Early Wednesday morning, Hurricane Otis became the strongest storm in recorded history to strike the Pacific coast of Mexico. The Category 5 hurricane made landfall near Acapulco, where its heavy rain and 265-kilometer-per-hour (kph) winds unleashed massive landslides and knocked out power lines, killing at least 2 dozen people and causing widespread devastation.
But just 2 days earlier, meteorologists doubted whether Otis—then a tropical storm—would even achieve hurricane status. Forecasters at the U.S. National Hurricane Center expected the storm to undergo “gradual strengthening,” with most computer models predicting maximum wind speeds of about 100 kph. Instead, as Otis careened toward Mexico’s coastline, its winds increased by 180 kph in 24 hours, a record amount of “rapid intensification.”
For meteorologists, it was a tragic reminder that although forecasting methods have drastically improved in recent years, predicting when a minor storm will suddenly explode into a catastrophic hurricane is another matter. “It’s difficult to forecast something like that,” says John Kaplan, a meteorologist at the National Oceanic and Atmospheric Administration (NOAA).
Scientists do know the major ingredients driving rapid intensification. Two keys are a warm ocean and moist air, which combine to fuel the convection forces at the storm’s center. Sharanya Majumdar, a hurricane researcher at the University of Miami, notes that ocean waters have been “unusually warm” throughout this year’s hurricane season, with the El Niño climate pattern channeling even more heat into the tropical Pacific Ocean. As Otis neared shore, it crossed over a patch of water that reached 31°C—several degrees above the average expected for late October.
For a hurricane to grow in strength, it must also be free of cross-cutting winds, known as wind shear, which can tear holes in the walls of the storm’s eye or throw it off kilter. “A hurricane doesn’t like to be tilted,” says Brian Tang, an atmospheric scientist at the University at Albany. Earlier in the week, when Otis still resembled an ordinary tropical storm, some forecasters did note that environmental factors had the potential to turn it into something more dangerous, but the storm’s structure didn’t seem organized enough to take advantage. And although wind shear was weak, Kaplan says it wasn’t extraordinarily low.
Environmental factors such as ocean temperature, humidity, and wind shear are relatively simple to measure and feed into forecasting models. But Majumdar says the “inner anatomy” of a storm, which is much more difficult to measure, ultimately determines whether rapid intensification will occur. If thunderstorms swirling around the center of a hurricane are symmetrical, then heat stays concentrated in the eye of the storm instead of dissipating over a larger area. Symmetry also leads to dramatic drops in pressure at the storm’s center that ramp up wind speeds. Once that happens, “nothing can prevent it from becoming a monster,” says NOAA meteorologist Sundararaman “Gopal” Gopalakrishnan.
But thick clouds block those internal conditions from the view of most satellites. Fed with the available data, computer simulations all predicted Otis would intensify gradually. It wasn’t until late Tuesday, when aircraft were able to fly inside the storm and take direct measurements, that forecasters began to realize their models had gotten things wrong. This delay spelled disaster for the people of Acapulco, who were caught largely unaware when Otis slammed the Mexican coast. “Our predictions just didn’t give enough lead time for residents to adequately prepare,” Tang says. “If you’re behind the ball just a little bit, it’s too late.”
As climate change causes oceans across the globe to heat up, some scientists are concerned that rapidly intensifying hurricanes like Otis will become more frequent. “It’s not easy to make a very clear prediction for the future,” Majumdar says, noting that whether such storms are truly becoming more common hasn’t been clear. However, he adds, recent evidence does point to an upward trend—and to warming oceans as the driving force.
If more storms resembling Otis are indeed on the horizon, then forecasters—and their computer simulations—will need to become better at predicting them. “There’s certainly a long way to go,” says Kaplan, who notes that the resolution of current models isn’t high enough to analyze smaller storms, which are especially susceptible to environmental factors and can either weaken or strengthen with incredible speed. Gopal, who served as the principal architect for NOAA’s state-of-the-art Hurricane Weather Research and Forecasting system, explains that models are also only as good as the data that go into them. Because rapidly intensifying hurricanes have so far been rare, such data are scarce.
Ultimately, Kaplan adds, there is a limit to how well any model—or any meteorologist—can perform. “Nature is hard to predict sometimes,” he says.
© OfficialAffairs
