A combination of high tides, large ocean swells and an El Niño-related deep-water wave could cause flooding in low-lying parts of the Bay Area this week, the National Weather Service warned Monday, Oct. 12. The weather service issued a coastal flood advisory through 3 p.m. Tuesday for minor coastal flooding along the Pacific Coast and around San Francisco and San Pablo bays. Some low-lying areas could see flooding of more than 18 inches, according to the forecast. Residents should avoid driving through flooded roads because the water’s depth may be difficult to judge, said Karleisa Rogacheski, a meteorologist with the agency’s Bay Area office. Flooding had already been reported in Marin County. UC Berkeley environmental planning professor Kristina Hill said Sunday that tides were running about 12 inches above normal and that Tam Junction had significant flooding. A Mill Valley-area bike path was also reported underwater. The elevated water levels reflect several factors acting together: swells from post-tropical cyclone Rachel, large surface waves, high tides and a Kelvin wave associated with El Niño. Kelvin waves carry warm water eastward across the Pacific and can temporarily raise sea levels. Warmer-than-average ocean water also expands, contributing to higher sea levels. Dillon Amaya, a North Carolina State University professor who studies climate change and coastal waters, said the current Kelvin wave may have raised Bay Area sea levels by about 6 inches. He said it is difficult to separate the wave’s effect from other influences. Michael Beck, who directs UC Santa Cruz’s Center for Coastal Climate Resilience, said the wave appeared to be peaking near San Francisco on Monday and that a similar ocean-level anomaly in Monterey Bay reached about 15 inches above normal. The added water becomes more dangerous when storms push surge and waves onshore. “When you have something like Rachel kicking up storm surge on top of the Kelvin wave, that’s when you get concerned,” Amaya said. The current conditions could offer a preview of risks later this year. The National Oceanic and Atmospheric Administration’s Climate Prediction Center said in an October outlook there was an 83% chance El Niño would become “historic” heading into winter. Amaya said another Kelvin wave was near the Gulf of California and could reach the Bay Area in about three weeks. El Niño-related storminess is still expected later in the year, he said. Winter storms could bring much larger waves to already elevated coastal waters, Beck said, increasing the potential for flooding along shorelines and inland. The immediate advisory is limited to minor coastal flooding, but forecasters and researchers are urging residents in vulnerable coastal areas to stay alert as tides, waves and storm conditions change.