NOAA’s Precipitation Frequency Atlas of the United States (Atlas) provides the statistical likelihood of an extreme precipitation event at a particular location in any given year, which — until recently — assumed a stationary climate. These statistics are the basis for planning and infrastructure design and for communicating the likelihood of extreme events, such as storms that have a 1% chance of occurring. 

Recognizing that extreme precipitation and nationwide flood risk are getting worse in a warming climate, NOAA is changing the methodology used to produce the Atlas by factoring the future state of the climate into official precipitation frequency estimates

Beginning with a pilot project for the state of Montana, on October 9 NOAA’s National Weather Service’ Office of Water Prediction released an early look  at the data used to produce Atlas 15 Volumes 1 and 2 to get stakeholder feedback before expanding nationwide. Volume 1 uses trends in observations and Volume 2 uses climate model output data to estimate future conditions. 

Novice and experienced users of Atlas tools are asked to review and compare both volumes and provide feedback on the utility of each interface. Doing so will be the first step in a robust peer-review process that Atlas 15 will undergo before it is published for the continental U.S. in 2026, and the rest of the country in 2027. Next year, NOAA will release preliminary estimates for the lower 48 states to begin the peer review process. When Atlas 15 is published, these data will provide communities across the country with information necessary to adapt to future climate conditions. 

The new Atlas will provide an update of the current Atlas 14 “point and click” system of spatially independent estimates of expected precipitation depth for specified storm durations from 5 minutes to 60 days, and is also presented as annual exceedance probabilities from 50 percent to 0.1% annual exceedance.  

The move to Atlas 15 will bring numerous benefits, including:

• Updated Historical Data: Atlas 15 will refresh outdated data to the present. Many Atlas 14 products are 10–20 years old.
• Seamless National Estimations: It will ensure consistent precipitation estimates across the entire country.
• Future Climate Projections: Atlas 15 will include estimates of future conditions based on different greenhouse gas scenarios going forward, and it will display these side by side for easy comparison.

Instructions for accessing the Montana Atlas 15 pilot can be found here, along with a six-minute video explanation of its features and a 36-page detailed explanation of methodologies employed in developing the Atlas. This represents a huge accomplishment that ASFPM has strongly supported over the past five years, as part of a larger effort to improve flood estimation and, ultimately, mapping accuracy. We encourage our members to review the pilot data and complete the NOAA survey. Feedback will be accepted through December 31, 2024

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