If you’re wondering why we’re getting so much heavy rain in Southern California -- and the state for that matter -- it has everything to do with what’s called an Atmospheric River.
An atmospheric river is a long, narrow band of water vapor in the sky, usually 250-350 miles wide. On their own, they bring beneficial rain and increase the California snowpack. The rain forms because as the "river" makes land, the glide from sea level to the mountains lifts the moisture, condenses it and turns it into rain or snow. In a typical year these rivers account for 30-50 percent of the West Coast’s annual rainfall.
But this season has been anything but typical.
On Jan. 22, 2017, 2.67 inches of rain fell on downtown Los Angeles — that’s the most rain in one day since December 2010. And an all-time one day record rain of 3.97 inches fell on Long Beach, causing portions of 710 Freeway to be flooded with water. And, of course, we’ve seen what has been going on at the Oroville Dam.
We’ve seen historic flooding in the state because of a few unique factors. First, the amount of moisture in this winter’s atmospheric river has been equivalent to the average flow of water at the mouth of the Mississippi River.
Last week, a satellite image showed the entire atmospheric river from Japan to Northern California. If the water vapor has its origin near the Hawaiian Islands, it’s called a Pineapple Express. Second, since January the atmospheric river has stalled and focused mainly on the state of California. Last, the intensity of our storms has been enhanced by deep areas of low pressure and cold fronts moving though at the same time. It all makes for record-setting rain. But we have seen these patterns in the past.
The Great Flood of 1862 devastated Sacramento and Southern California. This atmospheric river stalled for 30 days and brought wave after wave of heavy rain.
And the devastating flood of 1938 killed dozens of people in Southern California and destroyed more than 7,000 homes. This event prompted the transformation of the LA River into a series of channels.
What makes atmospheric rivers hard to forecast from a seasonal perspective is they can arrive in El Niño, La Niña or neutral patterns. They are most common in significantly strong El Niño patterns. In 2016, atmospheric rivers brought record-setting rain to the Pacific Northwest and started the work getting Northern California out of a five-year drought.