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.