Hurricane Hilary Poses Unusual Threat to Southern California© alfredo estrella/Agence France-Presse/Getty Images
Hurricane Hilary is charting a rare path to Southern California.
The storm sped up and has been losing strength, declining from a Category 4 hurricane on Friday to Category 2 by Saturday afternoon. While it is expected to weaken into a tropical storm this weekend, Hilary is on course to pummel the west coast of Mexico’s Baja California peninsula with heavy rains and a dangerous storm surge. On Sunday, it is forecast to bring hazardous flooding to an area unaccustomed to tropical storms.
Hurricane Hilary Poses Unusual Threat to Southern California
The heavy rains, coupled with high winds, could cause mudslides or landslides, and wash out roads in mountainous areas.
Residents and visitors with special needs on Catalina Island, about 22 miles southwest of Los Angeles, were advised to evacuate Saturday by the Los Angeles County Sheriff’s Department. Forecasters said Saturday the potential for “catastrophic flooding” in Los Angeles and San Diego was very high.
The hurricane’s predicted path is due to an unusual convergence of warm water, a high-pressure zone to the east and a jet stream to the west, according to Dan DePodwin, AccuWeather’s director of forecasting operations.
“You have to have the exact right atmospheric setup,” said DePodwin. “And that’s what we have.”
Hurricane Hilary Poses Unusual Threat to Southern California
Scientists use the term hurricane for storms originating in the North Atlantic and Northeast Pacific. Hilary formed in the East Pacific, off the coast of Mexico.
Many of the hurricanes that form in the Pacific every year go out into the ocean without affecting the U.S., as steering winds direct the storms away from land.
In this storm system, a ridge of high pressure that is causing heat waves in the middle of the U.S. is pulling Hurricane Hilary north, DePodwin said. A jet stream off the coast of California is keeping it from moving back into open waters. These sorts of jet streams are usually more prevalent in the winter.
Related video: Hilary grows to Cat.4 hurricane; SoCal braces for rain (KTLA-TV Los Angeles)
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“It’s squeezed between those two different atmospheric features and it has nowhere else to go but between these two,” said DePodwin. “And in this case, the direction is right into the Southwestern United States.”
Warmer water is also a factor. Hotter ocean temperatures act like high-octane gasoline, enabling a hurricane to be more powerful, according to Jamie Rhome, the National Hurricane Center’s deputy director.
Cool Pacific waters typically weaken storms before they can reach Southern California. But the water off the west coast of Mexico is around 85 degrees Fahrenheit, several degrees above the historical average for this time of the year, Rhome said.
“When you have a lot of strength, a lot of intensity, a storm can coast further,” said Rhome. Hilary won’t start losing intensity until it reaches waters below 80 degrees Fahrenheit, he added.
The weather phenomenon El Niño is contributing, too. It tends to warm the Pacific and help hurricanes such as Hilary acquire substantial strength, said Rhome.
Though this confluence of events is rare, it isn’t the first time the Southwest has faced the impacts of a tropical storm. Historical records dating back to the 1800s show an unnamed hurricane brought floods to San Diego in 1858. Four tropical storms hit the same region between 1938 and 1939, according to DePodwin.
In 1997, Hurricane Nora cut a similar path to what is predicted for Hilary, though the center of the storm was slightly farther east, in Arizona.
Forecasters say it is difficult to pinpoint exactly where Hurricane Hilary will make landfall and what path it will take toward the U.S. Currently, the storm is expected to hit San Diego County as a tropical storm Sunday afternoon, about four hours earlier than previously projected. It will first make landfall in Mexico.
The heaviest rainfall in Southern California is expected starting Sunday. San Diego County and Palm Springs in California and northern Mexico along the Tijuana River are likely to experience flooding from tremendous amounts of rain. As the storm moves up the coast, it is forecast to deliver a year’s worth of rain to California’s Death Valley and parts of southern Nevada.
Scientists are hoping Hurricane Hilary will be a learning opportunity. A military aircraft working with the National Oceanic and Atmospheric Administration will fly to the eye of the hurricane to learn more about where its center is and how strong it has become, said Alex Tardy, a warning coordination meteorologist with the National Weather Service.
The weather service, which usually launches a weather balloon twice a day across 90 locations in the U.S., will launch a special balloon every six hours to gather data on Hurricane Hilary’s location and strength to help make forecasts.
“We want to sample the atmosphere as best we can,” said Tardy.
Write to Suryatapa Bhattacharya at Suryatapa.Bhattacharya@wsj.com