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The driest winter on record will finally get a downpour of relief this weekend with a series of storms expected to deliver in four days double the amount of rainfall the Bay Area has seen all season.

In a state beset by drought, poor air quality and winter wildfires, the welcome weather forecast calls for 2 inches of rain on the valley floor and 4-to-5 inches in the mountains by Monday. Much more is needed, however, to alleviate a drought emergency gripping the Golden State.

"We need a lot more, several series of storms to catch up," said Marty Grimes, spokesman for the Santa Clara Valley Water District, where reservoirs are at extremely low levels and rainfall is at 21 percent of normal for the season.

This image provided by NOAA compares January 13, 2013 and January 13, 2014 snow cover in Northern California and Nevada as seen by the Suomi NPP satellite.
This image provided by NOAA compares January 13, 2013 and January 13, 2014 snow cover in Northern California and Nevada as seen by the Suomi NPP satellite. California, which is seeing its driest conditions in 500 years, is experiencing extreme drought in more than 62 percent of the state. ((AP Photo/NOAA))

A stubborn ridge of high pressure parked off the West Coast has been to blame for blocking normal weather patterns for months. It finally has started to move aside, allowing one weak weather system to break through last week and two stronger ones through this weekend.

Does that mean the door is wide open for storms to come?

"Will that ridge rebuild or will we be back to a more normal progressive pattern, where we have a ridge for a few days, then low pressure with rain?" asked Jan Null, meteorologist with the Golden Gate Weather Service. "The jury is still out with what's going to happen a week from now. But we're done with the ridge for a short period of time."

Rainfall that began Wednesday night across the East and South bay could drop a half inch at lower levels and an inch in the hills and mountains. A second, stronger system is forecast for Friday night through Sunday, bringing another inch to the valleys and twice that in the higher elevations.

This will get many Bay Area communities that have received only 20 percent of normal rainfall up to about 30 percent or so for the season.

The rain could get heavy at times, said Diana Henderson of the National Weather Service in Monterey, but no heavy winds or weather advisories are expected. Flash flooding is not expected. "The ground is pretty dry, so it should absorb quite a bit."

That dry ground and brush have been a menace for firefighters. In January alone, they battled nearly 500 wildfires across the state, which burned 11,000 acres. An average January sees closer to 80 wildfires that burn only about 150 acres total.

"The conditions we have been experiencing are equivalent to what we would see in August, as far as dry conditions," said Cal Fire spokesman Daniel Berland.

The rain and light winds will also improve air quality, another welcome sign after a winter that has already tied a record at 30 "Spare the Air" alerts, days when wood-burning is banned because of poor air quality.

Rainfall totals across the Bay Area so far this season, which begins July 1, are abysmal.

Santa Rosa, with only 2.75 inches of rainfall so far this season, is at 12 percent of normal. San Francisco, with 2.42 inches of rain so far, is at 17 percent of normal. Oakland, which normally would have received 12 inches by now, has received just 2.45 inches, or 20 percent. Normal rainfall for San Jose by this time would be 8.42 inches, but only 1.76 have fallen so far. This weekend's storm could bring the San Jose area total closer to 5 inches.

Not only is local rainfall important, but so is snow in the Sierra, which could get more than 2 feet of fresh powder this weekend.

East Bay Municipal Utility District's 1.3 million customers rely primarily on the Pardee Reservoir in the Mokelumne River Watershed, which relies on snowmelt.

"We have a somewhat decent amount of water in reservoirs, but what is terrible is the amount of rain and snow we are not getting," said EBMUD spokeswoman Andrea Pook. "It is the driest water year ever on our watershed. We have a little bit of time left here, but it's not looking that great."

Most of Santa Clara Valley Water District's reservoirs, which are also low, are in the Santa Cruz Mountains and surrounding hills, which are expected to get some of the heaviest rainfall this weekend.

"The rain last week didn't really make a dent," Grimes said. "The ground has been too parched, so we haven't seen any significant runoff into our reservoirs."

When it comes to the drought, the rainfall this weekend is certainly good news. But if the ridge builds back up, all bets are off.

"It gives more hope for the next couple of weeks," said meteorologist Null. "But after that, there's no way of knowing."

Contact Julia Prodis Sulek at 408-278-3409. Follow her at Twitter.com/juliasulek.