Climate Scientists Declare La Nina Season Ahead, Which May Or May Not Bring Extra Rain to Northern California – SFist

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It’s officially going to be a La Niña fall and winter, which could be disastrous for Southern California in terms of extending the dry season in an already extra fire-prone year. But it’s anyone’s guess whether it will noticeably affect our weather in the Bay Area or not.

As anyone who’s lived in California for any length of time should know, the El Niño/La Niña cycles can make for  more extreme (often wet, for us) winters. But the general guarantee is that they each create cyclical anomalies in the weather that impact everything from Atlantic hurricanes to rain in South Africa, and they recur every three to five years.

At his point, the National Oceanic and Atmospheric Administration’s Climate Prediction Center says that there’s a 75-percent chance this La Niña event will last through the winter.

In an El Niño, surface temperatures in the central Pacific Ocean are warmer than usual, often creating milder winters for Canada and parts of the U.S., and sometimes creating extra “rainpocalypse” or “atmospheric river” events over California that can go on for a day or two. (The last, most notable ones in the Bay Area were in December 2014 and December 2016, with an El Niño cycle that went on for two consecutive years.) There were El Niño events through the past two years (2018-2020), though this past winter the Bay Area saw below-average rainfall, and it was really Tahoe that got all the benefits of big snows in early 2018 and 2019 while Guerneville and the Russian River ended up underwater.

But La Niña cycles, in which cooler than normal water gathers in the Pacific, are harder to pin down in terms of NorCal weather. There hasn’t been what climatologists would call a “strong” La Niña event since the winter of 2010-2011. And as the NOAA shows us via the graphic below, we’re kind of right on the border of what is likely to be the wetter-weather region of the Pacific Northwest during a La Niña.

The New York Times presents the gloomy forecast today that a La Niña winter makes it likelier that the Southwest will see an extension on its ongoing drought, along with extending dry weather in SoCal.

The most immediate impact that has already been seen from this La Niña is its assistance in creating storms over the Atlantic. As Mike Halpert, deputy director of the Climate Prediction Center, tells USA Today, “La Niña can contribute to an increase in Atlantic hurricane activity by weakening the wind shear over the Caribbean Sea and tropical Atlantic Basin, which enables storms to develop and intensify. [And] The potential for La Niña development was factored into our updated Atlantic hurricane season outlook issued in August.”

Halpert and his team upgraded the hurricane forecast to 25 potential storms in August, and there have already been 17.

As the warmer waters of the Pacific essentially shift to the tropical areas to the west during a La Niña event, changes occur in the high-altitude air stream, separating warmer and colder air, which impacts winter weather in North America. La Niña events don’t tend to last as long as El Niños, as atmospheric scientist Dr. Emily Becker tells the Times.

So the long and the short of it is: This may be a perfectly average winter, or maybe we’ll get more rain.

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