Hurricane Florence, the first major hurricane of the 2018 Atlantic season, went from zero to hurricane in the course of about two days.
How did a tropical storm become a violent hurricane so fast?
On Sunday, Florence shifted from a tropical storm to a low-level hurricane. But in the span of a just few hours Monday, it jumped from Category 2 to Category 4, undergoing what scientists call “rapid intensification,” or a wind speed increase of more than 35 miles per hour in less than 24 hours. It’s now been called the “storm of a lifetime” by the National Weather Service, and is expected to be one of the most powerful storms that’s ever made landfall north of Florida.
Brian Tang, an assistant professor at the University at Albany’s Department of Atmospheric and Environmental Sciences, told The Daily Beast that Florence’s rapid escalation was the result of multiple changes in water temperature, air, and wind, all of which converged to give the storm its terrifying destructive power.
“When all these things come together,” Tang said, “it creates an environment where a storm can intensify really rapidly.”
Florence’s intensification was kicked off by its movement into warmer waters.
“The hurricane derives much of its energy from the ocean,” Sharanya Majumdar, a professor of atmospheric sciences at the University of Miami, told The Daily Beast. “When we have a relatively warm ocean, that promotes the transfer of energy through the water vapor, from the ocean to the atmosphere. That water vapor then condenses to water, that then releases latent heat, [which] provides a source for the hurricane to organize and intensify around.” The warmer the water, the more intense a hurricane like Florence can become.
That’s exactly what happened. Four days ago, Florence was traveling through water that was about 80 degrees Fahrenheit, said James Belanger, senior meteorological scientist at IBM’s The Weather Company. But by Monday, the storm had reached water that was slightly warmer, around 84 degrees.
“Four degrees doesn’t seem like a lot, but when you’re talking about a hurricane, that’s actually really significant,” Belanger explained. “For every degree Fahrenheit you’re able to warm the ocean temperatures, that really allows the maximum potential intensity of the storm to rise.”
When Florence moved to warmer waters, Tang added, it also moved away from harsh winds that would otherwise limit the storm’s strength. Vertical wind shear—the difference between the speed of the winds pushing on the top and the bottom of the hurricane—affects a hurricane’s intensity.
The more difference there is between the those two wind speeds, Tang said, the more the hurricane will start to “tilt” forwards or backwards, significantly limiting the storm’s efficiency and bringing in drier air that slows the evaporation process.
“A hurricane is a sensitive system,” Tang added. “Any little interference from the environment will cause the storm to weaken.”
That’s what happened last weekend, when Florence—which became the first Category 4 of the Atlantic hurricane season last week—moved into an area with colder water and higher wind shear. As a result of those conditions, the storm weakened to a tropical storm, a less severe classification than Category 1.
But when the storm moved into an area with less wind shear, that tilting stopped, allowing Florence to regain some of its strength and begin its rapid spike in intensity.
The lack of cold water and wind shear aren’t the only two factors that caused Florence to strengthen so quickly over the past two days, Tang notes. The surrounding air was moist, and the hurricane was surrounded by a ring of storms, accelerating wind speeds.
This combination of environmental and meteorological conditions is rare. But when it does occur, it’s disastrous: Hurricanes Harvey, Irma, and Maria all experienced a period of rapid intensification before striking land.
It’s because of this rapid intensification that the predictions of Florence’s wind speeds and storm surges are so dire. But Belanger noted that intensification doesn’t have much to do with the expected rainfall and flooding.
Most storms, no matter how intense, are pushed by heavy winds that prevent them from raining too much on any given place. But Belanger said that a ridge of high pressure will likely neutralize the winds that would push the hurricane across the country, causing the hurricane to “stall” somewhere over the Carolinas or Virginia, dumping devastating amounts of rain.
It’s that rainfall that’s causing meteorologists to call Florence the “Harvey of the East Coast.” And that’s a terrifying comparison: Hurricane Harvey, which devastated Texas in August 2017, stalled above Houston and caused deadly, catastrophic flooding. Harvey and its accompanying rainfall brought $125 billion in damage, impacting more than 100,000 homes and claiming 88 lives.
Florence’s projected 40 inches of rain in some states might not exceed Harvey’s 50-60 inches, but it would far outpace the region’s rainfall record (the highest, in Virginia, is 27 inches from 1969’s Hurricane Camille). It could also prove lethal on uneven terrain susceptible to flash floods and landslides.
More than anything, Majumdar said, the flooding poses the greatest risk.
“The main concern, as we saw last year with Harvey over in Houston, is that when a storm stalls, it rains consistently over the same place.” Majumdar said. “A stalling storm, regardless of its intensity, can be quite catastrophic for the amount of flooding it creates.”
Wind shear is projected to pick up right before Florence makes landfall, which could slightly reduce the storm’s intensity. But Tang was careful to note that this change would be minor and would not do anything to impact the predicted rainfall.
His message is clear: Residents living in the storm’s path should prepare for the worst.
“All indications are we’re expecting a major hurricane landfall in the Carolinas,” Tang said. “And any time you have a major hurricane making landfall over any stretch of populated coastline, it’s going to be a disaster.”