Flooding and snowmelt are more likely when intense weather systems come in quick succession. “It is clear we will have both of these issues this weekend, with a very warm atmospheric river following on the heels of a very cold atmospheric river,” says Daniel Swain, a fellow at UCLA’s Institute of the Environment and Sustainability. “We have a lot of substantial snowpack in the Sierras right now, and a lot of that at lower elevations might be washed away as rain. I would be surprised if there wasn’t a big contribution to snowmelt.”