Hurricane Harvey: Why Is It So Extreme?

By Mark Fischetti

Hurricane Harvey is drowning southeastern Texas for the fourth day, putting a vast area under feet of water. Experts say Harvey has been stuck longer in one place than any tropical storm in memory. That’s just one of the hurricane’s extremes; the storm is off the charts in many categories. Scientific American wanted to learn why, and asked meteorologist Jeff Masters for help. Masters is the co-founder of Weather Underground, a web site that meteorologists nationwide go to for their own inside information about severe weather. Masters also wrote a fascinating articleon why the jet stream is getting weird.

Why did Hurricane Harvey so quickly explode from a Category 1 hurricane to Category 4?

Last Wednesday night, August 23, Harvey was a tropical depression, but after just eight overnight hours it was forming a hurricane eye wall. “That’s remarkably fast,” Masters says. On Friday it rapidly ballooned from a Category 1 hurricane to Category 4. That’s because it happened to pass over a region of extremely warm ocean water called an eddy. This spot of hot water was 1 to 2 degrees Fahrenheit warmer than the Gulf of Mexico around it, which itself was already 1 to 2 degrees F higher than average, reaching 85 or 86 degrees F in places. The hotter the water, the more energy it drives into a storm. Hurricane Katrina, which destroyed New Orleans in 2005, also mushroomed to Category 4 in similar fashion because it, too, passed over a hot eddy in the Gulf.

Why is Harvey so stuck in place over Texas?

Hurricanes are circular structures with winds that spiral counterclockwise. But they are steered by larger wind patterns in the greater atmosphere that push them in one direction. In Harvey’s case, a big high-pressure system over the southeastern U.S. is trying to push the storm in one direction, but a big high pressure system over the southwestern U.S. is trying to push the storm in the opposite direction. “The systems have equal strength and are cancelling each other out,” leaving Harvey stranded, Masters says. “It’s highly usual to have two highs on either side of a hurricane of equal strength.” The only other time Masters recalls that happening to a huge storm system was Hurricane Mitch in 1998, which struck Central America and killed an estimated 7,000 people in Honduras.

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