The creeks, streams and rivers leading snowmelt out of the Sierra Nevada will likely flood this weekend as a powerful storm moves over Northern California and dumps inches of rain from the coast to the Nevada border.
The "atmospheric river," as the plumes of water vapor are called, is loaded with warm tropical moisture that will accelerate snowmelt at some of the highest elevations along the northern and central Sierra Nevada, likely lifting waters in the Merced and Truckee rivers, among others, beyond their banks.
The storm has sparked a wave of watches, warnings and closures across much of the region.
Yosemite Valley was closed Friday to the public and all overnight reservations were canceled through Saturday night as National Park Service officials expected "significant impacts" from the storm.
The Merced River running through the park flooded last spring amid warm weather.
The storm is expected to bring 3.5 to 4 inches of rain to Yosemite Valley through Saturday, with the heaviest rain coming late Friday and into the early morning hours, said Jim Bagnall, a meteorologist with the National Weather Service in Hanford, Calif.
Forecasters said the Merced River at Pohono Bridge is expected to rise close to 15 feet. Flooding occurs at about 10 feet, Bagnall said.
"That's one of the reasons that they're closing the campgrounds, getting the people out of there," Bagnall said. "With all that rain there's a possibility of flooding, mudslides and rock slides, rivers and creeks rising out of their banks."
The flooding risks also extend to the coast, the National Weather Service said. The Navarro River in Mendocino County is expected to wash out Highway 128, according to the California Nevada River Forecast Center.
Farther south, residents in the burn areas of October's wine country fires are being told to monitor the storm.
"Extreme rainfall rates over burn scars could potentially trigger flash flooding or mudslide/debris flows," Sonoma County officials cautioned residents on social media.
Up to 4 inches of rain could fall on some of the burn scars by Saturday.
Officials with the California Department of Water Resources, meanwhile, said they will be closely monitoring how much water the storm sends into Lake Oroville, where a patched-up dam spillway may be used for the first time since last year.
"Right now we're basing it on the most extreme versions of our forecast," DWR spokeswoman Erin Mellon said. "We do anticipate it will be able to handle the flows."
The spillway is the reservoir's primary tool for flushing out massive amounts of water in a quick, controlled way. It crumbled last year, triggering a massive evacuation downstream, and has been only partially rebuilt with engineers patching up the rest earlier this year. The rest of the spillway will be torn down and rebuilt after the current rainy season has passed.
Water resource managers have kept the lake well below its usual levels for the last year to avoid using the spillway amid repairs. But a string of late-season storms in March pushed the lake's level higher.
The lake spills over at 901 feet. If the lake rises to 830 feet, DWR officials said they will open the spillway gates, releasing water in a more controlled manner. It sat at about 792 feet Friday morning.
