The year 1862 began with a storm. It had been raging in Utah Territory for a week already, and John D. Lee, a Mormon bishop and president of the Harmony branch of the church, meticulously recorded its effects: How eight inches of snow fell one day only to wash away in the night's battering rain; how the swollen Rio Virgin made off with a neighbor's "house, Cane mill, (and) Blacksmith shop," plus 150 gallons of molasses. Lee tracked the shifting winds and noted the weather they brought with them; he measured the waters that rose in his basement and counted the days without sun (28 and a half as of Jan. 31). Even when two of his children were killed on Feb. 6, he carefully noted that the wind that blew down the wall that crushed them had gusted straight and sudden out of the north.
Perhaps Lee saw the storm as God's punishment. He was already a fugitive, a man who just five years earlier had participated in -- some say led -- the slaughter of 120 emigrants at Mountain Meadows. But if God was indeed unleashing His wrath, it wasn't upon Lee alone. As Lee moved his clan from one crumbling fort to the next, hoping to find walls to protect his 10 wives and their brood, settlers all along the western flank of North America were cowering under storms more intense and enduring than any of them had ever known. Rivers were changing course, the Pacific coast was changing shape, small towns along riverbanks were floating away in floodwaters. From British Columbia to the Mexican border, hundreds of thousands of people were struggling to bear up against blasting winds triggered by fast-moving air streaming up from the tropics, and carrying enough moisture to turn California's inland valleys into lakes.
The western edge of this continent has not felt a winter so devastating since. "It was the West's Katrina," says Cary Mock, a professor of geography at the University of South Carolina and student of the 1862 storms. But something that brought catastrophe once is likely to do so again. Only this time, millions of city dwellers will be at the mercy of engineered environments: New suburbs planted in floodplains; dream homes perched on precarious hillsides; long, heavily traveled bridges.
Marty Ralph, a research meteorologist with the National Oceanic and Atmospheric Administration in Boulder, Colo., blames 1862's bad weather on a phenomenon called an "atmospheric river." Unlike the jet stream, which moves extremely cold, dry air 30,000 feet above us, atmospheric rivers flow at altitudes of around 2,000 to 6,000 feet, pushing warm equatorial water vapor toward the poles. Shorter-lived systems of various intensities hit the West Coast a couple of dozen times a year, and they do have an upside: "Atmospheric rivers produce half the water supply of California," Ralph says. "A few big storms feed the snowpack that fills the region's reservoirs."
But once in a while -- no one knows why -- an atmospheric river comes on especially fierce and stalls when it hits the coast. In 1969, 21 inches of rain fell on Los Angeles in a single day; in 1986, an atmospheric river dumped 41 inches on the Northern Sierras in less than a week. That may be what happened for an extended period in 1862, says hydrologist and climate expert Mike Dettinger of Scripps Institution of Oceanography in La Jolla, Calif. "Or it may have been a string of storms, back to back -- or it could be both." The effect is pretty much the same, he says: "A month of unbroken rain."
In an effort to come to grips with the civic consequences should an 1862-type storm come again, the U.S. Geological Survey launched the "ARkStorm" project, the second phase of a five-year effort to assess the threat of various natural disasters, specifically in California. "The idea," says project manager Dale Cox, "is to bring people together around a big-concept storm and get them thinking about it in a way they otherwise wouldn't. Where would the worst flooding be? Where would landslides and debris flows occur? What systems do we depend on that lie in the path of those areas?
"This way we get a chance to ask in advance, ‘What do I need to do to prepare?' "
In 2008, Cox recruited Ralph and Dettinger to draw up a hypothetical "Frankenstorm." The two collaborators then convened a panel of experts in February 2009 -- meteorologists, climatologists, hydrologists -- and for three days they brainstormed about how to use recent data to craft an 1862-level storm. "We took a modern storm that was focused in Southern California, and another focused on Northern California and strung them end to end," Dettinger explains. "And because we see in the record all the time that these winter storms stall for anywhere from a few hours to a day, we took the Southern California part of the storm -- one that actually happened on Jan. 19, 1969 -- and allowed it to stall."
It's not quite the storm of 1862, Ralph admits: "We dialed it back a bit." Still, the computer models gave them three solid weeks of rain.
"Everything was in trouble by that time," Dettinger says. "We had massive flooding everywhere."
From there, ARkStorm is in part a mathematical problem, measuring how much rainfall will overtop a levee here, how much moisture will collapse a hillside there. But natural systems seldom follow a neatly engineered order, and human beings very rarely do.
That's where anecdotal accounts like Lee's come in: In the same way the USGS collects information from the public after an earthquake on its "Did You Feel It?" Web site, Cox wanted first-hand accounts to figure into ARkStorm's data set. "There's so much that's not known about these past storms," he says. "In 1861-1862, there were four rain gauges in the whole country." And so he contacted Cary Mock, who had been working with graduate student Stephanie Dodds to recreate an 1880 hurricane that happened in Sitka, Alaska, using ships' logbooks.
"I told him, ‘We're going to create our own storm, and I'd like to know what the storm actually did,' " Cox recalls. "And I come to find Cary Mock had been collecting stories about 1862 since 1988."