Percent of normal precipitation over the past two weeks.
The Bay Are is getting ready for the longest stretch of rain we've seen all year, which starts today and will continue through the weekend. Today and tomorrow are expected to drop at least a half an inch on San Francisco, with more to come by Saturday and Sunday—with a chance of even more storms on Monday.
That's all good news in helping ease—even if slightly—California's continued parched drought. Today alone, the Lake Tahoe region is expected to receive anywhere from two to twelve inches of snow. The state—and the Western part of the country—is still way below our average yearly precipitation. San Francisco, for example, is at just 42 percent of that total, and would need 12 more inches of rain by July to get back to the level, according to the Chronicle.
As always, Stanford graduate student Daniel Swain, writing at Weather West, has the most comprehensive meteorological analysis (though he really doesn't go in for nearly as many puns as the nightly weatherman does). According to Swain, although the Ridiculously Resilient Ridge of high pressure air in the Pacific that had been keeping rain from reaching us is no longer, a smaller, less-stable Ridge has formed over the northeaster Pacific. He expects that ridge to break down shortly, however, leading to the rain systems we're expecting this week.
Swain is no optimist though, about the possibility of respite from the drought: "Barring a truly exceptional barrage of extreme precipitation events between now and May [...] it appears that California’s ongoing extreme drought is unlikely to be substantially alleviated before next winter."
The only light at the end of the tunnel is that according to Swain, "all signs point to a continued evolution toward El Nino conditions" within the next few months. Presumably if a system of warm water in the Pacific were to develop, it would lead to increased rain in the Bay Area.