantitative precipitation estimates, and NOAA Atlas 14 for average recurrence internals (ARIs).

Impact and what's next

• Gathering extreme precipitation data in one location will greatly improve researchers ability to analyze past storms to better understand how they developed and ultimately impacted a specific area.
• The NOAA PMP team will continue to gather as much information as possible from past extreme precipitation events, including storms from the 20th century via National Weather Service and other historical sources, building the database into a robust source of extreme precipitation event data from past decades.
• They also will use these cases to determine how skillful new models and datasets are at representing past extreme events, which is critical to determining their value for future predictions and projections.
• The team is collaborating with the US Army Corps of Engineers to try to combine these storm resources with their databases, as well as planning for future integration of many physical historical documents that are currently awaiting digitization.
• Through this effort and the ongoing PMP estimation modernization initiative, researchers will better understand how future extreme precipitation events may impact vulnerable areas across the nation, and help planners, engineers, and decision-makers protect public safety and property.

Explore Historical Extreme Precipitation Events
Learn more about the PMP project

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