It hasn't happened since 1862, but California is due for another one.
Excerpt from this story from the Washington Post:
A mention of California may usually conjure images of wildfires and droughts, but scientists say the Golden State is also the site of extreme, once-a-century “megafloods” — and climate change could amplify just how bad one gets.
The idea seems inconceivable — a month-long storm that dumps 30 inches of rain in San Francisco and up to 100 inches of rain and/or melted snow in the mountains. But it’s happened before — most recently in 1862 — and, if history is any indicator, we’re overdue for another, according to a new paper published Friday in Science Advances that seeks to shed light on the lurking hazard.
In such an event, some in the Sierra Nevada could end up with 40 or 50 feet of snow, and most of California’s major highways would be washed out or inaccessible.
Swain is already working with emergency management officials and the National Weather Service, explaining it’s not a question of if a megaflood will happen — it’s a matter of when.
His paper built off the work of other scientists, who examined layers of sediment along the coastline to determine how frequently megaflood events occurred. They found evidence of extreme freshwater runoff, which washed soil and stony materials out to sea. Those layers of dirt became buried beneath years of sand. The depth of each layer, as well as the size of the pebbles and material contained, offers insight into the severity of past flood events.
On the West Coast, there are commonly atmospheric rivers, or streams of moisture-rich air at the mid-levels of the atmosphere with connections to the deep tropics. For a California megaflood, you’d need a nearly stationary zone of low pressure in the northeast Pacific, which would sling a series of high-end atmospheric rivers into the California coastline.
“These would be atmospheric river families,” Swain said. “You get one of these semi-persistent [dips in the jet stream] over the northeast Pacific that wobbles around for a few weeks and allows winter storm after winter storm across the northeast Pacific into California.”
The paper warns of “extraordinary impacts,” and reports that such an episode could “[transform] the interior Sacramento and San Joaquin valleys into a temporary but vast inland sea nearly 300 miles in length and [inundate] much of the now densely populated coastal plain in present-day Los Angeles and Orange Counties.”
