SACRAMENTO — Weather officials warn Californians to be wary of fast, cold rivers this weekend as the snowbound Sierra Nevada finally begins to melt in earnest.
The peak of snowmelt is coming about three weeks later than normal due to unusually cool weather this spring, said Rob Hartman, hydrologist in charge of the California-Nevada River Forecast Center at the National Weather Service in Sacramento.
The snowpack also is unusually large because of numerous late-winter storms: 215 percent of normal statewide as of Tuesday, according to state Department of Water Resources data.
On top of all that, yet another storm is expected to hit Northern California mountains late Friday. This warm storm could drop as much as 2 inches of rain on the mountains draining into Lake Shasta, which already is full.
After that, conditions will rapidly warm into early next week, reaching normal seasonal temperatures in the high 80s and low 90s, further accelerating snowmelt.
The weather service this week issued cautionary statements urging the public to be mindful of dangerous river conditions throughout Northern California. There's even a "high potential" for flooding on some rivers in Modoc and Siskiyou counties.
"As we get back into more normal temperatures, people are going to want to cool off," said Hartman. "There's a tendency to want to hang out around streams, and many of them are going to be kind of dangerous."
In the Sacramento Valley, major rivers are not expected to see flooding conditions. But the water may be faster than normal, and it will certainly be colder.
The same can be said for the Tuolumne and Stanislaus rivers as they make their way from the mountains and foothills through the Northern San Joaquin Valley.
That's a real danger for swimmers who may overestimate their stamina.
"They get in that cold water and the first thing you lose is your extremities, and that makes it harder to swim," said Scott Sackett, past-president of the seven-county Drowning Accident Recovery Team, based in Sacramento.
The American River at the Hazel Avenue Bridge in Sacramento, for instance, is only about 54 degrees. That's plenty cold enough to drain the energy of an incautious swimmer.
Sackett urged all adults swimming or boating in area rivers in the days ahead to wear life preservers.
Bill Center, owner of the Camp Lotus rafting and kayaking center on the south fork of the American River, said anyone boating on Sierra rivers this time of year should consider a wetsuit and life preserver mandatory.
Water officials this week are keeping a close eye on Lake Shasta. The surrounding watershed holds a snowpack nearly four times larger than normal. Warm rain on top of this snow could boost runoff by 15 percent to 20 percent, Hartman said.
As of May 28, the reservoir was just 1 percent short of its capacity of 4.5 million acre-feet, with that huge snowpack still waiting to melt.
Because of the storm predicted Friday, the U.S. Bureau of Reclamation in recent days gradually has doubled water releases from Shasta — to about 15,000 cubic feet per second — to make room for more snowmelt, said Peggy Manza, a Reclamation hydrologist. Releases into the Sacramento River could double again to nearly 30,000 cfs by this weekend, she said.