Rare “atmospheric river” storms to soak California this week

Two atmospheric river storms are expected to soak much of California and the Bay Area starting Wednesday, bringing double or triple the average May monthly rainfall in a week.

Dig out that umbrella, and even the tire chains. It’s mid-May, but a series of rare, winter-like storms will soak the Bay Area and much of California through next week and bring up to 2 feet of new snow to the Sierra Nevada.

Two atmospheric rivers are moving in from the Pacific, forecasters say. And although they are weak — expected to be a 1 on a scale of 1 to 5 — they will generate steady rainfall starting Wednesday and continuing through next Tuesday.

“We’ve got an active storm track moving in,” said Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego. “By May, standards the rain will be substantial.”

Commonly known as “Pineapple express” storms when they come from the tropics, atmospheric rivers are giant conveyor belts of water in the sky, moisture-rich storm systems that roll in from the Pacific Ocean a few times a year and account for up to 50 percent of the state’s annual precipitation.

Ralph, one of the nation’s experts on atmospheric river storms, said the first one, arriving late Tuesday night and early Wednesday, seems to be focused on Sonoma County. The second, coming in next Monday and Tuesday, seems to be headed for the San Diego area.

Rain from each storm will be widespread across the state state, but neither appears strong enough to cause major flooding or mudslides, he said.

“There will be some small stream urban high water, but for the most part they will be beneficial, from a water supply standpoint,” Ralph said.

Atmospheric rivers normally occur in the winter months. But every other year or so, they extend into May, Ralph said, adding that he isn’t aware of any unusual conditions in the Pacific that are generating the latest wave. Since October, Washington, Oregon and California have received 47 atmospheric river storms, with 12 of them being strong or extreme, he added.

Over the next week, Bay Area residents should expect three separate waves of rain. The first, on Wednesday and Thursday, will be the wettest, delivering up to 1 inch in San Francisco, Oakland and the Monterey Bay Area, about half an inch in San Jose, and up to 2 inches in the coastal mountains and the North Bay.

“The storm door is open through early next week,” said Steve Anderson, a forecaster with the National Weather Service in Monterey.

“People should expect a slow, slippery, accident-laden commute home on Wednesday evening,” he said. “Dirt, debris, oil that has accumulated on the road will mix with the water and create a slick layer at first.”

As the rains come through, winds should gust at up to 30 m.p.h., leading the National Weather Service to declare a small craft advisory off Central and Northern California, with seas up to 7 feet, through Thursday night.

“There may be some areas of power outages as tree branches come down,” Anderson said. “But those shouldn’t be too widespread.”

After a dry spell Friday, the next wave of rain is forecast to come in Saturday afternoon and continue through Sunday, again bringing up another 1 inch of rain in many Bay Area locations, and slightly more in the mountains.

A final wave will hit Monday night into Tuesday morning in the Bay Area, Anderson said, with half an inch in most Bay Area cities and 1 inch at higher elevations.

Altogether, some parts of the Bay Area could see 3 inches or more for the week. That’s highly unusual. The historic average rainfall total for San Francisco is .7 of an inch, going back to 1849.

The all-time monthly record, set in 1925, is 4.02 inches for the month of May in San Francisco, which is often used as a measure of Bay Area weather because it has the oldest set of accurate records of any city.

Temperatures during the storms will dip 5 to 10 degrees, Anderson said. That means 60s along the coast and 70s inland during the day, with cooler temperatures at night.

“Get that extra blanket on the bed,” he said. “If you are heading to the Sierras, put some chains in your car.”

The National Weather Service has issued a winter storm watch for the Sierra Nevada at elevations above 5,000 feet, with 1 to 2 feet of new snow expected at higher elevations through Thursday night.

The rain will cause headaches and cancel outdoor activities. But it also will delay the start of California’s wildfire season by soaking grasses, shrubs and trees.

And it adds to an already spectacular winter rain season. Rainfall totals since Oct. 1 are about 105 percent of normal in most Bay Area cities, and 125 percent in Los Angeles.

On Tuesday, the Sierra Nevada snowpack, the source of one-third of California’s water supply, was at 136 percent if its historic average for that date.

And most of the major state and federal reservoirs around California are full or nearly full, with Shasta Lake, the state’s largest, at 94 percent full; Oroville, the state’s second-largest reservoir, at 95 percent full; and Trinity Lake, the third largest, at 94 percent full.

“It’s not unheard of to get some rain in May,” said Anderson. “We do get these late season storms rolling through. But it’s a little unusual to have three of them back-to-back.”