The United States Geological Survey (USGS) issued a statement over the weekend saying a phenomenon called atmospheric river (AR) caused disastrous high winds and rains in northern California in October, but the information inadvertently touched a sensitive chord among anxious residents, triggering a flare-up of fears about massive natural calamities waiting in the wings.

USGS described atmospheric river as a meteorological phenomenon that draws water vapor from the Pacific Ocean near the equator and transports it to the U.S. West Coast with firehose-like ferocity.

For some time, scientists have been studying the link between atmospheric rivers and natural disasters in the U.S. West Coast. The connection between atmospheric rivers and phenomena like the El Nino has also been under scientific investigation. Initial studies have shown that massive thunderstorms in the western Pacific and the Indian Ocean have links with phenomena similar to the atmospheric rivers.

USGS release says while the atmospheric river that hit central California on October 13 and 14 was large, the impact could be larger and more devastating especially later in the winter.

The atmospheric river broke some daily rainfall records and streamflow records for this time of year on several California creeks and rivers, including the Salinas River, the Russian River, and the Merced River, the release quoted USGS Research Hydrologist Dr. Michael Dettinger as saying.

What perhaps spooked the people was a bit of history thrown in by the release which suggested that storms of proverbial might technically recur. For weather experts, storms this large always bring to mind the historically massive storms that impacted both northern and southern California in 1861 and 1862, flooding the Central Valley of California, obliterating at least one community in southern California, and causing the state capital to me moved from Sacramento to San Francisco. According to scientists, storms of this magnitude will eventually happen again.

NOAA'S DEFINITION

The National Oceanic and Atmospheric Administration (NOAA) has described the phenomenon at length in its website:
Although it probably never occurs to the average person gazing up into a clear blue sky on a tranquil day, the atmosphere can be riven by enormous channels, hundreds of miles wide that are very similar to the rivers that course through the landscape.

Even less likely are they to imagine that these so-called atmospheric rivers play a critical role in the global water cycle and are a key source of the moisture that falls as rain and snow in the Western states, ultimately providing fresh water through snowmelt in the dry season and during droughts to a huge swath of this semi-arid region.

While atmospheric rivers are natural phenomena which have a role in keeping earth well hydrated, it can also cause storms and cause havoc.

Many atmospheric rivers form in the mid-latitudes when cold fronts concentrate moisture into narrow ribbons as it is transported toward the poles. Occasionally, atmospheric rivers tap moisture directly from the tropics. ... In these situations, the likelihood of devastating rainfall and flooding—such as what occurred in La Conchita—increase significantly.

There have been recent examples of atmospheric rivers causing natural calamities, NOAA says.

HAVOC

Like their terrestrial counterparts, atmospheric rivers can also “cause flooding rains in coastal and inland mountains that can have devastating effects on people and landscapes, causing untold costs in property damage and even taking lives,” NOAA says.
USGS has pointed out that enormous storms are a recurring part of California's history, one that brewed on the eve of Christmas in 1861 and lasted well into the following year being the deadliest ever.

USGS quotes William Brewer, author of Up and Down California, to give a glimpse into the range of devastation that struck the West Coast at that time. The great central valley of the state is under water Ð the Sacramento and San Joaquin valleys — a region 250 to 300 miles long and an average of at least twenty miles wide, or probably three to three and a half millions of acres!

The Santa Ana River tripled the highest-ever estimated discharge, cutting arroyos in to the southern California landscape and obliterating the ironically named, Agua Mansa (Smooth Water), the largest community between New Mexico and Los Angeles. The storms wiped out nearly a third of the taxable land in California, leaving the State bankrupt, USGS adds.

The authority says such storms can recur. With the right preconditions, just one intense atmospheric river hitting the Sierra Nevada Mountain Range east of Sacramento, could bring devastation to the Central Valley of California.

The National Oceanic and Atmospheric Administration has documented a recent occurrence of atmospheric river causing calamities in the region. It says one such river produced more than 40 inches of rainfall in the mountains of southern California in only four days in early January 2005. This caused a massive mudslide in La Conchita, Calif., that took 10 lives and produced widespread flooding, it says.

WHAT IS ARkStorm?

ARkStorm is a hypothetical winter storm scenario created by USGS's Multi-Hazards Demonstration Project for Southern California (MHDP) where experts have tried to forecast and assess the scientifically plausible physical event of a disastrous storm and the consequential secondary hazards as well as physical, social, and economic consequences.

The hypothetical winter storm scenario would impact the US West Coast and be analogous to the intense California winter storms of 1861-1862, providing a reality check on what is historically possible.

The storm scenario is named, ARkStorm to represent an atmospheric river (AR) with a value of 1000 (k) Ð on a scale of atmospheric rivers being devised by atmospheric scientists. The scenario storm then will be an AR 1000, and other US West Coast storms will be scaled in comparison.

USGS has said one of the purposes of the creation of the ARkStorm scenario was to address storms of this magnitude and help prepare emergency responders and resource managers.

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