In 1861 an atmospheric river that brought storms for 43 days turned California’s Central Valley into an inland sea 300 miles long and 20 miles wide. Thousands of people died, 800,000 cattle drowned and the state went bankrupt. A similar disaster today would be much more devastating, because the region is much more populated and it is the single largest food producer in the U.S.
"So maybe 1861 was an oddity. Not really. Geologic core samples show that extreme floods like the one in 1861 have happened in California about every 200 years, since the year 200 A.D. So the next disaster could be coming around the bend. The West Coast has actually been slowly constructing large, specialized, meteorological observatories that can sense atmospheric rivers as they develop, so forecasters can give early warnings."