How a mix of natural and human-caused factors cooked up Tropical Storm Hilary’s soggy mess

Aug 21, 2023, 4:29 PM

Members of Cal Fire Pilot Rock 6 crew, out of Crestline, Calif., clear mud off the side of the road...

Members of Cal Fire Pilot Rock 6 crew, out of Crestline, Calif., clear mud off the side of the road in the aftermath of Tropical Storm Hilary, Monday, Aug. 21, 2023, in Yucaipa, Calif. Scientists figure a natural El Nino, human-caused climate change, a stubborn heat dome over the nation’s midsection and other factors cooked up Hilary’s record-breaking slosh into California and Nevada. (AP Photo/Marcio Jose Sanchez)

(AP Photo/Marcio Jose Sanchez)

A natural El Nino, human-caused climate change, a stubborn heat dome over the nation’s midsection and other factors cooked up Tropical Storm Hilary’s record-breaking slosh into California and Nevada, scientists figure.

Cooked up is the key phrase, since hot water and hot air were crucial in rapidly growing Hilary and then steering the storm on an unusual path that dumped 10 months of rain in a single weekend in normally bone-dry places. Nearly a foot of rain fell along a couple Southern California mountains while cities smashed summertime records.

“It was a combination of sort of a perfect situation of everything coming together in a way that made the storm possible,” said University of Albany atmospheric scientist Kristen Corbosiero, an expert on Pacific hurricanes.

It’s never easy to attribute a single event to climate change, especially so quickly and with El Nino being a prominent factor, said former National Oceanic and Atmospheric Administration hurricane and climate scientist Jim Kossin, now with the nonprofit First Street Foundation.

To understand Hilary’s unusual path, it’s best to go where the storm began.

Hilary formed in an area south of Baja California and west of Mexico. Many storms form in the Eastern Pacific there, but most move harmlessly west into the open Pacific or into Mexico and then eventually — weaker — into the U.S. Southwest.

It’s one of the most active birthing places for tropical cyclones, Corbosiero said. But the water — fuel for the heat engine that is a hurricane – was about 3.5 to 5 degrees Fahrenheit (2 to 3 degrees Celsius) hotter than normal at the surface and that warmth went deep, said UCLA western weather scientist Daniel Swain.

So Hilary rapidly intensified, gaining 75 mph in wind speed strength in just 24 hours — going from nearly nothing to a Category 4 hurricane in no time.

“We’ve been seeing (rapid intensification) more and more recently,” said Kossin, who did a study showing this phenomenon increasing.

“For a storm to intensify the way Hilary did everything has to be ideal,” Kossin said. There has to be warm water, it has to run deep and there has to be little to no crosswinds decapitating the storm, he said. Hilary checked all those boxes.

The water was warm both because of the natural El Nino, a warming of parts of the equatorial Pacific that changes weather worldwide, and because of long-term climate change that has been shattering records for heat deeper in the oceans, scientists said.

UCLA’s Swain said there are three main reasons storms that form where Hilary did don’t normally swamp Southern California.

First, unlike the hurricane-prone Atlantic coast where the warm Gulf stream is ideal for storms, the coast along California and Baja California is cold and it brings cold water up from the deep, Swain said: “That’s a real hurricane killer.”

The normal atmosphere in California is also a hurricane killer. It’s dry and has downward motion, while storms like upward motion, Swain said.

But Hilary had grown so strong and big that even though it rapidly weakened when it hit the cold water, it was still packing enough of a punch when it got to California, Kossin said.

The reason it got to California is that the third factor — usually prevailing winds pushing storms from east to west – failed to protect the Pacific coast this time, Swain said.

Hot air to the east and a low-pressure system to the west combined to push and pull Hilary up into California instead of the normal paths for eastern Pacific storms, Corbosiero and other scientists said. And a big hot air mass sitting over the middle United States blocked the storm from turning east.

What’s unusual is that big hot air mass just hasn’t been moving. Some scientists, including Woodwell Climate Research Institute’s Jennifer Francis, have theorized that especially in summer there are more and more situations where weather patterns get stuck and it seems to be connected to changes in the Arctic because of global warming. Other scientists disagree. It’s one of the biggest unresolved issues in mainstream climate science, Swain said.

“Hilary is a rare storm but almost certainly we will see equally bizarre and destructive but different events unfold as the globe continues to warm generally and this El Nino continues to strengthen,” Francis said.

Last October, MIT hurricane scientist Kerry Emanuel was at UCLA giving a guest lecture on the rare chance of a tropical storm or hurricane hitting Los Angeles. His computer models, factoring in climate change and other ingredients, found that the type of storm that would dump 15.7 inches of rain (40 centimeters) on downtown Los Angeles used to have a one-in-108-year chance of happening, at least until 2010. But now that type of storm has a one-in-30-year chance, he figured.

“Hilary was substantially more probable today than it would have been 20 or 30 years ago,” said Emanuel, who also calculated the likelihood of a storm flooding New York City, months before 2012’s Superstorm Sandy.

But it’s not just climate change, Emanuel said: “We do know for sure that El Nino tends to enhance” hurricane activity in that region.

And when storms like Hilary hit, the warmer air also holds more moisture and that means more rain falling down, Corbosiero, Swain and Emanuel said. Studies show that worldwide tropical cyclones are getting rainier.

For the next two to three weeks, expect the eastern Pacific hurricane basin to be active – peak season is near the end of the month – Corbosiero said. Other weather and climate conditions may provide the region a break in early to mid-September only to get busier again at the end of next month, she said.


Follow AP’s climate and environment coverage at


Follow Seth Borenstein on Twitter at @borenbears


Associated Press climate and environmental coverage receives support from several private foundations. See more about AP’s climate initiative here. The AP is solely responsible for all content.

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How a mix of natural and human-caused factors cooked up Tropical Storm Hilary’s soggy mess