NOAA Atlas 15 CONUS Preview: Why Stormwater Codes Will Need Rewriting in 2027–2028

NOAA Atlas 14 has been the operational reference for precipitation frequency in the United States for two decades. Every municipal stormwater code in the country that names a design storm by reference points at Atlas 14, either directly or through a state-level technical document that does. That dependency is about to become a transition problem.

The NOAA Office of Water Prediction has scheduled the CONUS preliminary release of Atlas 15 Volume 1 for September 2026. Atlas 15 had a brief funding pause in mid-2025 before being resumed; the September 2026 target is publicly stated as on-track. Once it lands, the operational federal precipitation frequency dataset begins to migrate, and the half-life of every Atlas 14 reference in the country starts to shorten.

This post takes a position on what that transition will look like, why it will not play out evenly, and what designers and program managers should be doing in the months between now and 2027.

Atlas 15 is structured as two volumes, and conflating them will produce poor analyses through 2027.

Volume 1 is a re-estimation of precipitation frequency from the historical record, using non-stationary statistical methods. The L-moment regional frequency analysis that anchored Atlas 14 assumed stationarity, that is, that the statistical properties of the precipitation record do not change in time. Volume 1 abandons that assumption and applies methods that allow precipitation distribution parameters to vary across the record. The result is a re-fit IDF estimate that is anchored entirely in observed data.

Volume 2 is a climate-adjusted future projection. It applies adjustment factors derived from climate model ensembles to the Volume 1 estimates, producing future-period IDF estimates under specified climate scenarios. Volume 2 is the more disruptive of the two for design practice because it explicitly builds future climate into design rainfall.

The two volumes will not land at the same time. Volume 1 CONUS preliminary is the September 2026 target. Volume 2 follows. Designs adopting Atlas 15 between the two releases use one set of numbers; designs adopting after both releases potentially use a different set. The transition window itself becomes a defensibility question.

A stormwater ordinance is not a hydrology document. It is a legal instrument. Updating it requires more than swapping a table.

Most municipal stormwater ordinances reference Atlas 14 either by document name and edition, by a state technical manual that references Atlas 14, or by a specific table embedded in the code itself. Updating any of those points means a code revision process, typically involving public comment, council or commission review, and coordination with the state DEQ if the municipality operates under a delegated NPDES program. In some jurisdictions the cost of conveyance infrastructure sized to a larger design storm has utility-rate implications that put the update into a budget conversation rather than a hydrology conversation.

The result is predictable. Federal programs that operate on national technical guidance can pivot quickly. Municipal ordinances cannot. The asymmetry between how fast Atlas 15 gets absorbed by FEMA versus how fast it gets absorbed by an MS4 ordinance is going to be the operational story of the next 24 months.

The likely sequence runs federal first, state DEQ second, municipal third, with a long tail.

Federal infrastructure programs will move fastest. FEMA flood map production, USACE civil works design, FHWA hydraulic design for federal-aid projects, and the federal flood risk management standard all sit on national technical guidance and can absorb a new precipitation frequency reference on a relatively short cycle. Expect adoption beginning in 2027 as the CONUS preliminary data stabilizes into final.

State environmental agencies will move second. State technical manuals (stormwater design manuals, drainage design manuals, MS4 program documents) are revised on multi-year cycles, with some states updating annually and others on a five-year cycle or longer. The leading states will reference Atlas 15 in technical manual updates in 2027–2028. Lagging states will reference Atlas 14 into the early 2030s by default.

Municipal MS4 ordinances will move third. Some progressive cities will run a code update process in 2027–2028 explicitly to incorporate Atlas 15. Most will not. The default will be that the ordinance continues to reference Atlas 14 by name while the federal design environment shifts underneath.

This asymmetry will produce projects where the federal floodplain analysis uses Atlas 15 design rainfall while the municipal site plan uses Atlas 14 design rainfall on the same parcel. That inconsistency is a problem worth flagging in scoping conversations now, not after it shows up in plan review.

The firms that come out of this transition cleanly are the ones treating it as a documentation and traceability problem now, not a hydrology problem later. A short list of practices that hold up across the transition:

• Document the Atlas version and edition used in every design report, by name and date, not by reference to a state manual that may have changed. A design that says "Atlas 14 Volume 9, 2014 edition" is durable across the transition. A design that says "per the [state] stormwater manual" is not.
• Run a parallel sensitivity check on infrastructure with a 50-year-plus design life. The Atlas 15 pilot product for Montana (released in 2024) gives a defensible bound on the direction and rough magnitude of change in that region. For other regions, use the published methodology to estimate the order of magnitude of the precipitation change a Volume 1 update would produce for the design event of interest. Flag designs where the sensitivity is large enough to matter.
• In capital planning conversations, name the shelf-life. Infrastructure being sized in 2026 to Atlas 14 design rainfall is defensible today but will be on a different basis than equivalent infrastructure sized in 2028. That fact belongs in capital planning memos, not in a footnote.
• Where a client has a choice between sizing to Atlas 14 plus an explicit climate adjustment versus sizing to Atlas 14 alone, take the climate adjustment seriously. Volume 2 will eventually formalize what is currently a judgment call. Designs that have already absorbed a defensible adjustment are easier to defend across the transition than designs that have not.

Atlas 15 is not a technical curiosity. It is the first major reset of the operational precipitation frequency reference for the United States since stationarity was declared dead in the hydrology literature in 2008. The eighteen-year gap between that declaration and the operational response is itself the story; the operational response itself has the next two years to land. The firms paying attention now are positioning for the work that follows.