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 science behind California’s extremely wet winter, in maps
But what was behind the endless barrage of storms? And how is the changing climate playing a role?
Departure from normal
precipitation, year to date
No
departure
or drier
+1%
+10%
+20%
+50%
and
greater
Seattle
Olympia
Portland
1
Boise
2
Salt Lake
City
San
Francisco
4
Pajaro
Las Vegas
3
Los
Angeles
Phoenix
While the Pacific Northwest has been hit by multiple storms, the region has been drier than average.
1
As the atmospheric rivers move inland, enough moisture has remained that Utah is seeing record-breaking snowpack.
2
The Sierra Nevada takes most of the moisture as snowpack. Mammoth Mountain reported the snowiest season on record.
3
During the deluge on March 10th, the 3,000 person town of Pajaro was severely flooded.
4
Sources: NOAA, Natural Earth Data
Departure from normal
precipitation, year to date
No
deptarture
or drier
+1%
+10%
+20%
+50%
and
greater
Seattle
Olympia
Portland
1
Salem
Eugene
Boise
San
Francisco
Salt Lake
City
2
4
Pajaro
Las Vegas
3
Los
Angeles
Phoenix
While the Pacific Northwest has been hit by multiple storms, the region has been drier than average.
1
As the atmospheric rivers move inland, enough moisture has remained that Utah is seeing record-breaking snowpack.
2
The Sierra Nevada takes most of the moisture as snowpack. Mammoth Mountain reported the snowiest season on record.
3
During the deluge on March 10th, the 3,000 person town of Pajaro was severely flooded.
4
Sources: NOAA, Natural Earth Data
Departure from normal precipitation, year to date
+1%
+10%
+20%
+50% and
greater
No departure
or drier
Seattle
Olympia
Portland
While the Pacific Northwest has been hit by multiple storms, the region has been drier than average.
Salem
Eugene
Boise
As the atmospheric rivers move inland, enough moisture has remained that Utah is seeing record-breaking snowpack.
Salt Lake City
Sacramento
San
Francisco
Pajaro
Las Vegas
The Sierra Nevada takes most of the moisture as snowpack. Mammoth Mountain reported the snowiest season on record.
Los Angeles
During the deluge on March 10th, the 3,000 person town of Pajaro was severely flooded.
Phoenix
Sources: NOAA, Natural Earth Data
Departure from normal
precipitation, year to date
Seattle
+50% and
greater
Olympia
Portland
While the Pacific Northwest has been hit by multiple storms, the region has been drier than average.
+25%
Salem
Eugene
+10%
Boise
+1%
No departure
or drier
As the atmospheric rivers move inland, enough moisture has remained
that Utah is seeing record-breaking snowpack.
Salt Lake City
Sacramento
San Francisco
Pajaro
During the deluge on March 10th, the 3,000 person town of Pajaro was severely flooded.
Las Vegas
The Sierra Nevada takes most of the moisture as snowpack. Mammoth Mountain reported the snowiest season on record.
Los Angeles
Phoenix
Sources: NOAA, Natural Earth Data
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.
Atmospheric rivers, by
intensity since October
Weak
Mostly
beneficial
rainfall
Moderate
Strong
Extreme
Primarily
hazardous
Seattle
Nov. 3
Dec. 23
PACIFIC
OCEAN
Dec. 25
Portland
Eugene
Dec. 26
Jan. 11
Sacramento
San Francisco
Jan. 4
Dec. 29
Jan. 9
March 9
March 14
Los Angeles
San Diego
Note: Location shown is the approximate latitude that experienced each individual atmospheric river’s strongest intensity of rainfall, Oct. 1 through present.
Sources: Scripps Institution of Oceanography, University of Oklahoma, Natural Earth Data
Atmospheric rivers, by
intensity since October
Weak
Mostly beneficial
rainfall
Moderate
Strong
Extreme
Primarily
hazardous
Seattle
Nov. 3
PACIFIC
OCEAN
Dec. 23
Portland
Dec. 25
Eugene
Dec. 26
Jan. 11
Sacramento
San Francisco
Jan. 4
Dec. 29
Jan. 9
March 9
March 14
Los Angeles
San Diego
Note: Location shown is the approximate latitude that experienced each individual atmospheric river’s strongest intensity of rainfall, Oct. 1 through present.
Sources: Scripps Institution of Oceanography, University of Oklahoma, Natural Earth Data
Atmospheric rivers, by intensity since October
Weak
Mostly beneficial
rainfall
Moderate
Strong
Extreme
Primarily hazardous
Seattle
Nov. 3
Dec. 23
Portland
Dec. 25
Eugene
Dec. 26
PACIFIC
OCEAN
Jan. 11
Sacramento
San Francisco
Jan. 4
Dec. 29
Jan. 9
March 9
March 14
Los Angeles
San Diego
Note: Location shown is the approximate latitude that experienced each individual atmospheric river’s strongest intensity of rainfall, October 1st through present.
Sources: Scripps Institution of Oceanography, University of Oklahoma, Natural Earth Data
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.
Accumulated rainfall by week
5 inches
10
24
36
70 and
more
Source: NOAA
Accumulated rainfall by week
5 inches
10
24
36
70 and
more
Source: NOAA
Accumulated rainfall by week
5 inches
10
24
36
70 and
more
Source: NOAA
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.
Satellite image from April 1st this year compared to April 1st last year shows the effects of this season’s extensive rain and snowfall. #CAwx pic.twitter.com/ufsFUUzk9t
— NWS Hanford (@NWSHanford) April 5, 2023
“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.