As rain pelted the Sierra Nevada over the last couple days, it percolated into the dry layer of snow blanketing the landscape and froze into ice. Over that short time period, the rain has notably increased the amount of water in the snowpack.

Experts welcomed the change, reporting that one essential metric, the median “snow water equivalent” or SWE, had jumped up by 5% since Monday.

“SWE is the most important metric for all of our water resources,” Andrew Schwartz, the director of UC Berkeley’s Central Sierra Snow Laboratory, wrote in an email to SFGATE. “… It’s the metric that we deal with the most and the one that the entirety of the snow research and operations community is working to get right. So, seeing an increase in SWE like that, even if it’s from mid-winter rain, is a great thing because that means we have more water stored in the snowpack moving forward.”

This good news for California’s water resources arrives as the overall snowpack remains low. Statewide snowpack levels, which will feed reservoirs and rivers come spring, are at 61% of average peak snowpack. The Southern Sierra has the healthiest snow supply, which is at 95% of normal for this date. But the Central Sierra is at 74%, while the Northern Sierra is at 54%.

The recent change in the median SWE – which rose from 70% on Monday to 75% on Wednesday – is the result of a warmer storm arriving this week on the heels of last week’s colder storm, which dumped significant snow.

“When colder storms occur, like the one we had last week, that colder air gets trapped and leads to an increase in snowpack cold content,” Schwartz added. “If we get rain after that, the rain can percolate into the snow and then freeze due to the colder, sub-freezing, temperatures in the snowpack.”

Over the last 48 hours, the storm system generally brought the most precipitation to higher terrain. The western slopes of the Sierra Nevada and the Southern Cascades, north of Interstate 80, received highs of 6 inches, according to the National Weather Service.

“It was definitely a much warmer system compared to the one last week,” Katrina Hand, a meteorologist for the weather service’s Sacramento office, told SFGATE. “This one just had a different storm track. The moisture came from a warmer air mass, so that’s why we didn’t really see much of any snow with this one. The majority of it was liquid precipitation.”

California’s rainfall overall looks more auspicious than its snowpack. Since state water managers began a new seasonal tally last October, the total precipitation averaged statewide is about 18 inches of rain, amounting to 116% of normal.

As for the SWE, Schwartz hopes the boost will last. “It will be interesting to see whether it stays up,” he wrote. “Occasionally, we’ll get a pulse up during the storm and then, after the snowpack warms a bit, we see some [run out] of it.”

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This article originally published at The surprising element that’s boosting California’s stubbornly low snowpack.