All the rain that has led to swollen rivers and flooding in parts of San Diego and large portions of Southern California has coincided with multiple snowstorms that blew across the Sierra Nevada in the northern half of the state.

That may translate to a second consecutive year of robust output from the state's hydroelectric power plants, which would help bolster the electric grid this summer. But officials at the California Independent System Operator, which manages the power system for about 80 percent of the state, aren't celebrating yet.

"It's always encouraging to have a wet winter and a good snowpack," California ISO spokesperson Anne Gonzales said, "but it's too early to tell the full impact of the recent rains and snowfall on electricity supplies through the summer and into fall."

After a very slow start, rain and snowfall totals are growing in the wake of a series of atmospheric rivers — columns of condensed water vapor that produce significant amounts of precipitation.

The UC Berkeley Central Sierra Snow Lab, located at nearly 7,000 feet elevation at the Donner Pass in the Sierra, recorded more than 5 feet of snow from a series of storms in the past week.

What's called "snow water equivalent" is a critical metric that refers to the overall amount of water the snowpack contains and then releases when it evaporates. Five weeks ago, the statewide snow water equivalent stood at just 28%; as of Thursday morning it had grown to 75%.

"We're a little behind, but overall the winter's been pretty normal," said Andrew Schwartz, the Snow Lab's lead scientist. "At the lab, we've had just over 5 feet of snow in the last eight days, so it's definitely helpful."

Healthy amounts of snow and rain fill reservoirs that feed the state's large hydroelectric plants. The power generated by those facilities adds much-needed megawatts of electricity to the state's grid.

In wet years, hydroelectricity can account for roughly 20% of California's energy mix. But in dry years, it can drop to around 6%. That means grid operators have to rely more heavily on other energy sources such as natural gas or out-of-state imports, especially during the hottest months of summer when electricity demand soars.

After multiple years of drought, last year was one big, wet blockbuster. The Snow Lab measured 754 inches of snowfall in the winter of 2022-23, making it the snowiest winter since 1951-52.

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