What is an "atmospheric river”?

This week, parts of the West Coast are going to be inundated with a storm system that is projected to bring anywhere from 3 to 7 inches of rain and up to 8 feet of snow in the mountains. This may sound like an astronomical amount of precipitation, but it’s actually more common than you may think for our friends on the West Coast. Here’s why.

California’s wet season is essentially the wintertime. For instance, Los Angeles and San Francisco both receive over half of their years’ worth...