Gulf Coast gutters: five cities, their rain and their storms

Five cities side by side

Annual figures are NOAA's 1991-2020 station normals and link to each station's data file. Intensities, in inches per hour, come from NOAA Atlas 14 through the Precipitation Frequency Data Server at the latitude and longitude shown.

For scale, a 1,500-square-foot section of roof in New Orleans sheds about 83 gallons a minute at its 100-year, 60-minute rate, using the U.S. Geological Survey's rain volume figure of 27,154 gallons per inch on an acre. The same section in Mobile sheds about 186 gallons a minute during a 10-year, five-minute burst.

Pensacola

Pensacola's airport station has the highest annual normal of the five, 68.31 inches. Hurricane Sally showed what the city's gutters face in a slow storm: the NHC's Sally report records 24.88 inches just west of Naval Air Station Pensacola, the highest Florida total in the storm, and says freshwater and storm surge flooding produced 2 to 4 feet of inundation in parts of downtown Pensacola. For short bursts, Atlas 14 puts Pensacola's 10-year, five-minute intensity at 10.9 inches per hour.

Mobile

Mobile has the most violent short bursts on this list. Its two-year, five-minute intensity of 8.71 inches per hour, the kind of downpour expected every couple of years, is higher than any other city here, and its 100-year, five-minute rate reaches 16.9 inches per hour. The same Sally report places landfall at Gulf Shores, Alabama, with maximum sustained winds of 95 knots, and records the event's highest rainfall, 29.99 inches, at Orange Beach on the Alabama coast.

New Orleans

New Orleans has the highest 100-year, 60-minute intensity of the five, 5.34 inches per hour, which is the kind of rate the International Plumbing Code names as its sizing basis. For the same roof, that is about 21 percent more design flow than Baton Rouge's 4.4 inches per hour. Sally's Louisiana maximum was far smaller than on the Alabama coast, 6.94 inches near Pearl River, according to NHC.

Baton Rouge

Inland Baton Rouge still averages 61.94 inches of rain a year at its airport station. Its Atlas 14 rates are the lowest of the five at every duration shown, but the 10-year, five-minute figure is still 9.84 inches per hour, or about 0.8 inch of rain in five minutes. The code math works the same way here as on the coast.

Lake Charles

Lake Charles sits in southwestern Louisiana, which turns up in two of the NHC reports this publication draws on. NHC's Allison report describes extremely heavy rainfall over much of southwestern Louisiana as well as the Houston area, and Harvey made its final landfall near Cameron, Louisiana, on August 30, 2017, with 40-knot sustained winds, per the Harvey report. Atlas 14 gives Lake Charles a 10-year, five-minute intensity of 10.8 inches per hour.

Planning gutters on the Gulf

• Size from the Atlas 14 point nearest your house, not from a regional rule of thumb.
• Check the five-minute rates as well as the hourly one; on this list, the 10-year five-minute rate runs about two to two and a half times the 100-year hourly rate.
• Clear outlets and downspouts before June. Ready.gov lists de-cluttering drains and gutters among its hurricane preparations.

When a storm damages the roof edge along with the gutters, the National Roofing Directory's storm damage roof repair guide explains the repair process. You can find a roofer in Alabama, Florida or Louisiana through the Directory, check each state's rules on the roofing licensing page, and follow storms on the NRD Storm Center. The full sizing method is on the downpour sizing page.

Frequently asked questions

Which Gulf city has the heaviest short downpours?

Mobile. Its Atlas 14 five-minute intensities, 8.71 inches per hour at the 2-year level and 11.9 at the 10-year level, are the highest of the five cities on this page.

How much rain did Hurricane Sally drop near Pensacola?

NHC's report gives 24.88 inches just west of Naval Air Station Pensacola, the highest Florida total in the storm. The event's overall maximum was 29.99 inches at Orange Beach, Alabama.

Do New Orleans gutters need more capacity than Baton Rouge gutters?

On the plumbing code's hourly basis, yes. New Orleans's 100-year, 60-minute intensity of 5.34 inches per hour is about 21 percent higher than Baton Rouge's 4.4, so the same roof produces about 21 percent more design flow.