How Hurricane Otis defied forecasts and exploded into a massive storm

Hurricane forecasts have improved, but Otis still surprised meteorologists with how rapidly it intensified.

On Tuesday morning, Otis was a mere tropical storm with sustained winds of 70 miles per hour. Six hours later, the storm’s wind speed had nearly doubled, and hours later it slammed into southwest Mexico as a Category 5 storm. Nearly 1 million people in Acapulco were directly in its path.

“A nightmare scenario is unfolding for southern Mexico,” the National Hurricane Center said at the time.

By sunrise on Wednesday, Otis had weakened to Category 2 strength, with sustained winds of 110 miles per hour, yet it had already carved a path of destruction. Mexican President Andrés Manuel López Obrador said Wednesday in a news conference that the regions of Guerrero, Costa Grande, Acapulco, and Técpan were “hit hard” and communications were “completely lost.” Forecasts warn of further heavy rainfall, flash flooding, and high winds over southern Mexico.

“Rarely, according to records, does a hurricane develop so quickly and with such force,” Lopez Obrador added.

Otis displayed one of the hallmarks of modern hurricanes: rapid intensification, when a hurricane gains at least 35 mph in wind speed over 24 hours. Otis actually blew past that threshold, gaining almost 100 mph in less than half that time, making it one of the fastest-intensifying hurricanes on record. Otis is another breathtaking data point in a year marked by unprecedented weather extremes. This year’s El Niño, coupled with ongoing climate change, has amplified many of the drivers of hurricanes, and their destructive elements.

Hurricane Otis was also especially tough to predict, making it harder to get people out of harm’s way. Scientists are urgently unpacking the factors behind this storm and others like it to better anticipate how future disasters will play out. But as the climate continues to change, hurricanes like Otis can still humble even the best forecasters.

How El Niño fueled Otis’s unprecedented surge

El Niño, the warm phase of the Pacific Ocean’s temperature cycle, has been especially strong this year, driving global air and ocean temperatures to levels never before recorded. Hurricanes require sea surface temperatures of 80 degrees Fahrenheit or more to form, so hotter weather tends to favor more hurricanes. In the Atlantic Ocean, El Niño also tends to create wind shear — wind coming from different directions or at different speeds — which usually suppresses hurricane formation, but the water temperatures have been so hot that hurricane activity in the Atlantic has still been above average.

In the Pacific, El Niño is much more favorable to tropical storms (In the eastern Pacific, major tropical storms are called hurricanes, while in the west, they’re known as typhoons). This year, the west coast of North America has already seen several hurricanes. Hurricane Hilary struck California in August, and the winds from Hurricane Dora fanned the flames of wildfires in Hawaii.

NOAA

Hurricane Otis, the 15th named storm in the eastern Pacific this year, spooled up right in the patch of warmer water around the equator that characterizes El Niño.

The area was also prime real estate for rapid intensification. For Otis, the “key ingredient” for intensification was warm water, with temperatures topping 86 degrees Fahrenheit, said Brian McNoldy, a hurricane researcher at the University of Miami. By this time in October, the Pacific Ocean had soaked up plenty of heat all summer long, so there was abundant warm water at the surface and several feet below to feed Otis. A day or two before the storm intensified there was some wind shear and patches of dry air — which can disrupt the formation of a strong storm — but they subsided just enough by late Tuesday for Otis to explode, McNoldy said.

Scientists are getting better at predicting rapid intensification, but the baseline is changing

The ingredients of rapid intensification can be difficult to measure with satellites in remote regions of the ocean where hurricanes form. These factors are also complicated to incorporate in models, making it difficult to predict when a disturbance will suddenly turn into a monster. But scientists have been steadily improving their foresight as they gathered data from hurricanes over decades.

“We’ve reached a point where we are starting to understand how storms undergo rapid intensification,” Sundararaman Gopalakrishnan, who leads NOAA’s hurricane modeling team, told Vox in August.

The National Hurricane Center did a good job forecasting the intensification of Hurricane Ian, for example, which rapidly intensified over 48 hours, said McNoldy. The Center’s first advisory, when it was still a tropical depression, indicated that it would rapidly intensify in the eastern Gulf of Mexico before slamming the Fort Myers region, he said.

“But there are still misses like this,” he said, referring to Otis. There were likely some smaller-scale meteorological phenomena playing in the eastern Pacific — beyond ocean temperatures, humidity, and shear — that even the best models failed to detect.

This year, NOAA has begun to deploy its Hurricane Analysis and Forecast System, which is designed to better predict rapid intensification and could anticipate hurricane tracks as far as seven days out. However, it’s still in its testing phase, and Gopalakrishnan said it’s “being closely monitored for performance by the modelers, and assessments will be used for further model improvements.”

Accurate forecasts can, of course, save lives. Yet as temperatures rise, the behavior of storms is changing. The question now is whether our predictive capabilities can keep up.