Slower-moving storms will rain more over a given area, will batter that area longer with their winds, and will pile up more water ahead of them as they approach shorelines, said Jim Kossin, a scientist with the National Oceanic and Atmospheric Administration and the study's author. Adding last year's storms would have made the slowdown a bit more prominent, he said.
Dr Kossin came to his conclusion by studying the tropical cyclone record, which spans from 1949-2016. But when Atlantic storms hit land - like Harvey - the study said the slowdown is a significant 20 percent.
"Inland flooding, freshwater flooding, is taking over as the key mortality risk now associated with these storms", Kossin said.
But Kossin can't say whether Harvey's rains are a model for the future.
"A lot of the initial research has been focused largely on numbers of tropical cyclones and then of course on intensity because that's the way that we feel the impacts", Christina Patricola, an atmospheric scientist at Lawrence Berkeley National Laboratory, told the Los Angeles Times.
There is considerable evidence that global summertime circulation patterns in the atmosphere are slowing as a result of global warming.
Tropical cyclones have generally slowed more in the Northern Hemisphere where they are also known as hurricanes and typhoons and where more of these storms typically occur each year.
In a warming world where atmospheric circulations are expected to change, the atmospheric circulation that drives tropical cyclone movement is expected to weaken.
Recent research also found that "blocking" of the jet stream can lead to extreme weather events such as hurricanes and floods.
The result is more rainfall and more damage to buildings as hurricanes hover over population centers for longer periods of time. It was also based exclusively on observations and didn't use computer models to simulate the Earth with and without warming.
To understand the relationship between climate change and hurricane speeds, Kossin analyzed the paths of 7,585 tropical cyclones from 1949 to 2016. "We'll need more formal attribution studies to disentangle these factors".