Southern California's extreme weather season is making a sharp turn, from a long, dangerous heat wave to the season's first widespread rain. Tropical Storm Rachel is churning toward the region, and forecasters are warning of coastal flooding, powerful surf and possible thunderstorms over the weekend.
The shift comes as an unseasonably cold trough of low pressure drops down the California coast and meets moisture and wind from Rachel. The National Weather Service called the setup "a very complex situation" and urged residents to keep monitoring forecasts.
Thursday was the predicted end of the heat wave and cooler air is on the way as onshore flow returns with the weekend expected to be at least 5 to 10 degrees cooler.
Rain chances build through the weekend
Showers could begin late Saturday. The weather service said widespread light to moderate rain is likely from late Saturday through Tuesday, with the greatest confidence for Sunday and Monday. Chances are highest south of Point Conception.
How much rain falls depends largely on Rachel's track, which forecasters called the main source of uncertainty as tracks have shifted farther south. Most computer model runs show at least some tropical showers south of Point Conception by Sunday, and about 10% show more than an inch of rain. The more likely outcome is a tenth to a quarter of an inch, with locally higher amounts in the mountains.
Sunday temperatures should be mostly in the 70s, but forecasters said a sharp rise in humidity from Rachel's moisture will likely make it feel much warmer. There is about a 10% chance of thunderstorms Sunday through Monday, though models suggest storms will stay mostly farther east and south.
An upper-level low is expected to move into Southern California late Sunday and Monday and could interact with leftover tropical moisture to produce more showers. Rain chances on Monday extend across the entire four-county forecast area, but most locations can expect only a tenth to a quarter of an inch. Highs Monday should fall to the mid-60s to low 70s.
Forecasters said a 20% to 30% chance of rain could linger into Tuesday, depending on how quickly the low moves out. Wednesday should be mostly dry, with 3 to 6 degrees of warming, though highs will still run about 6 degrees below normal.
Coastal flooding is the biggest local threat
A coastal flood advisory is in effect until 2 p.m. Tuesday for the Malibu coast, Los Angeles County beaches, Ventura County beaches and Catalina and the Santa Barbara islands. The weather service expects minor to locally moderate coastal flooding, elevated surf and dangerous rip currents.
High tides are forecast to reach 7 to 7.5 feet over the weekend, 1 to 1.5 feet above astronomical predictions. Forecasters said the greatest risk is Sunday, especially during the morning high tide. If Rachel takes a more northerly track, stronger wind-driven waves and lower air pressure could push tides even higher.
Seawater is likely to flood vulnerable low-lying areas around high tide, including beach parking lots, bike paths and harbor walkways. Forecasters said moderate to significant beach erosion is likely and isolated damage is possible.
The weather service warned of an increased risk of drowning. Rip currents can pull swimmers and surfers out to sea, and waves can wash people off beaches and rocks and capsize small boats near shore. Officials urged people to stay out of the water or near occupied lifeguard towers, stay off rock jetties and never drive around barricades or through water of unknown depth.
Conditions at sea are also expected to deteriorate. Winds will increase to at least 20 to 30 knots by Saturday afternoon, and there is a 40% to 50% chance of gale-force winds Sunday, mainly in nearshore waters off Los Angeles and Orange counties. Large, steep and choppy seas are expected from Saturday through at least Monday.
Rachel turns toward the border
As of Thursday, Rachel was about 915 miles west of the southern tip of Baja California, with maximum sustained winds near 70 mph, according to the National Hurricane Center. It was moving west-northwest at about 7 mph.
Forecasters expect a sharp turn to the northeast Saturday, with an increase in speed, as an approaching trough steers the storm. The center of Rachel is forecast to approach northern Baja California late Saturday and could make landfall Saturday night or Sunday somewhere in northern Baja California or near the U.S.-Mexico border.
Dry air and cooling waters have been weakening the storm, and the hurricane center expects gradual weakening to continue. Rachel is still forecast to be at tropical storm strength as it nears land. A tropical storm watch could be needed for parts of extreme northern Baja California, the northern Gulf of California and Southern California as early as Thursday night.
"There is still a high degree of uncertainty on if Rachel will be a tropical or post-tropical system as it approaches northern Baja California and southern California," hurricane center forecasters wrote. Either way, they said, it is expected to bring heavy rain and tropical-storm-force winds. Moisture from Rachel could raise the threat of flash and urban flooding across the Southwest.
Farther south, Tropical Storm Simon formed Wednesday off southwestern Mexico, about 175 miles south-southeast of Lazaro Cardenas. Forecasters said it could become a hurricane by late Thursday and possibly a major hurricane by Saturday, with up to 25 inches of rain possible in parts of Colima and Michoacan.
El Niño and a Kelvin wave raise the stakes
The high tides are being pushed up in part by a Kelvin wave, a slow-moving bulge of warm ocean water that forms when El Niño weakens the equatorial trade winds. After crossing the Pacific, it travels north along the coast.
The Climate Prediction Center said Thursday that a strong to very strong El Niño is likely through March.
Malibu is already preparing. The City Council on Sept. 28 declared a citywide local emergency over threats tied to El Niño, including flooding, mudslides, debris flows and coastal erosion.
"El Niño's impacts are already being felt," Mayor Bruce Silverstein said. "We are taking action now to be prepared so we can keep our community safe."