Are megastorms in Tahoe’s future?

1862 Sacramento flood waters. | Bancroft Library

Much of California, including the Tahoe Sierra, has a higher risk for catastrophic storms and flooding than any other region of the country, including the southeastern hurricane states. Experienced Tahoe locals know that extreme rain and snowfall events, floods and blizzards, are part of our mountain environment.

For the past decade, U.S. Geological Survey, Desert Research Institute (Reno) and other institutions have been assessing natural hazards in the United States. The California-focused portion of this national project is called ARkStorm, an ongoing research effort that aims to gain a better understanding of the risks we face from intensified weather systems in a warming climate.

ARkStorm utilizes new insight into the potent atmospheric rivers (AR) that regularly invade the West Coast with intense rain and snow. The colloquial term Pineapple Express has been used for decades to describe a stream of sub-tropical Pacific moisture, but this critical weather mechanism was first identified in 1998 when scientists flew instrumented aircraft into them.

Atmospheric rivers increasing
ARs are conveyor belts of dense water vapor surging at elevations between 5,000 and 6,000 feet. A strong one can carry as much liquid as the discharge of 10 to 15 times the Mississippi River, the largest waterway in the United States. Virtually every major winter flood in the Truckee River watershed, which is among the most volatile in the country, has been triggered by an AR event.

ARkStorm analysis paints a bleak picture of the impacts: possibly 2 million people evacuated in California, damage in the hundreds of billions and widespread destruction of agricultural lands and urban infrastructure.

California can expect about nine ARs annually, which bring beneficial rain and snow that provide up to 50 percent of a season’s precipitation over the course of just 10 days. Climatologists predict that in the future the average number of ARs will rise to 12 or so, which may mean more rain for the Golden State.

Unfortunately, supercharged AR weather systems have periodically inflicted destructive floods. To gauge present-day impacts from such an event, researchers used computer models to recreate a 500-year storm to assess the consequences on our region’s population, economy and infrastructure. A 500-year tempest at this level does not happen every 500 years but has a two-tenths of a percent chance of occurring in any given year. It can happen in back-to-back winters or not occur for many centuries. Climate change may accelerate these cyclical odds with warmer ocean and air temperatures that increase the atmosphere’s capacity to hold water to release as precipitation, thus concern regarding future megafloods.

There is precedent for such extreme hydrological events. Analysis of soil sediments indicate that during the past 2,000 years incredible superstorms have inundated California roughly every two centuries, the most recent being the winter of 1861-62, about 161 years ago. Statistically, the clock is ticking for the next megastorm.

43 days of rain, snow
The ARkStorm study highlights 1862 when it rained and snowed on and off for 43 consecutive days in California — historic flooding ranged from Canada to Mexico and into Nevada Territory and Utah.

The deluge produced a calamitous inundation of California’s Central Valley, creating an inland sea 30 feet deep that took months to drain into San Francisco Bay. Muddy flood water swamped the state capital Sacramento, forcing Governor-elect Leland Stanford to take a rowboat to his inauguration. During the early construction of Sacramento, local Native Americans warned the new settlers that in some winters the valley was engulfed by boundless overflow, but their sage advice was ignored.

The Alta California newspaper described the 1862 flood zone: “It extended from Tehama, 80 miles north of Marysville, to a point in the San Joaquin at least 50 miles south of Stockton, forming a lake 30 miles wide by some 250 miles long.”

On the San Joaquin River, ferries were swept away and thousands of sheep, horses and cattle drowned. Turbulence on the Feather and Yuba rivers washed away bridges, stores, saloons, restaurants and, tragically, a camp of Chinese miners. Geology professor William Brewer reported that the vast Central Valley was “a district of 5,000 or 6,000 square miles” where nearly every house and farm was obliterated. Household furniture such as chairs, sofas, tables and beds floated among the carcasses of drowned animals. The devastation forced Sacramento residents to raise their business district by 10 to 15 feet and initiated development of a costly levee system on the Sacramento River.

When it comes to precipitation, 1862 was a winter of superlatives. The epic rainfall was measured in feet by numerous observers equipped with gages. The unprecedented large-scale impacts suffered that winter rate the aggregate storms of 1862 with a cycle of return at 1,000 years. (Hence the acronym: Atmospheric River k [1,000-year] Storm.)

In 1862 San Francisco was deluged with 28.25 inches in 30 days — a rainfall event so severe that climatologists determined it should occur only once every 37,000 years. Down south in Los Angeles, 66 inches of rain hammered the city’s 4,500 residents that season — more than 400 percent above normal. Due to orographic enhancement of saturated air rising on the Sierra west slope, roughly 9 feet of rain and the water equivalent of snow pummeled foothill towns such as Sonora and Blue Canyon. An Army surgeon stationed at Fort Gaston in Hoopa Valley tallied 129 inches of rain that year, nearly 11 feet of water.

A bleak look ahead
The initial premise for the ARkStorm project was based on the extreme weather of 1862, a millennium event, but scientists dialed back the rainfall metrics about 50 percent to assuage critics who argued that the modeled scenario was unrealistic to expect today. To create a more reasonable ARkStorm framework, scientists retained the geographical extent of the 1862 megastorm in California, but combined January 1969 and February 1986, two particularly robust months weatherwise, to more closely represent modern-day repercussions.

In 1969 snowfall reached 300 percent of normal at Lake Tahoe and 800 percent in the high country south of there, with 40-foot drifts on Donner Pass. Multiple snowfall records were set in the Nevada portion of the Eastern Sierra. In 1986, the Tahoe Sierra watershed was inundated with torrential high-elevation rain. Reno was overwhelmed by the rampaging Truckee River and Tahoe-Truckee communities lost both electricity and natural gas for a week or more.

The 2011 ARkStorm analysis of weather, runoff, infrastructure engineering and economic modeling paints a bleak picture of the impacts from a weather event of this significance today: possibly 2 million people evacuated in California, damage in the hundreds of billions and widespread destruction of agricultural lands and urban infrastructure due to levee failures.

The simulation study did not predict death and injury, but one can imagine the scale of human suffering. Compared with similar research and projections based on a hypothetical magnitude 7.8 earthquake in the Golden State, a major temblor poses less than a third of the financial impact caused by an atmospheric river-fueled megastorm.

Stay tuned for Part II in the next issue and at