California Flash Flood Risk As Tropical Storm Hilary Approaches

California is facing heavy rains of up to two inches on Sunday and the potential for isolated flooding early next week as Tropical Storm Hilary bears down on the West Coast, forecasters have said.

The storm, which is currently strengthening off the coast of Mexico, is expected to make its way north-westerly along Baja California before reaching San Diego and Los Angeles on Monday. On Wednesday evening, the National Weather Service (NWS) said "probability of a very anomalous rain event is certainly increasing."

Hilary is expected to produce hurricane-force winds of up to 74 miles per hour over the next five days, and it remains unclear if it will transition from a cyclone into a less powerful storm by the time it makes landfall in California.

No tropical storm has made landfall in the state with hurricane-force winds in recorded history, while only a handful of tropical storms have reached the West Coast in any capacity. The most recent, Hurricane Nora, brought winds of up to 130 miles an hour in 1997.

"There remains a very large spread in the ultimate track [of the storm] so this remains a very low confidence forecast in terms of the track as well as the impacts," the NWS cautioned, but added that almost all models showed "moderate to heavy rain amounts."

"Rains could trigger flash flooding if the intensity is high enough, but otherwise most areas will be able to absorb several inches of rainfall," Eric Boldt, a warning coordination meteorologist at the NWS station in Los Angeles, told Newsweek.

In January, the state was battered by a series of deadly storms that brought severe flooding and led Governor Gavin Newsom, and later President Joe Biden, to declare a state of emergency.

The high-intensity storms hammered California for several weeks at the start of the year, which left thousands of homes without power across the state and thousands of families evacuated. A coast-to-coast storm in February then brought further flooding due to snow melt from mountain regions.

"The potential exists for isolated flooding and residents, particularly near burn scars should monitor the situation closely," the NWS station in Los Angeles said on Wednesday. "The most likely period for the heaviest rain is later Sunday into Monday, though again the speed and track of the storm will impact the timing and rain intensity."

"Significant flash flooding will be the biggest threat, especially in southeast California deserts," Colin McCarthy, a storm chaser, said.

Boldt said the "three factors of concern" for Los Angeles County were "heavy rainfall, high winds, and large ocean swells" as storm Hilary makes its approach.

"Winds over 40 mph and gusts to 80 mph will likely occur for higher terrain, and possibly stronger closer to the Mexico border," he explained. "We should also see large ocean swells for this time of year, 5-10 feet over the water between LA and Catalina Island will be possible."

While it is too soon to determine the location and magnitude of the rain and winds Hilary will bring to the West Coast, the National Hurricane Center has said that "large swells" are predicted to spread northwards over the coming days.

In the five days from Wednesday evening, it gives a 20-30 percent chance of tropical storm strength winds from Hilary reaching southern California. The coastline expected to face the strongest winds is Baja California in Mexico.

The National Hurricane Center has not yet issued any coastal watches or warnings. As of 11 p.m. ET on Wednesday, it said Hilary had winds of up to 50 miles per hour, but that a "rapid intensification is forecast" over the coming day and a half.

Hurricane season in the Pacific runs from mid-May to late November, but there is usually a spike in late August. Tropical cyclones tend to form along the jetstream—an atmospheric current that runs near to the equator—and take a north-westerly path, which is why the Atlantic coast and Gulf of Mexico tends to see more make landfall.

Update 08/18/23, 3:00 a.m. ET: This article was updated to include comment from Eric Boldt, meteorologist at the NWS in Los Angeles.