Nearly 28 million people in California spent Christmas Day under flood watch. As of the morning of December 25, an atmospheric river dumped over 3.5 inches of rain in parts of metro Los Angeles and over a foot of rain in the LA County mountains. Similar rainfall totals have been reported across large swaths of southern California, raising the risk of flooding, landslides, and debris flow. In response, California Governor Gavin Newsom declared a state of emergency in Los Angeles, Orange, Riverside, San Bernardino, San Diego, and Shasta counties.
The life-threatening conditions that arrived in southern California this week are all too familiar to millions of Americans in parts of the Pacific Northwest. From late November through mid-December, torrential rain was reported across the region, as an atmospheric river moved through parts of Idaho, Oregon, and Washington. Resulting flooding closed roads and highways, washed out rail lines, and forced tens of thousands of people to evacuate. In Washington State, heavy rainfall pushed the Naselle River to a near-record breaking depth of 20 feet, up from its normal 5 foot depth.
Whether or not the Christmas rainfall in southern California breaks historical records remains to be seen, but in the Pacific Northwest, over half a dozen weather stations reported record-high rainfall totals since late November…