Scientists believe a natural El Niño, human-caused climate change, a persistent heat dome over the center of the country, and other factors caused Tropical Storm Hilary to make a record-breaking sweep across California and Nevada.
Cunning is the key, as hot water and hot air were critical to Hilary's rapid growth and subsequent steer the storm on an unusual trajectory, dumping ten months of rain in normally bone-dry places in a single weekend.
Almost a foot of rain fell in parts of Southern California's mountains as cities broke records in the summer.
“It was kind of a perfect combination where everything came together to make the storm possible,” said Kristen Corbosiero, an atmospheric scientist at the University of Albany and an expert on Pacific hurricanes.
“It's never easy to attribute a single event to climate change, especially not so quickly and since El Nino is a major contributor,” said Jim Kossin, former National Oceanic and Atmospheric Administration hurricane and climate researcher who now works with the nonprofit First Street Foundation works.
The best way to understand Hilary's unusual path is to go where the storm began.
Hilary originated in an area south of Baja California and west of Mexico.
That's where many storms form in the eastern Pacific, but most move harmlessly west into the open Pacific or into Mexico and eventually — weaker — into the US Southwest.
It's one of the most active tropical cyclone birthplaces, Corbosiero said.
But the water — fuel for the heat engine that is a hurricane — was about 3.5 to 5 degrees hotter than normal on the surface, and that heat went deep, said UCLA western weather scientist Daniel Swain.
So Hilary quickly intensified, increasing wind speeds by 75 miles per hour in just 24 hours — going from nearly zero to a Category 4 hurricane in no time.
“We've been seeing (rapid intensification) more and more frequently lately,” said Kossin, who conducted a study showing the increase in this phenomenon.
“For a storm to intensify the way Hilary did, everything has to be ideal,” Kossin said. There must be warm water, it must go deep, and there can be little or no crosswinds to decapitate the storm, he said. Hilary ticked all of those boxes.
The water was warm due to both the natural El Niño, a warming of parts of the equatorial Pacific that is changing weather around the world, and long-term climate change that has broken records for heat deeper in the oceans, scientists said.
UCLA's Swain said there are three main reasons storms form where Hilary doesn't typically inundate 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, bringing cold water from the depths, Swain said, “That's a real hurricane killer.”
Even the normal atmosphere in California is a hurricane killer. It is dry and has downward movement while storms have upward movement, Swain said.
But Hilary had grown so strong and big that, although it quickly weakened upon hitting the frigid water, it was still strong enough when it arrived in California, Kossin said.
The reason it got to California is because the third factor — usually prevailing winds that drive storms from east to west — isn't able 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 toward California instead of along the normal paths for eastern Pacific storms, Corbosiero and other scientists said.
And a large mass of hot air over the central United States prevented the storm from turning east.
What is unusual is that the large hot air mass simply did not move.
Some scientists, including Jennifer Francis of the Woodwell Climate Research Institute, have theorized that stuck weather situations are becoming more common, particularly in summer, and this appears to be linked to changes in the Arctic due to global warming.
Other scientists disagree.
It's one of the biggest unsolved problems in mainstream climate science, Swain said.
“Hilary is a rare storm, but we will almost certainly see equally bizarre and destructive, but distinct events as the global warming continues and this El Niño continues to strengthen,” Francis said.
Last October, MIT hurricane researcher Kerry Emanuel gave a guest lecture at UCLA on the rare possibility of a tropical storm or hurricane hitting Los Angeles.
His computer models, which accounted for climate change and other factors, concluded that the probability of the type of storm that would dump 15.7 inches of rain across downtown Los Angeles by at least 2010 was one in 108 years lay.
But now this type of storm has a one in 30 year chance, he estimated.
“Hilary was significantly more likely today than it was 20 or 30 years ago,” said Emanuel, who also calculated the likelihood of a storm surge over New York City months before Superstorm Sandy in 2012.
But it's not just about climate change, Emanuel said, “We know for sure that El Nino tends to increase hurricane activity in this region.”
And when storms like Hilary hit, the warmer air also retains more moisture, and that means more rain, Corbosiero, Swain and Emanuel said. Studies show that tropical cyclones are getting rainier around the world.
Expect the eastern Pacific hurricane basin to be active for the next two to three weeks — peak season is towards the end of the month, Corbosiero said.
Different weather and climate patterns could give the region a lull in early to mid-September, only to pick up again late next month, she said.