The rainfall data that engineers have used for decades to size detention basins, design drainage networks, and satisfy permit requirements is being formally revised. For civil engineers, site designers, and land developers, that revision has direct consequences for how projects are designed, what permits require, and where professional liability exposure can arise.
The Federal Rainfall Standard Is Evolving
Stormwater infrastructure in the United States has long been designed against a common benchmark: NOAA Atlas 14, the precipitation frequency dataset published by the National Weather Service’s Hydrometeorological Design Studies Center. Atlas 14 provides intensity-duration-frequency (IDF) estimates for storm events ranging from the 2-year to the 1,000-year return period.
Engineers use these estimates to size pipes, calculate runoff volumes, and establish design storms for detention and retention facilities.
NOAA Atlas 15, funded in part through the Bipartisan Infrastructure Law, is expected to become the next-generation reference dataset for precipitation frequency in the United States. Atlas 15 will differ from its predecessor in a fundamental way: it will incorporate nonstationary statistical methods and climate model projections to account for future trends through the year 2100, rather than assuming a stationary historical climate.
A pilot dataset covering Montana was released in September 2024, with preliminary contiguous U.S. estimates targeted for release in 2026.
Importantly, NOAA provides datasets but regulatory adoption happens at the state and local level. Many jurisdictions will continue relying on Atlas 14 for years after Atlas 15 is released, and the transition will occur gradually and unevenly across the country.
NOAA’s National Water Center director has described the effort as a response to worsening extreme precipitation and flood risk, and as a tool to help communities design more climate-resilient infrastructure, but how and when that tool gets embedded into local design standards and permitting criteria is a separate question that will play out jurisdiction by jurisdiction.
The IDF Curve Problem
IDF curves are built on historical rainfall records and traditionally assume that the statistical properties of extreme rainfall remain stable over time. That assumption of stationarity is increasingly questioned in hydrology.
A peer-reviewed study in the Journal of Hydrologic Engineering found that in certain regions and scenarios, stationary climate assumptions can lead to substantial underestimation of extreme rainfall, with results varying widely depending on location, storm duration, and modeling approach. The research concludes that most existing stormwater infrastructure is ill-adapted to future rainfall conditions and that climate change adaptation needs to be built into design strategies.
A University of Florida IFAS Extension synthesis published in 2024 puts the practitioner challenge plainly: there is currently no consensus on the best approach for incorporating climate change into IDF curves. Engineers designing stormwater systems today are working in a period where the historical standard is under revision, the replacement standard is not yet final, and methodological guidance remains unsettled.
What This Means for Permitting on Construction Sites
Federal permitting requirements connect directly to precipitation frequency data. Under the NPDES stormwater program, any construction activity disturbing one or more acres requires coverage under EPA’s Construction General Permit (CGP) and the development of a Stormwater Pollution Prevention Plan. EPA’s 2022 CGP specifies that sediment basin sizing must provide storage for the calculated volume of runoff from a 2-year, 24-hour storm.
It is worth noting that the CGP references a storm event, not a specific dataset. The controlling precipitation data is typically set by state guidance or local drainage criteria manuals.
This means that whether an Atlas 14-based design remains defensible depends largely on what the applicable jurisdiction has formally adopted, not solely on what NOAA has published. As Atlas 15 rolls out, engineers will need to track adoption status at the local level.
Professional Exposure During a Transitional Period
Risk increases during periods when guidance is evolving faster than formal standards. The legal standard of care is tied to what a reasonable engineer would do in a given jurisdiction at a given time, not necessarily to the latest available science.
But as precipitation frequency data is updated and jurisdictions begin incorporating new estimates into their criteria, the gap between current practice and adopted standards will narrow, and design decisions made today may be reviewed against a more demanding benchmark in the future.
Documentation of the data sources used, the design storm basis, and any jurisdictional constraints should be part of every project file. When a detention basin overflows or downstream damage occurs, the adequacy of the design storm selection is among the first questions in any professional liability review.
