North Carolina Rainfall Data for Drainage Design: Storm Depths and Intensities by City

Why a drainage plan needs a rainfall number

Every sizing decision in a yard starts with the same question: how much water, how fast? A dry well or rain garden is sized on the depth of a storm — how many inches fall over the day — because it has to store the whole event. A pipe, a grate or a downspout is sized on the intensity — how hard it rains for the worst five or fifteen minutes — because it only has to keep up with the peak. Those are different numbers, and most drainage advice quietly uses one value for both.

The figures on this page are the official ones: NOAA Atlas 14, the dataset engineers and stormwater departments use, read at the centre of each city. They are the numbers behind the "Southeast, NC" presets in the site's calculators and the city lookup in the roof runoff calculator.

24-hour storm depths by city

Inches of rain in 24 hours for storms of each recurrence interval. The 1-year column is an ordinary bad day; 2-year is the hard rain you plan a dry well or rain garden around; 10-year is the storm that finds every weakness in a yard.

Peak intensities for pipes, grates and downspouts

Inches per hour. The 5-minute figure is the burst a downspout or a small paved area has to keep up with; the 15-minute figure suits a larger lawn catchment that takes longer to drain to the low point. The 60-minute columns are the depth of one hard hour, useful for a sanity check.

Which number each calculator wants

The calculators' regional presets are deliberately a little under the 5-minute figures above — 6 in/hr against Raleigh's 7.3 — because a residential catchment never delivers its whole peak to one point at once. If you want to be conservative, type the 5-minute value in.

Coast, Piedmont, mountains

Three North Carolinas show up in the tables. The coast — Wilmington, Jacksonville, New Bern — sees the biggest storms by a wide margin: Wilmington's 10-year day is 7.2 inches against Raleigh's 5.1, and its peak intensity is a third higher. Tropical systems drive that. The Piedmont — Raleigh, Durham, Charlotte, Greensboro, Winston-Salem — is remarkably uniform, within a few tenths of an inch across the whole region, which is why one "Piedmont" figure serves most of the state's population. The mountains vary the most over short distances: Asheville's valley reads lower than Raleigh at 3.8 inches, while Boone on the escarpment reads higher than the coast for long-duration storms. In the mountains, look up your own point rather than borrowing a neighbour's.

The Piedmont's problem is not the rain — it is the clay soil under it, which turns a moderate storm into runoff that has nowhere to go.

How to look up your own point

NOAA's Precipitation Frequency Data Server returns the same table for any latitude and longitude in the state. Search for it, click your property on the map, choose precipitation depth and partial duration, and read the 24-hour row for storage tools or the 5-minute row (multiply a 5-minute depth by 12 for inches per hour) for pipe and grate tools. The roof runoff calculator accepts any value you type.

FAQs

What rainfall should I use to size yard drainage in North Carolina?+

For anything that stores water — a dry well or rain garden — size for the 2-year 24-hour storm, about 3.5 inches across the Piedmont and 4.5 on the coast, and make sure the overflow can handle the 10-year storm. For anything that carries water — pipes, grates, downspouts — use the 10-year peak intensity, about 7 inches per hour inland and 9 to 10 on the coast, or the calculators' Southeast preset of 6 in/hr for a residential catchment.

What is a 10-year storm?+

A storm with a 10 percent chance of happening in any given year — not one that happens every ten years on schedule. Two can arrive in the same season. It is the standard design storm for residential drainage because it is severe enough to expose an undersized pipe without forcing everything to be built for a hurricane. The 100-year storm has a 1 percent annual chance and is used for flood mapping, not yard drains.

Why are the coast and the mountains so different?+

The coast gets tropical systems that drop several inches in a day and short bursts of very intense rain, so both depths and intensities run a third higher than inland. The mountains get long, orographic rain on the slopes facing the weather and much less in the valleys behind them, so two towns twenty miles apart can differ more than Raleigh and Charlotte do. The Piedmont in between is flat enough, meteorologically, that one set of numbers serves it.

What does rainfall intensity in inches per hour mean?+

The rate rain is falling at that moment, expressed as how deep it would be after an hour at that rate. A 10-year 5-minute intensity of 7 inches per hour does not mean 7 inches fall — it means the worst five minutes of the storm drop about 0.6 inches, which is what a grate or downspout has to pass in real time. Storage tools want depth; conveyance tools want intensity.

Where do these numbers come from?+

NOAA Atlas 14, the federal precipitation-frequency atlas, read from the Hydrometeorological Design Studies Center's data server at the centre-of-city coordinates in September 2026, using the partial-duration series in English units. They are point estimates; the confidence interval around each is roughly plus or minus 10 to 15 percent, which is another reason every calculator here treats them as planning figures rather than design values.