Yesterday, Harvey was a tropical storm. Today, it’s a Category 2 hurricane, and it may strengthen to a Category 3 by the time it hits southeastern Texas on Friday night. That would make it the strongest cyclone to strike the mainland United States in 12 years. What’s behind this storm’s sudden increase in power?
Harvey’s winds quickened from about 35 to 109 miles per hour over the past day and a half. That’s called “rapid intensification,” says Suzana Camargo, an ocean and climate scientist at the Earth Institute’s Lamont-Doherty Earth Observatory. She says the hurricane’s fast power-up was influenced by two factors.
The first is warm water. Warm water dumps heat energy into a developing storm. More heat allows the low-pressure circulating system to draw in more air from its surroundings, which makes the storm spin faster and grow. “A thick layer of warm water provides more fuel for the storm,” Camargo explains. The warm waters of the Gulf of Mexico, which Harvey is moving across, make it something of a breeding ground for hurricanes.
A satellite view of Hurricane Harvey as the storm approaches the Texas Coast, via Corpus Christi National Weather Service
But warm water isn’t enough to cause a rapid intensification; it also depends on wind speeds throughout the troposphere (the lowest region of the atmosphere). If you think of the storm as a big tower, explains Camargo, you can’t have the winds blowing in one direction at the bottom of the storm but a different direction at the top—that would topple the column of air and the storm would be caput. The same thing happens if the winds have different strengths at different heights. Basically, having uniform winds from the bottom to the top of the storm allows it to grow in strength.
Rapid changes in strength like Hurricane Harvey’s are not all that rare. A recent study led by Chia-Ying Lee, a postdoctoral researcher at the International Research Institute for Climate and Society, observed that out of about 500 hurricanes that formed in the North Atlantic, 96 underwent rapid intensification—so about one in five. The phenomenon is more common in the Pacific Ocean.
As Hurricane Harvey continues moving over the warm waters of the Gulf, it may continue to gain energy and speed. By the time it reaches Texas, forecasters think the storm will reach Category 3 status, which means its winds could whip up to 130 miles per hour, with the potential to cause devastating damage. Making matters worse, the hurricane will be moving slowly, giving it extra time to dump nearly three feet of rain onto some areas of the Lone Star State. Mandatory evacuations are underway in seven Texas counties due to flood concerns.
“A lot of people are taking this storm for granted, thinking it may not pose much of a danger to them,” Texas Governor Greg Abbott told Houston news station KPRC. “Please heed warnings and evacuate as soon as possible.”