As California enters the sixth year of its historic drought, something unusual is happening: It’s raining. And raining. Rainfall is expected across much of the Bay Area again Sunday, with another storm coming Halloween night. Marin, Sonoma and other North Bay counties should get the biggest soaking. Meteorologists stress that it’s only the very beginning of California’s rainy season, so there are no guarantees that a wet October will bring a wet November, December, January or February. So far, though, October has been surprisingly wet across the northern part of the state, raising the hopes of drought-weary Californians.