Thursday May 8, 2025
San Francisco Chronicle —
A prolonged spell of relatively warm and dry conditions across California is rapidly melting the state’s snowpack into creeks, streams and rivers.
Hot weather this week will accelerate the melt. Several rivers fed by snowmelt, mainly in central and southern Sierra Nevada, are expected to hit their spring peak flows in the coming days. The Merced River at Pohono Bridge and the Tuolumne River at Hetch Hetchy, both in Yosemite National Park, are forecast to reach maximum flow on Sunday.
It’s not just above-average temperatures driving the melt, but that in tandem with direct, strong sunlight warms up the snowpack, said David Rizzardo, hydrology section manager at the California Department of Water Resources.
“Solar radiation is a key driver in snowmelt,” Rizzardo said. Using airborne data and hydrology models, Rizzardo explained, “we can see that the snow itself has warmed up to the point that it is ready to melt. Snow insulates itself very well, so it takes weeks for the snow to warm up enough that it will yield runoff.”
In the late spring, temperatures more commonly rise above freezing during the day and nighttime, accelerating the melt through mid-May.