https://www.scientificamerican.com/article/hurricane-harvey-why-is-it-so-extreme/

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 article on 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.