A menacing storm taking shape over the Pacific is poised to pound California this weekend, causing what could be the worst flooding in parts of the state in more than a decade, forecasters said.
The atmospheric river of warm and highly concentrated water will begin to deliver its payload across the northern two-thirds of California on Saturday, overwhelming rivers, drenching urban areas and likely eviscerating much of the Sierra snowpack.
But while drought-stressed California has begged for such soaking storms in recent years, the rain may do more harm than good — especially in the mountains — when it surges Sunday.
“In terms of the way things look meteorologically, this is shaping up to be a significant event,” said Scott McGuire, a meteorologist with the National Weather Service’s Reno office, which monitors the central Sierra. “We’re going to have the highest levels on main-stem rivers in 11 years.”
The problem is that a significantly cooler storm dumped several feet of snow and rain in the Sierra on Tuesday and Wednesday, bringing the snowpack up to an encouraging 84 percent of normal, while soaking lower-lying areas.
Snow will fall at 9,000 feet and higher over the weekend, and up to a foot of rain is forecast to drench everything below that.
“This is what we talk about with bigger flood events,” McGuire said. “Not only are we dealing with a rather significant precipitating event, but the snow is also completely saturated.”
When all that fresh snow melts in the downpour, nearly all major rivers in the central Sierra will rise to their highest levels in years, with many expected to flood.
The National Oceanic and Atmospheric Administration’s early computer models show that the Merced River in Yosemite National Park will surge to near or above a record 23.7 feet. That’s well above its 10-foot flood level, and surpasses even its previous record of 23.4 feet set during the devastating January flood of 1997.
That event was one of Yosemite’s worst-ever natural disasters, and did an estimated $176 million in damage to the park.
“Meadows turned into lakes, streams became rushing rivers and the Merced River spread in all directions. Yosemite Village’s parking lot was flooded, cabins in low-lying areas were damaged, and a two-story building at Yosemite Lodge had to be evacuated,” said a Jan. 4, 1997, article in The Chronicle.
Park officials were closely monitoring the situation on Wednesday and coming up with a plan of action as the storm develops.
“We have a team of park personnel, law enforcement and emergency responders looking at the situation,” said Jamie Richards, a spokeswoman for the National Park Service at Yosemite. “We are prepared to take actions to ensure visitor safety.”
On top of the trouble in the Sierra, things will likely be a mess in the Bay Area. Streams and rivers are already swollen, and the heavy rain may cause them to spill over.
“We’re thinking this weekend storm is going to pack a pretty good punch,” said Brian Mejia, a meteorologist with the weather service’s Monterey office. “We won’t know the extent until the next few days, but the potential is there to flood.”
In the Santa Cruz and Santa Lucia mountains, where the summer’s Loma and Soberanes fires lefts swaths of earth scorched, hydrologists are concerned about the potential for debris flows. More than half a foot of rain is forecast to dump on those areas between Saturday and Monday.
Atmospheric rivers — which are especially wet air masses accompanied by high winds — are critically important to California, said Daniel Swain, a climate scientist at UCLA.
The past five years of drought have in part been characterized by an overall decrease in such storms.
“They can be beneficial, but they can also cause problems,” Swain said. “The one this weekend is raising some eyebrows.”
For one, the Sierra snowpack provides about 30 percent of California’s water for drinking, farming and other needs over the dry season when it typically begins melting around April 1.
But much of the snow that has accumulated so far will likely wash into the reservoirs this weekend — and those reservoirs are already flush.
The U.S. Bureau of Reclamation — which manages California’s water supply in conjunction with the state — in recent weeks has even been discharging water from several reservoirs like Shasta Lake that have already reached flood-release levels.
The weekend storm will do damage to more than just the snowpack. State officials are bracing for havoc on roadways and in urban areas as rain and winds cause streets to flood and trees to topple.
Several trees and branches fell during Tuesday’s heavy storm that brought just under an inch of rain to San Francisco and 3½ inches to San Rafael.
Forecasters estimate that around 3 inches of precipitation will fall in Oakland and San Francisco from Saturday to Monday.
In the next two relatively dry days, weather watchers are bracing for the potentially devastating conditions while the serpentine system continues to develop.
“At the moment it looks like this could be the most significant flood in six years, and more significant than that in other parts of the state,” Swain said.
Evan Sernoffsky is a San Francisco Chronicle staff writer. Email: email@example.com Twitter: @EvanSernoffsky