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Tropical Cyclones Are Moving Slower, And That's Not Good News

At a Glance

The speed of tropical cyclones around the world has decreased globally by about 10 percent since 1949.
Atlantic Basin storms have slowed down by 6 percent over water but by 20 percent over land.
The reduced speed leads to heavier rainfall and an increased risk of flooding.

Tropical cyclones – including tropical depressions, tropical storms, hurricanes and typhoons – around the world are moving at a slower pace over land and water, according to a new study published in Nature on Wednesday.

Their forward speed has decreased globally by about 10 percent since 1949, the study said, making these storms more likely to drop heavier rain as they spend longer amounts of time over the affected areas, increasing the risk of flooding.

(MORE: Hurricanes Will be Wetter, Stronger and Slower in the Future)

NOAA's National Centers for Environmental Information (NCEI) scientist Dr. Jim Kossin, author of the study, found a 20 percent slowdown in a storm's forward velocity over land for Atlantic storms, a 30 percent slowdown over land for western North Pacific storms and a 19 percent slowdown over land for storms affecting the Australian region.

The slowdowns were a bit less over the ocean in some regions, as can be seen in the map below from NOAA/NCEI.

Percentages indicate the observed slowdowns in tropical cyclone forward speed by ocean basin from 1949 to 2016. Slowdowns over land were higher in some regions (a 20 percent slowdown over land for Atlantic storms, a 30 percent slowdown over land for western North Pacific storms and a 19 percent slowdown over land for storms affecting the Australian region). Local tropical cyclone rainfall totals would be expected to increase by the same percentage due to the slowing alone.
(NOAA/NCEI)

“The slower a storm goes, the more rain it’s going to dump in any particular area,” Kossin told the Associated Press. “Hurricane Harvey last year was a great example of what a slow storm can do.”

Harvey dumped 60.58 inches of rain in Nederland, Texas, from Aug. 24 to Sept. 1. That rainfall total is the heaviest from any tropical cyclone in the United States in records dating to 1950, topping the 48-inch storm total in Medina, Texas, from Tropical Storm Amelia in 1978, according to NOAA's Weather Prediction Center meteorologist David Roth.

A total of 68 people, all in Texas, lost their lives as a direct result of Hurricane Harvey's devastating flooding, the National Hurricane Center reported.

(MORE: NOAA Maps Show the Torrents Harvey Unleashed on Texas)

A tropical cyclone moving 20 percent slower over land, which the study found for Atlantic storms, has the potential to dump up to 20 percent more rain, "increasing the flood risk for flood-defense systems designed for a 20th century climate with less extreme precipitation events," said Dr. Jeff Masters in a Category 6 blog post for Weather Underground.

Slow-moving tropical storms and hurricanes also lead to strong winds blowing for a longer duration over the same place and possibly more storm surge, Kossin told the AP.

(MORE: Three Reasons Slow-Moving Tropical Storms and Hurricanes Are the Worst)

Kossin's research discovered the slowdown in both the Northern and Southern hemispheres and in every ocean basin except the North Indian Ocean. Eastern Pacific, North Indian and western South Indian Ocean tropical cyclones that affected land areas showed no significant trends in forward speed while they were over land.

This analysis stopped at the end of 2016 and did not include any storms that formed in 2017, so Harvey was not factored in to the study. It was also based solely on observations and didn't use computer models to simulate the Earth with and without warming.

Kossin concluded that the trend has all the signs of human-induced climate change.

Brian Donegan is a meteorologist at weather.com. Follow him on Facebook, Twitter and Instagram.