LOS ANGELES • Scientists call it California's "other big one", and they say it could cause three times as much damage as a major earthquake ripping along the San Andreas fault.
Although it might sound absurd to those who still recall five years of withering drought and mandatory water restrictions, researchers and engineers warn that California may be due for rain of biblical proportions - or what experts call an ARkStorm.
This rare mega storm - which some say is rendered all the more inevitable because of climate change - would last for weeks and send more than 1.5 million people fleeing as floodwaters inundate cities and form lakes in the Central Valley and Mojave Desert, according to the US Geological Survey.
Officials estimate the structural and economic damage from an ARkStorm (for Atmospheric River 1,000) would amount to more than US$725 billion (S$983 billion) statewide.
One such storm hit in 1861-62. Over 45 days, intense storms hammered the state and dropped 90cm of rain on Los Angeles. So much water fell that it was impossible to cross the Central Valley without a boat, and the state capital was moved temporarily from Sacramento to San Francisco.
This year, in the heavily populated areas of the Los Angeles Basin, epic run-off from the San Gabriel Mountains could rapidly overwhelm a flood control dam on the San Gabriel River and unleash floodwaters from Pico Rivera to Long Beach, according to a recent analysis by the US Army Corps of Engineers.
In a series of recent public hearings, corps officials told residents that the 60-year-old Whittier Narrows Dam no longer met the agency's tolerable-risk guidelines and could fail in the event of a very large, very rare storm.
In addition, unusually heavy rains could trigger a premature opening of the dam's massive spillway on the San Gabriel River, releasing more than 20 times what the downstream channel could safely contain within its levees.
The corps is seeking up to US$600 million in federal funding to upgrade the 5km-long dam, and says the project has been classified as the agency's highest priority nationally because of the risk of "very significant loss of life and economic impacts".
Standing atop the 17m-tall dam recently, lead engineer of the project Douglas Chitwood surveyed the sprawl of working-class homes, schools and commercial centres about 22km south of Los Angeles and said: "All you see could be underwater."
A government study used computer models to estimate the effects of 900-, 7,500-and 18,000-year storm events. In each case, officials say as many as one million people could be affected.
Climate change could upend 20th-century assumptions on future flooding estimates.
Some researchers argue that, in a warming world, regions such as California will experience more whiplashing shifts between extremely dry and extremely wet periods - similar to how California's long drought was quickly followed by the wettest rainy season on record in 2016-17.
These intense cycles will seriously challenge California's ability to control flooding as well as store and transport water.