Why is Hurricane Dorian moving so slowly?
Hurricane Dorian has been a slow-moving disaster.
The powerful hurricane, which weakened slightly to a Category 2 storm Tuesday morning, had churned for 15 hours at a glacial pace of just 1 mph and was locked at a standstill over the northern Bahamas. Its fixed position allowed it to clobber the island chain with sustained winds of up to 185 mph for nearly two full days after making landfall Sunday as a deadly Category 5 storm.
Now, Dorian has finally begun to leave the Bahamas and is forecast to begin turning north — with hurricane- and tropical storm-force winds offshore but still menacing Florida and the East Coast of the U.S. over the next few days.
Even if Dorian doesn't produce significant damage in the U.S., the storm has been historic because of its wind speeds, making it one of the strongest Atlantic hurricanes ever recorded.
Why has it been moving so slowly?
The storm is traveling sluggishly because atmospheric conditions that dictate its movement collapsed on Sunday night. Those steering currents are driven by the other players in the atmosphere that are larger than the hurricane, including high pressure and low pressure systems, troughs and the jet stream. (It's the wind blowing in relation to all of these that steers the weather.)
A high-pressure ridge over the Atlantic that was moving Dorian over the weekend calmed completely, and that weakness in the ridge on Monday caused Dorian to stall.
Air — high off the ground and closer to where people live — is often stagnant in the summer and this is an extreme version of that, said Jeff Masters, a meteorology director at Weather Underground who used to fly into hurricanes.
Two years ago, Houston was deluged by extreme rainfall from Hurricane Harvey, which made an initial landfall as a Category 4 storm before it stalled over Texas because the steering currents collapsed.
Meanwhile, Dorian finally got moving again Tuesday and will continue advancing later this week, when a low-pressure trough swinging out of the continental U.S. will pick it up, push it northeast and accelerate it up the East Coast.
Troughs can act like bulldozers in the atmosphere, generally pushing weather systems from west to east. The timing of this trough is the most important variable because the sooner it starts pushing the storm, the farther it will stay offshore.
Are these slow storms becoming more common?
A study published in June by federal scientists suggests the average speed of tropical cyclones has slowed considerably since the mid-20th century, and in the North Atlantic, they have "become increasingly likely to 'stall' near the coast, spending many hours in confined regions."
And a 2018 study in the journal Nature found that over the past 70 years, the speed of hurricanes and tropical storms has slowed about 10 percent on average.
The reason why is unclear, although researchers believe an overall slowdown in atmospheric circulations is playing a role.
As the Earth's atmosphere warms, it is likely to cause those atmospheric circulations to change, which in turn influences the speed of hurricanes. Climate change can be attributed more to these steering currents becoming blocked or increasingly slow-moving, and that would then result in slower-moving hurricanes.
Could Dorian pick up in intensity again?
Some forecast models are suggesting Dorian could strengthen slightly in the next 48 hours.
But any strengthening, if it does occur, will be slight, and the storm is forecast to remain a strong hurricane over the next few days.
As Dorian moves north, it will travel back out over warm water in the western Atlantic, which could help it to recharge.
The hurricane still has several days to spend there before it moves north enough into cooler waters that would not support further intensification.