Why has Hurricane Dorian stalled over the Bahamas?
Thursday 5 September 2019 12:20, UK
by Tom Acres, technology reporter
Hurricane Dorian may have been downgraded from the highest category five to a category three storm, but that has been no comfort to thousands of people living under its eye in the Bahamas.
The tropical cyclone has stalled over the region, hovering dangerously for more than 40 hours, killing several people and unleashing massive flooding.
It is expected to move on to the east Florida coast by Wednesday evening, before heading north to coastal Georgia and South Carolina. The US is braced for potential devastating damage there, but the storm's slow-moving speed has already been catastrophic for the Caribbean islands.
Some of the most devastating hurricanes in recent memory have caused more damage because they have lingered in one place.
Sky News takes a look at just why that is.
Why do hurricanes stall?
Experts say the increasing warmth of the ocean and air is making hurricanes wetter and slower.
Data from the US National Oceanic and Atmospheric Administration (NOAA) shows the average global sea surface temperature has increased by approximately 0.13C per decade over the past 100 years, with the ocean absorbing more heat because of a larger concentration of greenhouse gases in the atmosphere.
James Baldini, a hurricane expert and professor of earth sciences at Durham University, told Sky News: "Often when a hurricane does stagnate, it pulls up warm water, and it needs warm water to grow and maintain strength.
"But eventually wind will pick up colder water from the depths, which will cool the surface and weaken the surface. But it did not happen with Dorian because its a part of the world with really warm water."
The warmer temperature of the ocean and the air was cited in a study by scientists from NASA and the National Oceanic and Atmospheric Administration (NOAA) earlier this year, which found that hurricanes are 10% slower now than they were 70 years ago.
Over land in the North Atlantic and Western North Pacific, they are 20-30% slower than they used to be.
The researchers have suggested that warming has contributed to general slowdown of atmospheric circulation, which means how fast global wind speeds are generally.
Discussing a slowdown in wind speeds in relation to hurricanes might sound odd, but these storms are steered and carried by far larger-scale wind flows. When these slow down or change direction, it can impact how quickly a hurricane moves, and where it will go.
Dorian has generated wind speeds of 185mph - but has been clocked moving west at just 1mph because winds higher in the atmosphere are not strong enough to shift the storm.
Professor Baldini told Sky News: "It was completely stagnant for a while, then creeping along at one or two miles per hour but with very strong wind speed.
"Usually people rely on the Bermuda High, which are high pressure winds in the North Atlantic, because they generally go clockwise around Bermuda. But those winds were very weak in the case of Dorian."
Compare that to Hurricane Nate in October 2017, which was less powerful than Dorian but at one point was recorded traversing the Gulf of Mexico at a record-breaking 28mph.
While the storm was the costliest natural disaster in Costa Rican history, causing $562m (£467m) in damage and killing 14 people, it's fast pace meant US cities such as New Orleans avoided an extended period of severe rainfall.
Louisiana, Florida, Alabama and Mississippi all declared a state of emergency prior to the hurricane, but total damage across all four states was a relatively modest $22.3m (£18.6m).
What are recent examples of hurricanes stalling?
Hurricane Dorian is not alone in having battered one area for a sustained period of time.
Hurricane Florence
In September 2018, Hurricane Florence lingered near the North Carolina coast for 53 hours, becoming the wettest cyclone on record for the Carolinas.
The storm was only category one when it made landfall, but its stubbornness in moving on saw it dump huge amounts of rain in concentrated areas over the course of several days.
The US National Weather Service recorded an unprecedented storm surge of up to 13ft and rainfall that peaked at 36 inches, which produced catastrophic flooding.
Hurricane Harvey
Hurricane Harvey in 2017 stalled for more than 100 hours in the northern Gulf of Mexico, pumping huge amounts of rain on to Texas.
Between 30 and 50 inches of rain was the norm in the city of Houston, and one station to the east registered 51 inches.
Even though it peaked at category four, Harvey is tied with the historic Katrina from 2005 as the costliest tropical cyclone on record at $125bn (£103.9bn, not accounting for inflation).
Professor Baldini said Harvey "took people by surprise in how severe it ended up being".
When will Dorian start moving?
Dorian itself might have generated winds of 185mph, but that atmospheric circulation mentioned earlier is what is keeping it in place over the Bahamas.
Jeff Masters, meteorology director at Weather Underground, who used to fly into hurricanes, said: "This is unprecedented. We've never had a category five stall for so long in the Atlantic hurricane record."
The storm is caught between a high pressure system and a low pressure system trying to pull it north, with neither generating enough power to win the battle.
Mr Masters says hurricanes that get stuck usually end up seeing themselves off by churning up colder water from below the surface - as it is warmer water on the surface of the ocean that fuels the storms.
But the Bahamas and the Gulf Stream are one of the few places where warm water runs really deep.
Mr Masters says the longer Dorian stalls in the Bahamas, the more the low pressure system has a chance to erode the high pressure and pull the hurricane north and away from Florida.
It means the US east coast could avoid the full force of the storm, but the Bahamas remains in real danger.
Professor Baldini said: "Dorian is not predicted to make landfall in the US, its path puts it out to sea, but just barely, and land is within the ancillary of the path.
"There is still potential for it to strike Florida, the Carolinas, and the Outer Banks off the coast of North Carolina are often hit by hurricanes because they jut eastwards out to the Atlantic. But hopefully it will stay out to sea."
