Study: Cyclones have been slower in recent decades
Lingering storms such as Harvey can devastate regions.
With wind speeds that have approached 140 mph, Hurricane Florence isn’t exactly slow. But forecasters don’t expect the storm to blow through quickly once it reaches land.
Instead, they think it will stall, much as Hurricane Harvey did over Houston last year, besieging the area for days with wind and rain. That is part of the reason Florence is expected to be so dangerous.
Unfortunately, Florence and Harvey are not alone. Tropical cyclones, which include hurricanes, have grown more sluggish since the mid-20th century, researchers say.
A study published this summer in the journal Nature focused on what is known as translation speed, which measures how quickly a storm is moving over an area, say, from Miami to the Florida Panhandle. Between 1949 and 2016, tropical cyclone translation speeds declined 10 percent worldwide, the study says. The storms, in effect, are sticking around places for a longer period of time.
Lingering hurricanes can be a problem, as Texans learned when Harvey caused devastating flooding and billions of dollars of damage. The storm dropped more than 30 inches of rain in two days and nearly 50 inches over four days in some places. A report issued in June by Harris County, which includes Houston, found that Harvey’s rainfall exceeded every known flooding event in U.S. history since 1899.
“The really, really high rainfall totals were because the storm moved so slowly,” said Deanna Hence, a professor of atmospheric sciences at the University of Illinois at Urba- na-Champaign, who was not involved in the research. “The large amount of rain that is going to come out of a trop- ical storm or hurricane anyway fell in the same place over a long period of time.”
To analyze the changes in translation speeds, James Kossin, a climate scientist with the National Centers for Environmental Information at the National Oceanic and Atmospheric Administration, tapped into a global data set on past tropical storms. The data included estimates of the latitude and longitude of each named storm’s center at six-hour intervals.
By compar i ng a given storm’s positions over time, Kossin was able to measure how quickly it had moved across the landscape. He then calculated the average speeds of the storms from year to year, and found that tropical cyclones had been slow- ing over time.
There were limitations to the analysis, Hence said. The study did not say how much of the extra rainfall produced by a storm like Harvey was caused by the storm staying in one place, as compared to other contributing factors, like the unusually warm ocean waters that fueled that hurricane.