A menacing storm taking shape over the Pacific is poised to pound California this weekend, causing what could be the worst flooding in parts of the state in more than a decade, forecasters said. The atmospheric river of warm and highly concentrated water will begin to deliver its payload across the northern two-thirds of California on Saturday, overwhelming rivers, drenching urban areas and likely eviscerating much of the Sierra snowpack. But while drought-stressed California has begged for such soaking storms in recent years, the rain may do more harm than good — especially in the mountains — when it surges Sunday.