Then an experienced human being sits down and looks at them all.
“The official track is—you have a human being sitting there, weighing runs, usually mentally, with which ones he trusts the most and which ones agree,” Dorst told me. “You always want to have a human being somewhere in the loop when you’re issuing forecasts saying, this is where [the hurricane] will go.”
That researcher will draw the anticipated-track line, based on the model runs and their own experience. Then they add the cone of uncertainty around the anticipated track. This cone doesn’t have anything to do with what the models say: It’s an average of how forecasters have gotten other tropical cyclones wrong over the past five years. “That’s why it spreads over time,” says Dorst. “In 12 hours, they might have a 20-mile error, and at 24 hours, a 40-mile error.”
Those errors have gotten smaller over time when it comes to storm track. But predicting a hurricane’s intensity is a less certain matter.
“Although the track forecasts have steadily improved over the last 30 years, there’s been no tangible improvement in our ability to forecast how strong a storm will be,” Emanuel told me.
“The forecaster’s worst nightmare is: There’s a storm in the Gulf that’s 12 or 24 hours from landfall, and you go to bed at night, and it’s a tropical storm. Then you wake up the next day and it’s a Category 3. By then there’s just no time to get people out of harm’s way,” he said.
That is more or less what happened when Hurricane Audrey, which strengthened from a tropical depression to a Category 3 cyclone over 24 hours, struck northeastern Texas and western Louisiana in 1957. Because it intensified so close to shore, few evacuated from its path. Audrey ultimately killed more than 400 people, making it one of the deadliest hurricanes ever to strike the United States.
“Even though weather forecasting has improved since then, I think even if you talk to a hurricane forecaster today, they’ll tell you this is what keeps them up at night,” says Emanuel.
Earlier this year, he found in a paper in the Bulletin of the American Meteorological Society that climate change will make this kind of rapid intensification more likely. “There’s a lot more cases of rapid intensification [in a warmed climate], and that includes the accident of it rapidly intensifying just before it makes landfall,” he said.
At the same time, it’s “immoral” to order an evacuation for any tropical storm that could strengthen, he said—since people often die in car accidents during mass evacuations.
Hurricane Harvey exemplifies the difficulty of examining both of these symptoms together. The recent hurricane intensified up until the moment it made landfall, setting it apart from all hurricanes in the Gulf of Mexico observed in the modern-day period. But its track was what really made it devastating: Had it not stalled out above Houston for several days, it would have been a major disaster, but not a multibillion-dollar catastrophe.