Almost every stormwater and floodplain design starts with the same question: how much rain falls in the design storm? For most of the United States, the answer comes from NOAA’s Atlas 14, the official precipitation frequency estimates. For Texas, the Atlas 14 volume was published in 2018, and it raised design rainfall across much of the state.
What Atlas 15 changes
Atlas 14 assumes rainfall patterns are stationary: the past is a guide to the future. Atlas 15 drops that assumption. It accounts for trends in the record and adds projections of future rainfall, so designs can plan for the storms a project will actually see over its life.
The timeline
- September 2024: a pilot for Montana was released for public review.
- September 2026: preliminary estimates for the lower 48 states go out for peer review.
- 2027: published estimates, ready for operational use.
When it’s published, Atlas 15 will replace Atlas 14 as the national standard. Until then, Atlas 14 remains the official source.
What it means for projects in design now
- Design to what your reviewing agency requires today, and ask whether it plans to adopt Atlas 15 early.
- For long-lived infrastructure, it can make sense to test designs against higher rainfall now.
- Watch for local criteria updates. Agencies adopt new rainfall on their own timelines.
Draft written for Creel’s review before publishing. General information, not engineering advice for a specific site.