It’s been a warm winter, and California’s snowpack shows it

Measurements taken across the Sierra Nevada show that California’s snowpack, which typically supplies nearly a third of the state’s water supply, now stands at 85% of average for this time of year.

The latest state data also show the amount of snow in the mountains varies dramatically depending on the region. The northern Sierra has lots of snow in most areas, while the central and southern Sierra have far less than average amounts — the result of weather patterns that have brought the heaviest atmospheric river storms to Northern California while leaving the southern mountains drier.

Alongside these largely random weather conditions, scientists are also seeing a trend linked to human-caused climate change: The snowpack this year is significantly smaller at many lower-elevation sites in the mountains after months of warmer-than-average temperatures.

“That’s really a signature of warmer temperatures,” said Daniel Swain, a climate scientist at UCLA. “There has been precipitation in the mountains in many cases, but that has been more in the form of rain than snow for much of the season.”

Across the central and southern Sierra mountains, average temperatures over the last three months have ranged from 2 to 5 degrees Fahrenheit warmer than the 30-year average. Swain said that has led to less snow accumulating in many areas below around 7,000 feet elevation, even as some higher-elevation locations have recorded above-average amounts of snow.

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In research published in 2023, scientists found that mountain snow lines in California have already crept higher because of rising temperatures, and they projected the mountains could lose more than half of their seasonal snow cover in the second half of the century if nothing is done to slow the pace of global warming.

The shift to less snow at lower elevations, Swain said, is “exactly what we increasingly have seen in a warming climate,” underlining a continuing need for California to change how water is managed to adapt.

The current snowpack levels, which state water managers plan to discuss during a monthly snow survey Friday, represent one of several metrics for gauging California’s water outlook. Another is the amount of water stored in the state’s reservoirs, which are well above average levels.

Extremely wet weather in 2023 and a moderately wet 2024 have left reservoirs brimming, which for the time being puts California’s stored water supplies in relatively good shape heading into the spring and summer.

The state’s largest reservoir, Shasta Lake, sits at 78% of capacity, while the second largest, Lake Oroville, is 84% full. Both reservoirs are expected to completely fill in the spring as they are replenished with snowmelt and runoff from rains.

Other large reservoirs, from New Melones Lake to San Luis Reservoir, are similarly at or above average levels for this time of year.

“We’re in good shape because we had two preceding wet years. That really makes a big difference,” said Jeffrey Mount, a senior fellow at the Public Policy Institute of California’s Water Policy Center.

The Metropolitan Water District of Southern California, which delivers supplies for 19 million people across six counties, has 3.8 million acre-feet banked in various reservoirs and underground storage facilities. That’s about three times the total amount of water the district has delivered annually over the last few years.

“At this point, there is really not going to be any immediate concern with urban water supplies,” Swain said.

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Although the snowpack is below average, it has improved after a series of atmospheric river storms. A month ago, the Sierra snowpack measured just 65% of average.

More storms are forecast to arrive in early March, bringing more rain and snow before the end of the wet season. Swain said it looks likely to be a “decent storm cycle.”

Throughout the winter and early spring, state water officials use a network of snow sensors combined with manual surveys across the Sierra Nevada to measure the snowpack. The final snow survey of the season is scheduled around April 1, when the snowpack typically reaches its peak.

This winter, the atmospheric river storms have followed something of a “corridor” over Northern California and southern Oregon, Swain said.

This has led to a persistent pattern of unusually wet, snowier weather in Northern California and unusually dry weather over Southern California and much of the Southwest. Swain said the current La Niña conditions, the cool phase of the cycle in tropical Pacific Ocean waters, are probably contributing to the pattern.

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As of Thursday, the snowpack in the northern Sierra measured 106% of average for the date, while it stood at 81% in the central Sierra and 69% in the southern Sierra.

“There is a large disparity from north to south,” Swain said, adding that this pattern of a north-south “dipole” appears likely to persist.

This week, the U.S. Drought Monitor website shows that nearly 42% of the state, covering Southern California and the San Joaquin Valley, is experiencing at least moderate drought conditions.

Severe drought has also persisted throughout the Colorado River watershed, another major water source that Southern California’s cities and farms depend on. Lake Mead, the river’s largest reservoir, is 35% full, and the snowpack across the upper Colorado River Basin has been below average this year.

In the coming months, Swain said, drought conditions are likely to expand and intensify in the lower Colorado River Basin as well as across Southern California.

But the snow and rain elsewhere, Swain said, appear likely to limit the advance of drought in Central and Northern California, which also have adequate supplies banked in reservoirs.

“It’s good news if you start to enter another dry period and reservoir levels are still above average,” he said.