Hurricane Harvey: a quintessential example of a stalling storm
Hurricane Dorian hit the Northwestern Bahamas as one of the most powerful Atlantic hurricanes on record. Then it did something worse.
For roughly 40 hours, Category 5 Dorian crawled across the Abacos and Grand Bahama Island, it’s pace so incremental that the National Hurricane Center’s assessment of its speed and heading for much of one day was: “Stationary.” That movement — or lack of it — reminded tropical cyclone scientist Timothy Hall of another monster storm: Hurricane Harvey, which drowned much of Houston in 2017.
“Harvey didn’t go as far north but it was also very slow-moving, meandering back and forth from its predicted path, and then it just stalled; like Dorian, it grew to a massive system that simply sat on a limited area for a long time,” said Hall, a hurricane researcher at NASA’s Goddard Institute for Space Studies in New York City. “These stalling events exacerbate rain-driven flooding.”
Dorian and Harvey join a list of other slow-moving or stalling storms that have turned what were expected to be wind-driven disasters into deadly flooding catastrophes.
Last year, Hurricane Florence hovered near the coast of North Carolina for more than 50 hours, becoming the wettest tropical storm on record for the Carolinas and killing 53 people. Hurricane Harvey in 2017 stalled for more than 100 hours in the northern Gulf of Mexico, pounding Texas with unprecedented rainfall that triggered record flooding in the Houston area. The death toll there reached 68.
In 2008, Tropical Storm Fay moved erratically around the Caribbean and then spent almost three days zigzagging from water to land in the Florida peninsula, becoming the first storm in recorded history to make landfall in Florida four times. Thirty-six deaths were recorded as a result of intense rainfall-driven flooding in different parts of the state.
Dorian — which killed at least 20 in the Northwest Bahamas, a death estimate expected to rise — is the latest piece of evidence supporting Hall’s thesis that hurricanes are getting slower, stalling more frequently and producing more rainfall-related damage as they increasingly linger over a limited area. The storm’s excruciatingly slow track through the Bahamas fits a pattern described in a recent study that analyzed hurricane data for the past 73 years.
Hall and Jim Kossin, an atmospheric research scientist with the National Oceanic and Atmospheric Administration, concluded that North Atlantic tropical cyclones have become increasingly likely to stall near the coast and become stationary for longer periods of time. The scientists also found that the average speed of tropical cyclones has slowed by 17 percent since the mid-20th century, to 9.6 miles per hour in 2017 from 11.5 mph in 1944.
Their study, titled “Hurricane stalling along the North American coast and implications for rainfall” and published in June, found that the stalling is driven not just by a slower forward speed for hurricanes but also by an increase in abrupt changes of direction.
In the study, Hall and Kossin looked at all tropical cyclones from 1944 until 2017. Using data from the National Hurricane Center, they mapped the positions of each storm center in six-hour intervals, calculating the average forward speed and changes in storm track direction. They opted to use data from the post-World War II era because of its consistency, the period when aircraft reconnaissance data about hurricanes and related atmospheric conditions became available.
Before that, weather information was mostly collected by ships in a sporadic manner.
The research explores possible mechanisms that could explain the lower storm speeds and stalling frequency but doesn’t reach a firm conclusion, Hall said.
“A lot of questions remain unanswered, but we have some suspected factors that might be causing these trends,” Hall said.
It may be related to a slowdown in large-scale wind patterns in the atmosphere. And it could be linked to climate change, though that connection isn’t very firm still.
Some models suggest that general atmospheric circulation patterns — the westward steering flows that typically guide North Atlantic hurricanes in subtropical latitudes, for example — are weakening. Hurricanes are guided and steered by huge wind flows, “like a cork in a stream,” said Hall.
He also said that research suggests that rising temperatures in the Arctic may be affecting the wind patterns that guide hurricanes in the mid-latitudes. The difference in temperature and pressure in the Arctic and the equator is a key driver of wind flows: It’s that contrast that leads air masses to move and contribute to hurricane patterns. Because temperatures in the Arctic are warming faster than those in lower latitudes, the contrast is decreasing, producing slower wind speeds, Hall said.
“These weaker and more variable winds provide less robust steering flow for tropical cyclones and allow blocking patterns to persist longer,” the study said.
The connection between slowing storms and global warming is still a matter of debate, and scientists have not linked climate change to the path or intensity of tropical storms. But they agree on a few things: Increased ocean temperatures, a result of global warming, are making storms more intense. It’s also a consensus among hurricane scientists that warmer air holds more humidity, so hurricanes are becoming wetter as a result of climate change, Hall said.