La Niña is brewing. Here's what it means for California weather

San Francisco Chronicle​

a day ago

Forecasters expect La Niña conditions to develop this fall and winter, according to an update Thursday by the National Weather Service's Climate Prediction Center. The agency has issued a La Niña watch, indicating that conditions in the tropical Pacific Ocean are favorable for the climate pattern's formation in the coming months.
La Niña is defined by cooler than average sea surface temperatures in the central and eastern Pacific along the equator. The ocean waters affect atmospheric activity, tilting the odds toward drier than normal conditions in Southern California and wetter than average conditions to the far north, especially in the winter.
The Climate Prediction Center's latest seasonal outlook for November, December and January hints at such a pattern. But the forecast isn't guaranteed: La Niña may not emerge at all. And, even if it does, other factors could outweigh La Niña's influence.
The recurring climate phenomenon involving La Niña and its counterpart, El Niño, is 'the great nudger,' said Climate Prediction Center scientist Michelle L'Heureux. 'It won't always be successful.'
Sea surface temperatures in the central and eastern tropical Pacific are currently near normal, following a brief stretch of La Niña conditions last winter.
The Climate Prediction Center outlook gives a 53% chance that La Niña conditions will develop in the fall. Forecasters expect the cooler than average waters to disappear shortly thereafter, with sea surface temperatures returning to near normal, or neutral, conditions in a few months.
During the most recent fall and winter, California precipitation mimicked the expected La Niña pattern: Northern California logged above-average rain, while southern regions remained parched. A lack of rain in Los Angeles set the stage for the devastating Palisades and Eaton fires, and more recent blazes that broke out in the region.
But during the 2022-23 La Niña winter, Central and Southern California faced a flurry of storms fueled by atmospheric rivers, ribbons of water vapor in the sky. Downpours brought flooding across California and snowstorms produced one of the state's largest snowpacks on record.
La Niña and El Niño aren't the only factors behind California weather. A recent study reported atmospheric rivers are 'wild cards' that can disrupt the patterns expected during La Niña or El Niño years.
'One or two atmospheric rivers can turn it into a wet year, but a weak atmospheric river season can turn it into a dry year,' said author Rosa Luna-Niño, a postdoctoral scholar at UC San Diego's Scripps Institution of Oceanography, in a statement. 'This means we can't trust El Niño and La Niña completely to make accurate water year predictions.'
L'Heureux explained that La Niña's influence is on seasonal time scales, rather than the day-to-day time scale of individual weather events. It's 'playing on a slightly different ball field,' L'Heureux said. One effect, for example, could be shifting atmospheric rivers slightly northward over the course of an entire season.
Signs that La Niña could potentially form have appeared in the past month, with shifts in ocean temperatures and wind patterns, L'Heureux said. But how the coming months will play out remains an open question.
'We need to wait … to see if a significant La Niña is going to develop this winter,' said Shang-Ping Xie, a climate scientist at UC San Diego's Scripps Institution of Oceanography, by email.