This has been the wetest stretch for San Francisco since the 1860s and they seem about 12.37 inches of rain in 15 days time back on December 31st. At least two or three more systems were expected through the middle of next week before things finally taper off. And what is an atmospheric river? Well, exactly what it sounds like. It might only be 100, 200 miles wide, but it can stretch thousands of miles long like it can bear belts all the way from Hawaii.