California's Superstorm: The USGS ARkstorm Report and the Great Flood of 1862

California’s ‘Superstorm’

On January 14th the U.S. Geological Survey (USGS) released a report prepared by its Multi Hazards Demonstration Project (MHDP) concerning a potential ‘super storm’ that could impact California at some future date and cause the costliest weather-related disaster in American history. The report references the most extreme rainfall event in modern Californian history (the floods of January 1862) as a warning of what might occur again.

The USGS Report

The report (see: http://pubs.usgs.gov/of/2010/1312/) is titled the “ARkStorm Scenario” with “ARk” standing for ‘Atmospheric River 1,000’. It is not clear what the ‘1,000’ represents (aside from k=1,000) but the ‘Atmospheric River’ refers to the long fetch of sub-tropical moisture that sometimes stretches across the Pacific Ocean and takes aim at the California Coast. This event is also known as the “pineapple express” since it often originates from the Hawaiian Islands region.





Two examples of ‘atmospheric rives’ of water vapor impacting California. The first chart is from February 16, 2004 and illustrates a ‘pineapple express’ scenario. The second chart is from October 13, 2009 when moisture from a dissipating typhoon in the western Pacific was entrained into the atmosphere and carried all the way across the Pacific Ocean to California. Up to 20” of rain in 24 hours deluged the Central California coast during this event.

On occasions, as it presumably did during December 1861-January 1862, this stream of moisture becomes a persistent feature lasting for days and even weeks and funneling storm after storm towards the West Coast of the United States. The abbreviation of “ARk” is obviously a play on the biblical story of Noah and his Ark and the mythical 40 days and 40 nights of rain.

The USGS suggests that up to 120” of rain might fall in California over the course of such an event (in favored orographic locations) the run-off from which would flood the entire Sacramento and San Joaquin Valleys as well as the basins of Southern California. A very detailed analysis from the report predicts damage to exceed $300 billion with up to 225,000 people permanently displaced (in terms of complete destruction of dwellings) and a further 1.2 million forced into evacuation.

Casualties aren’t predicted since, and as we’ve seen in Australia recently, they probably would not be significant given the long and slow unfolding aspect of such an event, unlike, as say from an earthquake. But the report’s #1 key finding is titled: “Megastorms are California’s other ‘big one’”.



A map of California from the USGS ARkstorm report showing (in blue) the regions of the state that would flood. In fact, the map very closely resembles the areas that actually DID flood during December 1861 and January 1862.

HOW PLAUSIBLE IS THIS SCENARIO?

From an historical vantage point, USGS sediment research in the San Francisco Bay Area and also near Santa Barbara indicate that ‘ARkstorm-like’ floods have occurred in the past in the following years A.D.: 212, 440, 603, 1029, 1418, 1605, and then during the modern era in December 1861-January 1862. So we see a pattern of reoccurrence once every 165-400 years.

Thus, in theory a repeat may occur as soon as 15 years from now or as late as 250 years in the future. Like major earthquakes, it is not a question of ‘if’ but ‘when’.
Fortunately, unlike earthquakes, an ARkstorm-series can be fairly accurately predicted (at least a week or more in advance) so life- and property-saving precautions could be undertaken. Even longer term forecasting may also be applicable since such a scenario is most likely to occur during a strong El Nino phase (although this certainly proved not to be the case this past December or, as it seems in 1861-1862!).

However plausible the ARkstorm scenario may be, however, it is obviously a ‘worst-case scenario sort of disaster’ and so planning for and undertaking costly preventative measures may not be cost effective. We know, for instance, that Manhattan could be swamped by a 15-20-foot storm surge should a major hurricane sideswipe the city, yet few would recommend building a 15-foot seawall around the southern perimeter of the island in anticipation of such a possibility (although there are suggestions to create some kind of barriers at certain ‘choke points’).

In any case, below is what we do know about the one time an event of ‘ARkstorm’ strength actually did swamp California.

THE DELUGE OF DECEMBER 1861-JANUARY 1862

In 1860 the population of California was 380,000 according to the official U.S. census (this figure may not have included Chinese immigrants). There are 38 million residents in the state now. In 1861 there was no formal weather service in existence, just a handful of individuals who kept data in such places as San Francisco, Sacramento, San Diego, and Los Angeles (a small village at that time). San Francisco was far and away the largest city in the state accounting for about 50,000 of the 380,000 state residents.

The first rain of the 1861-1862 season occurred on November 10th according to San Francisco records. By the end of November 4.10” of rain had fallen, well above the average of 3.20”. The first half of December brought an additional 3.27” and then the rains began in earnest on December 23. Between then and January 22 an amazing 29.28” of rain was recorded in the city. An additional 1.35” fell the last of week of January (the calendar month of January total being 24.36” and the Dec.-Jan. total being 33.90”) and February produced another 7.53”. San Francisco’s normal annual rainfall is 22.28”. Sacramento recorded 23.68” during the two-month period of December-January (annual average is 19.87”). In San Diego 8.76” was recorded (annual average 10.77”) and estimates of 35” accumulated in the Los Angeles area (annual average 15.14”). In the Sierra Nevada foothills truly extraordinary amounts of precipitation were reported including 102” in the mining town of Sonora over the two-month period. Flooding that had begun during the December deluges increased in scope and intensity throughout January. The capital city of Sacramento was flooded by ten feet of water and the new governor had to travel to his inauguration in a rowboat.



A photograph of downtown Sacramento at the height of the flood in January 1862. Photo from the Bancroft Library collection, Univ. of California, Berkeley.

It is unclear how many lives were lost. The New York Times reported rumors from San Francisco’s Chinatown suggesting 1000 Chinese miners and laborers perished in the vicinity of Yuba when their makeshift town on a bar of the Yuba River was washed away.

One strange meteorological aspect of the event was the wild swings of temperature that occurred during both December and January. Although the only actual temperature record we have was from a resident of San Francisco (Mr. Thomas Tennent), we know from the constant accumulation and subsequent melting of the Sierra snowpack that some of the storms were very warm and some very cold; snow melt reported as high as 8000-feet in the mountains and yet snowfall reported near sea level in the Sacramento Valley floor itself over the course of the storm period. ¾” of snow was actually reported in Sacramento itself on January 29th. Mr. Tennent, in San Francisco, recorded nine days with below freezing temperatures in January alone, including a 22° reading on January 28, a full 5° colder than any temperature ever measured in the modern era of the city.
Massive runoff from the mountains during the warm storms filled the Sacramento and San Joaquin Valleys almost from the foothills of the Sierra on the east to the hills on the west side of the Great Valley. A giant lake 250-300 miles long and 20 miles wide apparently formed, some 5,000-6,000 square miles (of what is now some of the most valuable agricultural land in the world and home to about 2 million people).

Curiously, the event may not have happened during an El Nino year according to research done at Oregon State University by climatologists Victor Neal and William Quin. Their analysis indicates a polar jet that swept down to central California in December and then fluctuated north and south over the Northwest and California for two months interacting with a persistent Alaskan low that bowled one storm after another into California. However, one must assume, given the prodigious rainfall amounts, that a subtropical stream of moisture must have been entrained into this system. It is unfortunate that we don’t have any temperature records from California to confirm this aside from Mr. Tennet’s records of the minimums from San Francisco.

REFERENCE: For the best detailed account of the California 1861-1862 flood event see the article in Weatherwise magazine by Jan Null and Joelle Hulbert, volume 60 no. 1, January-February, 2007.