THE SCIENCE BEHIND CALIFORNIA’S EXTREMELY WET WINTER, IN MAPS ~ The Washington Post

By Diana Leonard and

April 7, 2023

Atmospheric rivers and cold winter storms have pummeled California since December, deluging the state with rain and building up extreme snow totals. The state’s snowpack is one of its largest ever, with the southern Sierra sitting at more than 300 percent of normal levels

The moisture parade didn’t stop at the coast — it also buried the interior West in snow. This week, Utah broke a 1952 statewide record for the amount of water contained in snowpack. Alta Ski Area in Utah topped 875 inches of snow for the season — its snowiest on record.

But what was behind the endless barrage of storms? And how is the changing climate playing a role?

The wet winter and spring ushered in dramatic drought improvements across much of the West, but especially in California, where it wiped out drought in areas hardest hit during the state’s driest three years on record, a stretch from October 2019 through September 2022. But during the three previous drought years, moisture like this often bypassed the state, blocked by a zone of high pressure parked off the West Coast. However, from late December to mid-January, storms — including nine atmospheric rivers — marched directly into California from the west, thanks to a broad southward shift in the storm track.

Then beginning in late February, very cold systems dove into California from the far north.

The relentless stormy weather was a surprise for California this year. Seasonal outlooks for December through February were based largely on a La Niña climate pattern. Those outlooks favored drier than normal conditions in Southern and Central California and wetter conditions in the Pacific Northwest.

El Niño and La Niña, oscillating warm and cold ocean temperatures in the eastern tropical Pacific, are considered strong predictors of western winter precipitation. However, there are no guarantees in any given year, and recent departures from expected wet and dry patterns have called into question their reliability in seasonal outlooks.

“The east Pacific high pressure that is so typical of La Niña winters shifted a bit to the west, which allowed the jet stream to dip very far to the south,” said Alexander Gershunov, a research meteorologist at the Scripps Institution of Oceanography in San Diego.

The resulting high-amplitude trough “sent a series of midlatitude cyclones often associated with atmospheric rivers into Central and Southern California,” he said. “This brought moisture and cold air — ingredients that resulted in extreme snow accumulations.”

The onslaught continued through March, and an active weather pattern is forecast to remain in place for at least the first part of April.

In just over a decade, three notably wet California winters — 2011, 2017 and now 2023 — have unfolded during La Niña.

According to Alan Rhoades, a hydroclimate research scientist at Lawrence Berkeley National Laboratory, the cold, snowy conditions in California are not necessarily inconsistent with La Niña, which was in its third consecutive year in 2022-2023.

“I think what has been lost in the discussion … is that La Niña brings a wider range of outcomes in terms of precipitation than El Niño does,” Rhoades said in an email. “So, although this water year is not what is expected on average, it was not outside the realm of possibility, just the lower probability outcome.”

Outside of those opposing climate patterns, there are other factors that can influence precipitation in the western United States, he said.

One important player is the Madden-Julian oscillation (MJO), a disturbance of clouds and rain in the tropics that circles the globe in roughly 30 to 60 days. In a recent study, Jiabao Wang, a research analyst with the Center for Western Weather and Water Extremes in San Diego, found that a strong MJO in the western Pacific tends to produce storminess and early winter precipitation extremes in the western United States — a setup that was in place during the December-January storm onslaught.

While the drivers behind the 2022-2023 wet season will be the subject of much future research, the elephant in the room is climate change, and how that might be influencing traditional winter weather patterns, said Gershunov, with Scripps.

“El Niño and La Niña are just elements in the climate state which may or may not result in predictable precipitation anomalies,” he said. “There are other things out there in the warming globe that could throw a wrench into climate predictability.”

For example, researchers are eyeing unusually warm water in the north Pacific, which may have had a hand in this year’s storminess by disrupting established climate patterns or pumping more moisture into the atmosphere.

“The north Pacific has been very warm lately and that is bound to have an impact,” Gershunov said.

State climatologist Michael Anderson of the California Department of Water Resources sees this year as part of a broader trend toward more wet and dry extremes during this century.

“This year’s extreme weather is certainly an outlier due to both the number of atmospheric rivers that have made landfall in California and the amount of precipitation they have produced,” he wrote in an email. “It certainly fits into what would be expected as the climate warms and has more energy for storms to develop, creating new extremes.”

Still, the phenomenal snow year in 2023 is happening amid an ongoing decades-long decline in mountain snowpack in both the western United States and around the world, which could culminate in persistent snowless conditions later this century.

“It’s an anomalous year and it’s been really positive in a lot of ways, but it doesn’t disprove what the trends have been and where we’re headed in a warmer world,” Rhoades said.