Tropical Storm Hilary: Heavy Rain Pelts Southern California’s Desert Regions

Gaya Gupta

Catalina Island, Calif., in September 1939. A violent storm made landfall in Long Beach, Calif., that month and tore through Los Angeles County and the surrounding area.Credit…Robert C. Lyon.

Southern California’s natural protection against tropical storms and hurricanes — a combination of a cooler ocean, an atmosphere unfriendly to tropical depressions and easterly prevailing winds — has been upended this year by some unusual weather patterns, experts say.

Generally, there are three barriers to hurricanes forming along the West Coast, said Daniel Swain, a climate scientist at the Institute of the Environment and Sustainability at U.C.L.A.

First, the ocean along California’s coast is much cooler than on the East Coast — waters in Southern California are in some cases 10 to 20 degrees colder than on the equivalent latitude in the Atlantic Ocean.

“Warm water is essentially hurricane fuel, so if you don’t have it, you essentially have no hurricane fuel,” Mr. Swain said.

Another reason is that prevailing winds, known as the tropical easterlies, tend to blow from east to west, steering storms along the west of Mexico toward the open ocean and away from California’s coast. Along the East Coast, the same tropical easterlies bring hurricanes toward land.

The final reason is that the atmosphere of California is simply unfavorable for the development or the maintenance of tropical systems in general, Mr. Swain said.

But this year, two of these three barriers are either gone or weakened.

The most important one, Mr. Swain said, is the easterly winds. Instead, winds have shifted to blow almost directly from south to north right now, because of an unusually strong ridge of high pressure centered over middle of the United States, from Colorado to Ohio.

This high pressure has combined with unusually low pressure off the coast of California to produce a wind pattern that is “quite anomalous,” Mr. Swain said, drawing the storm almost directly northward over Southern California and Southern Nevada.

Ocean water temperatures in the region where the hurricane formed are also much warmer than usual, as much as 3 to 6 degrees.

That rise in water temperature is likely a product of both the temporary ocean warming effects of El Niño — intermittent conditions associated with warmer years on average — and the long-term warming associated with climate change, Mr. Swain said.

Still, Hurricane Hilary is expected to weaken to a tropical storm before it reaches California because the ocean remains too cool and the atmosphere too unstable, Mr. Swain said. The main danger is flooding caused by heavy rain.

While climate change and rising ocean temperatures raise the risk of tropical storms in Southern California region, it is unlikely hurricanes will become common there, simply the other two barriers aren’t going away, Mr. Swain said.

“The tropical easterlies will still be tropical easterlies,” he said. “The atmosphere in summer and fall in California is still generally going to be stable and unfavorable for hurricanes.”

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