Tropical Storm Melissa is riding the brakes. It’s a new, troubling tendency for Atlantic storms

Tropical Storm Melissa is expected to meander the Caribbean at an agonizingly slow pace in the coming days, even as it’s forecast to strengthen into a hurricane, covering as little as 2 miles in an hour. This is slower than an average person’s walking speed.

Hurricanes are powerful storms, but they require a tug or pull from other weather systems, such as a cold front or a dip in the jet stream, to determine how fast they move and where they go.

When those influences are very weak or absent, however — as is the case with Melissa — a storm can meander for days, dumping torrential amounts of rainfall on anyone unlucky enough to be nearby.

Slow movement and weak upper-level winds can also help a storm like Melissa intensify into a major hurricane — Category 3 or stronger. Right now, the Caribbean Sea has some of the most unusually hot waters of any part of the Atlantic Ocean, and the storm could have the opportunity to tap into that energy.

“The potential intensity is quite elevated in the Caribbean, compared to the 1979-2023 average for this time of year,” said Kerry Emanuel, a meteorologist specializing in tropical cyclones at the Massachusetts Institute of Technology. There has not been much storm activity in the Caribbean so far this year due to strong upper-level winds that have inhibited nascent storms from intensifying, he pointed out.

But now, conditions are expected to be primed for Tropical Storm Melissa to keep at its snail’s pace.

Are hurricanes slowing down?

There is evidence that slow-moving tropical storms and hurricanes are occurring more frequently in the Atlantic Basin, particularly near and over land masses, experts say.

Studies in the past several years have showed a slow-down in forward speed at landfall. That’s a critical change, since a slower-moving storm typically means more flooding rainfall, and potentially more storm surge.

Some research has pointed to global warming as a potential cause of this trend, but this is not yet fully established, said meteorologist Kieran Bhatia of the global risk and reinsurance company Guy Carpenter.

Some studies, Bhatia said, show tropical storms and hurricanes have slowed down in recent years over the continental United States and near-coastal US regions as well.

However, “these observed changes have not yet been confidently linked” to human-caused climate change, he said. The most recent major report from the UN Intergovernmental Panel on Climate Change found a trend that is not explainable solely by natural variability with “medium” confidence, Bhatia said.

Research from hurricane expert James Kossin and his colleagues have in fact pointed to global slowdowns in tropical cyclone forward speeds. They found this trend was related to changes in upper-level air circulation that are consistent with expectations from global warming.

Other work from Kossin, a former scientist at the National Oceanic and Atmospheric Administration who teaches at the University of Wisconsin, stands out for showing a trend toward slower-moving or even stalling North Atlantic tropical storms and hurricanes at or near landfall.

Additional research by other scientists provides further evidence of recent slowdowns in the forward movement of tropical cyclones, while some modeling studies project that future climate change is likely to result in more meandering storms.

But the broader community of meteorologists and climate scientists are not yet fully in agreement.

Regardless of any climate change ties, any tendency to slow down, like the crawling forward speed of Tropical Storm Melissa, has huge implications for impacts because it allows such storms to dump heavier rainfall totals over land.

Melissa, for example, poses a major flooding threat to Jamaica and Hispaniola in particular, due to its extended proximity to land, the presence of tall mountains that tend to wring out moisture from the atmosphere and the likelihood that heavy rains will fall for many consecutive days.

While the tie between slower-moving storms and climate change is a topic of active research, studies have conclusively shown tropical storms and hurricanes now produce more rainfall than they used to. This is due to warming ocean and air temperatures, since warmer air holds more moisture.

A devastating example of a slow-moving, water-laden storm was Hurricane Harvey in 2017, which produced 60.58 inches of rain in southeastern Texas. This was the highest rainfall total from a tropical cyclone in US history.

As storms slow down, there is potential for more hurricanes to be like Harvey.

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