This winter may seem colder than previous warmer winters Californians have experienced in recent history, because a moderate to strong La Niña is forming over the pacific.
But La Niña, an annual weather pattern off of the Pacific Ocean that often dictates California’s drier conditions in the winter, doesn’t buck global warming trends, according to Michelle Mead, a warning coordination meteorologist with the National Weather Service in Sacramento.
Mead says since California is this long skinny state, La Niña’s impact will differ depending on where you live, just like the storm moving across Northern California this week.
“We have a weather system that's coming in tomorrow,” she said. “While a majority of the energy is hitting the Pacific, Northwest Washington and Oregon, we are benefiting from the southern fringe of this storm, and we're going to see some widespread rainfall.”
La Niña is a weather pattern that occurs in the Pacific Ocean, in which strong winds blow warm water at the surface of the ocean from South America to Indonesia. As the water moves west, cold water moves to the surface near the coast of South America.
But La Niña’s being cold and dry is not a hard and fast rule, Mead said.
“We did a few studies a couple years ago, looking at the La Niñas and El Niños across the state, and all the La Niñas across California actually have about a 55% chance of being below average,” she said. “Now that's not to say we haven't had many new winters that have been average and above average.”
The odds are that a large swath of the state, from Tahoe all the way to Southern California, will likely have a drier-than-average winter, said Mead. But Northern California is a little different. Here she says how wet the season will be depends storm by storm.
“With La Niña well established and expected to persist through the upcoming 2020 winter season, we anticipate the typical, cooler, wetter North, and warmer, drier South, as the most likely outcome of winter weather that the U.S. will experience this year,” said Mike Halpert, deputy director of NOAA’s Climate Prediction Center.
NOAA Climate.gov, using NWS CPC data
That’s because La Niñas typically mean drier winters for Southern California and wetter storms across the Pacific-Northwest, and Northern California can reap the benefits of storms that dip lower.
“I like to say get out your dartboard because it really is storm by storm dependent,” Mead said. “If we're lucky enough [we’ll] get atmospheric rivers like this first one we're seeing tomorrow.”
A weak atmospheric river is moving across Northern California, and Mead says these kinds of storms are welcomed during winter months, because each storm could mean lots of snow in California’s snow reservoir.
“California's wettest timeframe is typically December through February, that's when we see our most beneficial rain,” she said. “Last week’s system compared to this week's system is a perfect example. Last week, we saw snow levels down to 3,500 feet. And tomorrow the snow levels should be up around 7,000 feet.”
But the cooler weather that California may experience this winter is not very cold when compared to the historic record, because our winter temperature baseline has shifted in California. And as UCLA Climate Scientist Daniel Swain points out, 2020 is a contender for the warmest year on record, even with a strong La Niña.
As a climate scientist, Swain says even though it may feel cooler this winter, it’s “not cold in an absolute sense relative to the 20th century” because of current warming trends in the state and globally.
“So to me, scientifically, and in terms of the long-term climate trends, that is particularly striking, one might even say alarming,” he said. “I think it points to this sort of inexorable warming in one direction that even with a really strong La Niña event we're not getting a cold year globally.”
And he says even a drier-than-average winter this year may seem wetter than last year, because last winter in Northern California was one of the driest on record in some places.
“But if I had to put money on it, I think I would put money on a drier than average winter for California overall, although it's less clear if that's going to be true across the northern Sierra this year,” he said.
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