Hurricane rain, storm by storm, and the math for your gutters

What the NHC reports record

Each storm name links to its Tropical Cyclone Report, the post-storm write-up the NHC publishes.

NHC's report on Allison is titled as a tropical storm report, not a hurricane report, a reminder that flooding rain is not limited to hurricanes.

Slow storms, big totals

Three of these reports tie the extreme rain directly to a storm that barely moved. Florence crawled toward the coast at around 5 knots, and NHC describes rainbands training over the same area from Wilmington to Elizabethtown. Sally's slow motion across the northern Gulf coast produced its high totals. Harvey's Houston rain came while a front hardly moved on August 27-28, feeding warm, humid air off the northwestern Gulf.

Helene broke the pattern. It was moving fast at landfall, about 27 knots, yet NHC reports that a predecessor rain event ahead of the storm and its interaction with a cut-off low produced a prolonged period of heavy rain, with historic totals in the southern Appalachians.

Turning totals into gallons

The U.S. Geological Survey's rain volume figure, 27,154 gallons per inch of rain on an acre, scales to about 623 gallons per inch on 1,000 square feet of roof. On that basis:

• Sally's 29.99 inches at Orange Beach is about 18,700 gallons off every 1,000 square feet of roof.
• Helene's 30.78 inches at Busick is about 19,200 gallons per 1,000 square feet.
• A 2,000-square-foot roof under Sally's Orange Beach total shed roughly 37,400 gallons, all of it through the gutters and downspouts or over their edges.

Totals versus intensity

Spread evenly over the 72 hours NHC cites, Busick's 30.78 inches works out to about 0.43 inch per hour. That is well below the two-year, 60-minute intensity in NOAA Atlas 14 for any of the Southeast cities this site tracks, which range from 2.03 inches per hour at Charleston and Savannah to 2.43 at Pensacola on the Precipitation Frequency Data Server.

Rain never falls evenly, of course, and that is the point. The storm total tells you how long your gutters have to keep working, and how much time a single clogged outlet has to back water onto the fascia. The short, heavy bursts inside the bands are what size the gutter itself. The sizing guide shows how to work from those burst rates.

What gutters cannot do

The NHC reports are blunt about where the deaths came from. In Harvey, all but three of the direct deaths came from freshwater flooding. In Florence, 17 of the 22 direct deaths in the United States were due to freshwater flooding. Allison's report attributes 27 of its 41 direct deaths to drowning in freshwater floods.

Gutters move roof water away from walls and foundations; they are no defense against rising water. The International Plumbing Code's storm drainage chapter requires rainwater from roofs to drain to an approved place of disposal, and for one- and two-family homes it permits discharge onto lawns or similar flat areas only when the water flows away from the building.

Before and after the storm

Ahead of hurricane season, Ready.gov lists de-cluttering drains and gutters among its home-strengthening steps. After a storm, it advises documenting property damage with photographs and contacting your insurer. The National Roofing Directory's storm damage roof repair guide and its advice on documenting roof damage for a claim cover what to do next, and you can find a roofer through the Directory. The Gulf cities are covered on the Gulf Coast page.

Frequently asked questions

What were the peak rain totals in Helene and Sally?

NHC's reports give 30.78 inches at Busick, North Carolina, for Helene and 29.99 inches at Orange Beach, Alabama, for Sally. Florence's report describes a narrow swath of totals exceeding 30 inches between Wilmington and Elizabethtown, North Carolina.

How much water comes off a roof in a 30-inch storm?

About 18,700 gallons per 1,000 square feet of roof, using the USGS figure of 27,154 gallons per inch of rain on an acre.

Can any gutter handle hurricane rain?

A gutter is sized for a design intensity, not a storm total. Averaged over three days, even Helene's 30.78 inches at Busick fell far more slowly than the short bursts Atlas 14 predicts for ordinary summer storms; the risk comes from the heaviest bands and from outlets that clog during a long event.

Do gutters protect a house from hurricane flooding?

No. They carry roof water away from the walls, but the NHC reports for Harvey, Florence and Allison show that most direct deaths came from freshwater flooding, a hazard gutters do nothing to stop.