An ARK STORM would flood the entire valley from Redding to Bakersfield.
SACRAMENTO, CA - The devastation sounds like it's from a natural disaster movie.
It's a storm so powerful that it dumps 10 feet of rain in a month over northern California, overwhelming the levees. The Sacramento and San Joaquin valleys turn into an inland sea, 250 miles long, up to 20 miles wide and 20 feet deep.
It's the "other" big one, the ARkStorm, and many experts said California is unprepared for it.
"The Ark Storm would be considered a catastrophe," US Geological Survey Hazards Coordinator Dale Cox said. "We are horribly unprepared for something of this magnitude. You can't really blame anybody at this point because no one really saw it coming."
Cox has helped communities and emergency managers prepare for earthquakes and tsunamis, and now ARkStorms.
Based on new studies, some experts believe that an ARkStorm will hit the U.S. west coast sometime in the next 40 years.
The chance of an ARkStorm hitting California is about the same as a major earthquake. But the damage would be three to four times great. Losses would amount to hundreds of billions of dollars and much of the damage would be uninsured, making it difficult for property owners to rebuild or make repairs.
READ MORE: Overview of the ARkStorm Scenario
ARkStorms are caused by atmospheric rivers, similar to a Pineapple Express. A long stream of moisture in the atmosphere blows across the Pacific, picking up speed before it slams into the Sierra Nevada mountain range.
READ MORE: 5 FAQs about storm that could destroy California
The result is weeks of steady rain in the valley and foothills. Put a couple of really strong atmospheric rivers back to back, and you've got an ARkStorm.
"Enough water to equal the Mississippi River, or several Mississippi Rivers," Cox said. "And it brings it in firehose-like fashion, causing high winds and massive floods. There would have to be massive evacuations."
Many parts of California would be flooded for months. An ARkStorm could bring 30 to 45 days of steady rain, rising flood waters and hurricane force winds.
Scientists digging into California's geological past have recently discovered that ARkStorms happen about once every 200 years. And California is way past due.
VIDEO: USGS creates a video about the ARkStorm
"And now that we know that these are a part of our future, maybe we can do something to better prepare," Cox said. "If forecasters say it's coming, what is our plan to do something about it? It might be a good start right now to start thinking about making those plans."
Two large atmospheric rivers combined to cause the Great California Flood of 1861-62. But not even that event was strong enough to be considered an ARkStorm.
It's not uncommon for atmospheric rivers to hit the California. In fact, scientists believe a record number of "A-R's" hit the west coast last winter. And now, with every one that approaches, Cox wonders "Is this the start of the big one?"
"These extreme events will be a part of our future," Cox said. "It may not be a matter of if an ARkStorm is coming, but a question of when."
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