Tropical Storm Rachel's swell floods Southern California's coast ahead of a rare landfall

Most of the damage so far is far from where Rachel will come ashore. The weather service expects the storm's direct effects to fall mostly on San Diego County, with Orange and Los Angeles counties seeing widespread rain and isolated coastal flooding ^[13]. "Even though the center of Tropical Storm Rachel will be well south of Los Angeles County, the significant swell in the ocean coming initially from the south, and then also phasing with a swell from the west, will bring coastal flooding concerns from any time from now through Tuesday, peaking on Sunday," said Ariel Cohen, the service's meteorologist-in-charge ^[6]. Forecasters single out south-facing beaches as most at risk ^[21].

That swell is arriving on an already high sea. Friday's damage came as the morning high tide made the surf worse ^[1]. On top of the tide, a Kelvin wave, a slow-moving surge of water along the West Coast up to 18 miles wide, can lift sea level by as much as a foot, AFP reported ^[14].

On Long Beach's Alamitos Peninsula, strong waves began Wednesday night ^[22]. The homes there were left vulnerable by last month's Hurricane Marie, and city crews are still repairing a 30-foot section of damaged seawall ^[4]. If Cohen's window holds, that shoreline will have taken swell for about six days by Tuesday ^[23]. The city plans to start moving up to 100,000 cubic yards of sand back onto the beach on Saturday, and a larger dredging project is scheduled to begin the week of October 19 ^[9].

Friday also tested the swell forecast, and the forecast came in low. In Newport Beach, officials said a swell double the size initially predicted blasted through 20-foot berms at the Wedge and washed out roads from about 21st Street to 6th Street ^[10]. Carol Ciliberti, a weather service meteorologist in Southern California, told AFP the flood risk should peak around Sunday morning ^[8]. "This is not even the worst that is expected," said Georgia Rios, a Newport Beach spokesperson. "It's forecast to get even worse on Sunday with rain." ^[5]

The forecast strengthening has a physical basis. Storms that reach the cooler water of the eastern tropical Pacific usually weaken significantly ^[16]. Rachel did lose hurricane status, and the latest advisory put its maximum sustained winds near 45 mph, with higher gusts ^[7]. But a marine heat wave and El Nino conditions have pushed ocean temperatures 10 to 13 degrees above normal, the Los Angeles Times reported ^[16]. El Nino also weakens vertical wind shear in the Pacific, letting storms there grow stronger ^[18]. During a briefing Friday, Swain said the event was unusual, made possible by the coincidence of an early-arriving low-pressure trough off the California coast and record warm ocean temperatures off the coast right now ^[17].

Marc Alessi, a climate scientist with the Union of Concerned Scientists, tied most of the contributing factors to warming. "All of these factors are kind of combining at the same time, and the majority of them, of course, are influenced by fossil fuel-driven climate change," he said ^[20]. Alessi was speaking about the factors in general. The reporting does not cite an attribution study for Rachel itself, and Swain's own account names coincidence, the early-arriving trough, alongside the record warm water ^[17].