The storms expected to finally bring Northern California a desperately needed deepdrenching this weekend after the driest year in state history aren’t just random showers.

They are the result of a developing situation that scientists call “an atmospheric river,” and recent research has shown that they have played a significant role in breaking droughts in the past.

Sometimes known as “the Pineapple Express,” these rivers of rain are long, narrow bands of highly-concentrated moisture that are formed in the Pacific Ocean and barrel eastward until they hit land, bringing downpours and flooding.

When they hit California, they pack an amazing punch. Just one can carry 15 times as much water as the Mississippi River.

“It’s essentially a fire hose of water brought up from the tropics that comes up and crashes into the West Coast,” said Michael Dettinger, an atmospheric scientist with the Scripps Institution of Oceanography in La Jolla.

And one is hitting now, moving up from Hawaii.

By Monday, forecasters expect 2 to 4 inches of rain to have fallen over San Francisco and the East Bay, 2 inches over the South Bay, 4 to 6 inches over the Santa Cruz Mountains and Marin County, and a soaking 7 to 9 inches over parts of Sonoma and Mendocino counties. The Sierra could get 3 feet of snow.

That will almost certainly rank as the biggest storm to hit the Bay Area in 14 months, since San Francisco received 1.39 inches of rain on Nov. 30, 2012. It may even be the biggest in four years, when 2.48 inches drenched San Francisco on Oct. 13, 2009.

Another storm is expected on Wednesday, but that doesn’t mean the drought is over. Far from it.

“We need a bunch of these. It’s a decent beginning, but we need more,” said meteorologist Jan Null, with Golden Gate Weather Services in Saratoga.

“The fire danger will go way down, and you’ll probably start seeing some green in the hills,” he said. “But hopefully people won’t think they can go back to their old ways of using water. We are still so far behind.”

Deep deficit

To put the lack of rain in context, San Jose, Oakland and San Francisco all have received between 2 and 3 inches of rain since July 1. That puts them at about 25 percent of normal for this time of year. Even if this weekend’s powerful storms double that total, which is a real possibility, Bay Area cities will still be just 50 percent of normal.

“It’s really great. It’s so needed. But it doesn’t mean people should stop conserving,” said Andrea Pook, a spokeswoman for the East Bay Municipal Utility District, in Oakland.

The district’s board on Tuesday is expected to vote to ask customers for a voluntary 10 percent reduction in water use, similar to other large Bay Area water agencies.

This weekend’s storms will offer a reminder of what winter looks like after California suffered in 2013 through the driest year since it became a state in 1850, with reservoirs at record-low levels, farmers fallowing fields and fire danger high.

“Most likely we’ll get ponding on the roadways, and urban and small stream flooding,” said Larry Smith, a meteorologist with the National Weather Service in Monterey. “You could see some minor mud slides. It’s definitely going to be a messy weekend.”

The rain will provide residents a chance to put out empty garbage cans or other large containers to catch rainwater to use later on plants. It also should begin to fill depleted reservoirs.

Atmospheric rivers

On Friday, a team of scientists from NOAA, Scripps and other institutions flew in a plane loaded with weather instruments from Travis Air Force Base in Fairfield out 800 miles off the California coast into the heart of the atmospheric river now heading east.

“The goal is to better forecast these events: the location, the amount of precipitation and where they make landfall,” said Ryan Spackman, with NOAA’s Earth System Research Lab in Boulder, Colo., who is working on the project.

The term “atmospheric river” was coined just 15 years ago, when new technology allowed scientists to put microwave equipment on satellites that better measured water vapor patterns in the air.

Since then, scientists have come to realize that such systems transport huge amounts of water in warm, moist air like conveyor belts to California. Even though they are only a few hundred miles wide, a few such storms often provide the bulk of the state’s annual water and snow.

Atmospheric rivers are responsible for nearly 50 percent of all the precipitation on the West Coast. They have been the cause of historic storms, Dettinger’s research shows, from the 1861 floods that forced Gov. Leland Stanford to take a row boat through the streets of Sacramento to his inauguration to the downpours of 1997-98, which flooded Yosemite Valley.

Unfortunately, this weekend’s atmospheric river doesn’t make future storms more likely. But it does show that the stubborn ridge of high pressure off the West Coast that has blocked storms for more than a year can break down, said Daniel Swain, a Ph.D. candidate at Stanford University who closely studies the subject.

“There’s no guarantee that it won’t come back,” Swain said of the ridge. “It’s possible this is just a temporary break, but I’m cautiously optimistic we’ll have more typical rainfall conditions for the rest of the winter.”

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/PaulRogersSJMN