The rain is back, and it seems to be coming all at once. A band of dense, airborne moisture from the tropics — known as an atmospheric river — swirled into Northern California on Saturday and was expected to linger through Monday. In just 48 hours, the system dropped up to four inches of rain across much of the region, with as much as nine inches in the foothills and mountains, where warmer air meant snow fell only at the highest elevations.