Probable Maximum Precipitation Estimation Modernization

Updating national PMP estimates through specialized research to improve the safe design, operation, and maintenance of high-risk structures


About this project

The Bipartisan Infrastructure Law (BIL), signed into law in 2021, along with the PRECIP Act, passed in 2022 , directed NOAA to modernize probable maximum precipitation (PMP) estimates. PMP estimates have remained relatively static over the past several decades. The PRECIP Act specifically calls for a PMP estimate modernization effort to bring estimates more in line with a changing climate.

The World Meteorological Organization defines probable maximum precipitation as “the greatest depth of precipitation for a given duration meteorologically possible for a design watershed or a given storm area at a particular location at a particular time of year." PMP is defined for areas such as watersheds and can cover up to 10,000 square miles (Office of Water Prediction , Hydrometeorological Report No.57 ).

PMP estimates are used by engineers when designing large, critical facilities such as dams and nuclear power plants to help gauge the “worst-case scenario” precipitation for the area in which they are being built. This approach in turn ensures that the facility can withstand this maximum precipitation, reducing the risk of a catastrophic event.

PMP vs Precipitation Frequency

PMP estimates are not the same as precipitation frequency estimates. Precipitation frequency is the probability (in a given year) of observing a precipitation event of a defined depth and duration at a particular location (i.e. 10% chance of observing 6 inches of rain over 3 hours). Precipitation frequency estimates are disseminated by the NOAA Atlas 14 product and its future evolution, NOAA Atlas 15 , which will be informed by climate variability.

How Probable Maximum Precipitation and Precipitation Frequency differ

The following table compiled by the NOAA Office of Water Prediction compares PMP and precipitation frequency via the NOAA Atlas 14 product in three key areas: definition, development, and use.

Comparison Probable Maximum Precipitation Precipitation Frequency
Definition The greatest depth of precipitation meteorologically possible for a given duration and design watershed, or a given storm area at a particular location, at a particular time of year.

  • Defined for areas such as watersheds; can cover up to 10,000 square miles.
  • Considered to represent the “worst case” maximum rainfall to be able to occur.
The probability (in a given year) of observing a precipitation event of a defined depth and duration at a particular location (i.e. 10% chance of observing 6 inches of rain over 3 hours in any given year)

  • Defined for finite points on the earth surface.
  • Not intended for use beyond 1,000-year average recurrence interval, or 1/1000 annual exceedance probability.
Development Storm-based approach that uses archived and projected storms to assess extreme precipitation events that can be geographically transposed to the study area.

  • Incorporates meteorological and statistical methods
  • Modernized approaches will account for climate change.

Method summary in Extreme Storm Events Work Group Recommendations Report , Section 4.2.2.

Point-based approach that uses observed precipitation at a point (e.g., rain gauges) without regard to specific storms or storm types (e.g. hurricane, cold front etc.)

  • Incorporates statistical methods.
  • NOAA Atlas 14 implements a stationary approach. Method details in Atlas 14, Volume 11 Section 4.
  • Modernized NOAA Atlas 15 will implement a non-stationary approach that accounts for climate change.
Use Used for the design and maintenance of large-scale, critical facilities and assets (e.g. dams, nuclear power plants) to address high-hazard risks for events involving catastrophic failure. Used for the design of engineering projects, planning and development (e.g. transportation, stormwater management, small-scale infrastructure, flood risk) to design to an acceptable risk level.


NOAA roles

NOAA Research will conduct PMP research that will help inform the development of new datasets and methods that may be used to modernize PMP estimation models. The Physical Sciences Laboratory out of NOAA Boulder will lead a collaborative team that incudes the Global Systems Laboratory and NOAA's Cooperative Institutes in this research to advance PMP estimation.

The National Weather Service’s Office of Water Prediction will be the operational home for modernized PMP estimates and associated material.

Project steps

The PRECIP Act outlines a specific process regarding PMP modernization by NOAA. The first step is a study by the National Academies of Sciences, Engineering, and Medicine recommending the best scientific approach to modernizing PMP estimates.

National Academies study

NOAA sponsorship of a National Academies study was authorized by the PRECIP Act to establish a committee of experts responsible for authoring a consensus study to “consider approaches for estimating probable maximum precipitation (PMP) in a changing climate, with the goal of recommending an updated approach, appropriate for decision-maker needs.”

The National Academies has released their PMP estimation modernization consensus study report

For more on the National Academies’ goals for their study, including committee members and past meeting information, visit their Modernizing Probable Maximum Precipitation Estimation page.

Post-study

Following the publication of the National Academies’ PMP report, NOAA will review the recommendations in the report and, over time, establish and implement a PMP research and modernization approach. Because PMP estimates have historically been static, new research is necessary to identify the best method for accurate estimation moving forward. This deliberative research will take time.

Per authorized legislation, NOAA is to issue updated PMP estimates within six years of the publication of the National Academies’ report. If fully funded, the first updated estimates can be expected by June 2030.

The legislation then stipulates that these estimates be updated no less than every 10 years following the initial updated estimates.


Page Last Updated: June 18, 2024