ARkStorm is coming.

The name is derived from an acronym for the term “atmospheric river 1,000,” a unique kind of rain-heavy storm that a U.S. Geological Survey report warns could devastate California.

The Golden State, only recently relieved of fears that a brutal five-year drought would last for decades, now faces the prospect of the opposite problem: way too much rain.

A severe ARkStorm “is estimated to produce precipitation that in many places exceeds levels only experienced on average once every 500 to 1,000 years,” the 2011 government report states.

The Los Angeles Times on Monday pointed out that some weather experts say the “rare mega-storm ... is rendered all the more inevitable due to climate change.”

As with the possibility of a huge earthquake, this “other big one” could happen tomorrow -- or next year or 100 years from now. The U.S. Army Corps of Engineers recently told Los Angeles-area officials and residents that the Whittier Narrows Dam would fail if such a storm happens, “unleash[ing] floodwaters from Pico Rivera to Long Beach.”

A storm of this magnitude -- maybe as much as 10 feet of rain in a month -- is difficult to see as actually being possible. But it’s not just something dreamed up by worrywarts at the U.S. Geological Survey.

Sacramento was inundated for weeks in 1861-62. (Wikimedia Commons)

The last time such a megastorm hit California, from December 1861 through January 1862, it killed hundreds of people in the then sparsely populated state, along with tens of thousands of cattle. The storm overwhelmed Sacramento, “turning the Sacramento Valley into an inland sea” and forcing California’s government to temporarily relocate to San Francisco.

And California wasn’t the only state to feel the storm’s fury. Oregon also saw extensive flooding. So did Arizona, Nevada and Utah.

More to the point: It was not a one-off. Geological evidence indicates that similar massive California floods happened in the years 212, 440, 603, 1029, 1418 and 1605, according to the government report.

The U.S. Geological Survey describes “atmospheric rivers” as “jets of warm, moist air that originate over the mid-latitude north Pacific Ocean and transport that moisture to California where much of the moisture turns to rain and snow that falls on the state.” These “rivers” can slam into the Sierras and other mountain ranges and dump out across the West Coast.

During the 1861-62 storm, wrote marine meteorologist John Lindsey last March, “Los Angeles recorded nearly 36 inches of rain while Sonora in the Sierra Nevada foothills measured more than 100 inches!”

California was effectively cut off from the rest of the country during the “43-day storm.” Oregon, then a new state, received news of the flood in brief spasms.

“The water is rushing in very fast and the lower part of the city is inundated,” stated one dispatch from Sacramento. “The water is now up to L street and still rising.”

By then parts of Oregon also were under water.

“Our own State, generally prosperous, advancing in improvement and wealth, has been grievously afflicted by an unprecedented flood, which has destroyed the property and present hopes of many of our citizens -- sweeping away the results of many years of untiring labor,” The Oregonian wrote January 4, 1862.

Still the rain kept falling.

“In Oregon, two and a half weeks of solid rain caused the worst flooding in this state’s history,” Scientific American wrote in 2013. “Deluges covered huge portions of the lower Willamette Valley where Oregon City is located. Oregon City was the terminus of the Oregon Trail, and it was the state’s capital, where George Abernathy, an Oregon pioneer and the state’s first elected governor, lived and ran a thriving business. The flood destroyed his home, forcing him (and many others) to leave.”

"The Islands of Portland" by Jeffrey Linn (Spatialities.com)

The U.S. Geological Survey report assumes the next ARkStorm will put a bull’s-eye on California again, but it could redirect to the north and cause even more flooding in Oregon than it did in 1861-62.

Another possibility: A major ARkStorm won’t materialize -- or won’t reach into Oregon. Instead, extreme, disaster-movie-level flooding doesn’t happen in the Beaver State until the distant future when global warming significantly raises ocean and river levels.

In 2014, cartographer and urban planner Jeffrey Linn created a map based on predictions of what Portland would look like if “all the world’s ice sheets melted.” It shows a dramatically enlarged Columbia River feeding “Hawthorne Bay,” the “Sea of Tualatin” and the “Willamette Sea.” The map’s name:

“Islands of Portland.”

-- Douglas Perry

@douglasmperry

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