- Hurricane Dorian is one of the two most powerful storms to ever hit land in the Atlantic. It’s also incredibly slow, moving just one mile per hour as of Monday afternoon.
- That slow pace means more rainfall and a longer period of hurricane-force winds wherever the storm goes. It has lingered over the northwestern Bahamas since Sunday night.
- Dorian’s sluggishness is part of a trend of slower storms due to warming oceans and atmosphere.
- Visit Business Insider’s homepage for more stories.
Hurricane Dorian is moving over the Bahamas at just one mile per hour — slower than most people walk.
Dorian slowly churned through the Caribbean over the weekend, tying for the strongest Atlantic hurricane landfall ever when it struck the Bahamas as a Category 5 storm with sustained wind speeds of 185 mph on Sunday.
It has since weakened to a Category 4 with 150-mph winds, but it has lingered over Grand Bahama for over 12 hours, where it first made landfall at 11 p.m. ET on Sunday. The National Hurricane Center (NHC) expects it to remain there through Monday night.
Satellites used by the National Oceanic and Atmospheric Administration (NOAA) show just how slow the hurricane’s movement has been.
The eye of catastrophic Cat. 5 #HurricaneDorian lingered over #GrandBahamaIsland overnight and into this morning, as seen in this close-up GeoColor loop from #GOESEast. Follow the storm via our hurricane tracker: https://t.co/rAbHgL0qkB pic.twitter.com/1EE9gdp6lD
— NOAA Satellites (@NOAASatellites) September 2, 2019
“The slower you go, that means more rain. That means more time that you’re going to have those winds. That’s a long period of time to have hurricane-force winds and tropical-storm force winds,” NHC director Ken Graham said in a Facebook live video on Friday, before the storm had reached the Bahamas. “All of a sudden we have trees down, more power lines down. So, a very serious situation.”
Caribbean weather systems slow Dorian down
Dorian started out slow, but it has only gotten slower as it churned through the Bahamas over the weekend, grinding to a near halt at 1 mph on Monday. That’s due to weakness in a pocket of high atmospheric pressure called the Bermuda High, which has been steering the storm west.
“That’s what’s been moving it. We’ve been forecasting that to basically collapse,” Graham said in a live video Monday morning. “As a result, there’s nothing to steer the hurricane. So when you lose that high pressure there’s nothing pushing that hurricane, so it’s just sitting there waiting, wobbling, waiting for something to pull it in a different direction.”
The NHC expects a growing low pressure system over the US to start pulling Hurricane Dorian north. That’s why its projected path has moved north towards Georgia and the Carolinas.
Dorian’s sluggishness is part of a growing trend of slower, more powerful storms.
Ocean warming is making storms slower
Hurricanes use warm water as fuel, normally weakening as they move over land or colder water. But as the Earth’s oceans and air get warmer — last year was the hottest on record for the planet’s oceans — tropical storms overall are getting stronger, wetter, and slower.
Over the past 70 years or so, the speed of hurricanes and tropical storms has slowed about 10% on average, according to a 2018 study. Over land in the North Atlantic and Western North Pacific specifically, storms are moving 20% to 30% more slowly, the study showed.
A slow pace gives storms more time to dump rain and pummel an area with powerful winds. Overall, that can lead to more flooding and more damage.
To make matters worse, a warmer atmosphere can hold more moisture, so a 10% slowdown in a storm’s pace could double the amount of rainfall and flooding that an area experiences. The peak rain rates of storms have increased by 30% over the past 60 years — that means up to 4 inches of water can fall in an hour.
Hurricane Harvey in 2017 was a prime example of this: After it made landfall, Harvey weakened to a tropical storm, then stalled for days. That allowed the storm to dump unprecedented amounts of rain on the Houston area — scientist Tom Di Liberto described it as the “storm that refused to leave.”
Dorian’s refusal to move on from the Bahamas is yet another example.
Hurricanes like Dorian start out over warm ocean water near the equator, where the sea’s surface temperature is at least 80 degrees and the air is humid. The rising moisture creates a cluster of thunderstorms, which form a vortex, like water circling a drain.
The churning winds then create an area of low pressure over the ocean’s surface, which allows more air to enter. Once the winds in that spinning storm system reach 74 miles per hour, a hurricane has formed.
“It’s pretty well understood that if the water is warmer and it’s causing more moist air to come up, you have the potential of a storm to grow quickly and intensely,” Brian Haus, a researcher who simulates hurricanes at the University of Miami, told Business Insider last week. “The question is how much that’s going to happen.”
A storm’s wind speed is influenced by the temperature of the water below. A one-degree Fahrenheit rise in ocean temperature can increase a storm’s wind speed by 15 to 20 miles per hour, according to Yale Climate Connections.
Over the weekend Hurricane Dorian outstripped forecasts, reaching Category 5 strength with sustained wind speeds of 185 mph on Sunday. Friday NHC projections had expected it to be a Category 4 with 140-mph winds.
In the Bahamas, reports of Dorian’s impact are still preliminary as it continues to churn over Grand Bahama. So far it has destroyed or severely damaged an estimated 13,000 homes, according to the Red Cross. Parts of Abaco and New Providence islands have no power or cell service. At least one person has died in the Bahamas.