Where do I get design rainfall depths for a US project?

NOAA Atlas 14, served through the Precipitation Frequency Data Server (PFDS) at hdsc.nws.noaa.gov. Click the project location on the map (or enter coordinates) and it returns depth-duration-frequency estimates — 5-minute through 60-day durations, 1-year through 1000-year average recurrence intervals — with 90% confidence bounds. It supersedes the old TP-40, HYDRO-35, and (where published) NOAA Atlas 2 values.

Which states are NOT covered by Atlas 14?

As of the mid-2020s, the Pacific Northwest and Northern Rockies gap — Washington, Oregon, Idaho, Montana, and Wyoming — still relies on NOAA Atlas 2 (1973) and state-specific studies. Everywhere else in the US and its territories has an Atlas 14 volume. NOAA's Atlas 15 program is developing nationwide updated estimates including those states; check the governing agency for what's currently accepted.

How do I convert a rainfall depth to an intensity for the Rational Method?

Intensity is depth divided by duration: i (in/hr) = P (in) / D (hr). Read the depth at the duration equal to the time of concentration — for tc = 15 min, use the 15-minute depth: a 0.9-inch depth over 0.25 hr is 3.6 in/hr. PFDS also outputs intensity tables directly. Never divide the 24-hour depth by 24 for Rational work — short-duration intensities are several times higher.

Which storm distribution goes with the 24-hour depth?

For NRCS/TR-55 hydrograph methods, the 24-hour depth is shaped by a design distribution: the legacy Type I, IA, II, and III curves by region, or the newer NOAA-based NRCS regional distributions derived from Atlas 14 data itself. Most of the interior US is legacy Type II; Gulf and Atlantic coasts Type III. Many state and local manuals now mandate the NRCS Atlas-14-based distributions — check before defaulting to Type II.

What are the upper and lower bounds on the PFDS output?

The 90% confidence interval on the frequency estimate. The spread is substantial — often ±20–30% at the 100-year level — and reflects gauge record length and regional variability. Standard design uses the mean estimate; risk-critical work (dams, levees) sometimes carries the upper bound or performs sensitivity runs across the interval.

Do Atlas 14 depths account for climate change?

No — Atlas 14 is a stationary analysis of the historical record through each volume's publication date. Several jurisdictions apply escalation factors or require checking a rarer event as a proxy. NOAA Atlas 15, in development, is planned to include nonstationary (future-condition) estimates. Follow the governing manual; don't apply ad-hoc factors without a basis.

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Design Rainfall — NOAA Atlas 14 / IDF Reference

By Michael Flynn, PE · water resources & dam safety engineer

Where design storm depths come from, which precipitation-frequency source covers your state, and how to move between depth, duration, and intensity without the classic mistakes.

The Workflow

Step	What to do
1	Open the PFDS (hdsc.nws.noaa.gov/pfds), click the site or enter lat/lon.
2	Read the depth-duration-frequency (DDF) table — depths by duration (5 min–60 day) and ARI (1–1000 yr), with 90% bounds.
3	Rational Method: read depth at D = t_c, convert i = P/D. NRCS methods: take the 24-hr depth and shape it with the applicable storm distribution.
4	Record station/grid, volume, and retrieval date in the drainage report — estimates change when volumes are updated.

Coverage by Source

Region	Source
Ohio Valley & mid-Atlantic/upper Southeast (NC, SC, VA, WV, TN, KY, OH, IN, IL, PA, NJ, MD, DE, DC)	Atlas 14 Vol. 2 (2004)
Deep Southeast (AL, AR, FL, GA, LA, MS)	Atlas 14 Vol. 9 (2013)
Midwest (CO, IA, KS, MI, MN, MO, ND, NE, OK, SD, WI)	Atlas 14 Vol. 8 (2013)
Northeast (New England, NY)	Atlas 14 Vol. 10 (2015)
Texas	Atlas 14 Vol. 11 (2018)
Semiarid Southwest (AZ, NV, NM, UT), California, Alaska, islands	Atlas 14 Vols. 1, 6, 7, 3–5
WA, OR, ID, MT, WY	NOAA Atlas 2 (1973) + state studies

NOAA Atlas 15 (in development) will supersede all of the above with nationwide, climate-informed estimates — watch the governing agency for adoption.

Depth ↔ Intensity

i (in/hr) = P (in) / D (hr)

Duration	Example depth P	Intensity i
5 min (0.083 hr)	0.45 in	5.4 in/hr
15 min (0.25 hr)	0.90 in	3.6 in/hr
1 hr	1.8 in	1.8 in/hr
24 hr	4.8 in	0.2 in/hr

Illustrative depths for a single hypothetical return period, to show the duration effect — pull the actual values for your site from PFDS. The 24-hr average intensity is useless for Rational Method work; the short-duration cell is where storm sewers live.

Three classic pitfalls. (1) Using the 24-hr depth ÷ 24 as a Rational intensity — understates i by 5–15×. (2) Grabbing depths from an old drainage manual table instead of current PFDS — many manuals predate Atlas 14 updates. (3) Pairing an Atlas 14 depth with the wrong storm distribution — several states have moved from legacy Type II/III to NRCS Atlas-14-based regional curves. The storm distribution card covers which applies where.

Sources: NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Vols. 1–11, NWS Hydrometeorological Design Studies Center; PFDS at hdsc.nws.noaa.gov. NRCS National Engineering Handbook Part 630 Ch. 4 (storm distributions). NOAA Atlas 2 (1973) for the Northwest gap.

Have the intensity? Rational Method → · Have the 24-hr depth? NRCS runoff depth · Need t_c first? Time of concentration.

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Pair with the SCS storm distributions card, the t_c methods card, and the composite curve number tool for the full NRCS chain.

Design Rainfall — NOAA Atlas 14 / IDF Reference

The Workflow

Coverage by Source

Depth ↔ Intensity

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