Tropical Storm Rachel raises flood and rain risk for Southern California

A tropical storm that was a Category 1 hurricane on Wednesday has weakened and is expected to bring heightened flood risk to Southern California’s south-facing beaches, including the Long Beach Peninsula, Catalina Island and the stretch from Malibu to Ventura. The National Weather Service’s Ryan Kittell said the greatest threat of coastal flooding will occur on Sunday, when tides are projected to be 1 to 1.5 feet above normal due to unusually warm ocean temperatures linked to El Niño.

Kittell noted that “it seems pretty likely that we’re going to have an additional round of significant beach erosion, especially in that Sunday time period,” and added that isolated damage could occur, though it is expected to be less severe than the impacts from Hurricane Marie earlier this year. The storm is forecast to turn eastward later Thursday or Friday, interacting with two low-pressure systems and residual moisture, but the chance it will retain tropical-storm strength by the time it reaches the coast is low.

Rainfall chances are highest from Sunday night through Monday, with a 70 %–90 % probability of measurable rain in Los Angeles and Ventura counties. Expected amounts are modest, roughly a quarter to three-quarters of an inch, with higher totals in the mountains. The National Weather Service office in San Diego expects increasing rain odds for its county and the Inland Empire, while the Monterey office warned of up to 1.2 feet of inundation in low-lying areas of the San Francisco Bay region.

Sunday is also projected to be the windiest day, with gusts of 20-35 mph and strong south-to-west winds, creating hazardous surf conditions and rip currents. Kittell cautioned that “there is a low but present risk for some locally strong winds as well as a thunderstorm or two, coinciding with that coastal flooding risk is also some risk for some high surf and strong rip currents at the beaches.”

Scientists highlighted the role of a “coastally trapped” Kelvin wave, a warm-water feature associated with El Niño, which can raise sea levels by 6-12 inches and extend elevated tides for months. Oceanographer Michael Jacox explained that the wave follows the coastline, keeping sea levels higher than normal. Researcher Dillon Amaya noted that similar waves have accompanied past strong El Niño events, and that long-term sea-level rise from climate change adds to the flooding hazard.

The Climate Prediction Center now assigns an 83 % chance that the current El Niño will become historic, a rise from the previous 75 % estimate. While strong El Niño episodes have historically produced above-average rainfall in Southern California, the upcoming pattern may instead manifest as coastal erosion and modest precipitation.

Separately, an extreme-heat warning remains in effect through Thursday, with temperatures expected to fall from the mid-90s-low-100s to the 70s by early next week. Officials advise limiting outdoor activity to early mornings and staying in air-conditioned spaces during the hottest periods.