Should California Be Bracing for a Possible ARkStorm?

The storm currently hitting California has not produced as much precipitation as was anticipated, irritating some Weather West readers (see yesterday’s post) while reducing flood concerns for the present. But the forecast remains extremely wet over the next week and beyond. As the maps posted below show, rain in the lowlands and snow in the mountains could fall in prodigious quantities, which would probably cause extensive flooding (note the 256 inches of snow over roughly two weeks forecasted on one of the maps posted below). If the current seven-day forecast verifies, and if the wet pattern remains entrenched, California might even experience what is known as an ARkStorm, an event that occurs on average once every several hundred years. In an ARkStorm, much of the Central Valley, California’s agricultural heartland and home to millions of people, could be inundated for weeks.

As noted in the previous post, California has been locked in a persistent drought, experiencing only two wet years out of the past 12. An abrupt end to a long-term drought by devastating floods would not be unprecedented. Indeed, this is precisely what happened in the mid 19th century. As reported by EarthDate:

In the 1840s and 1850s, California was exceptionally dry, so by the fall of 1861, California ranchers were hoping for rain in late November they got what they were wishing for and – then some. It didn’t stop raining for 43 days and by January 1862 the Central Valley was filled with an inland sea.

The Great Flood of 1862 was that an extraordinary event, one that affected much of the western United States. The Wikipedia article on it provides a good summary. As it notes:

The event dumped an equivalent of 10 feet (3.0 m) of water in California, in the form of rain and snow. .. An area about 300 miles (480 km) long, averaging 20 miles (32 km) in width, and covering 5,000 to 6,000 square miles (13,000 to 16,000 km2) was under water over a period of 43 days.

Although this was the biggest flood in California’s recorded history, geological evidence shows that even larger floods have occurred over the past several thousand years. Of particular note were the years 440, 1418, 1605, and 1750. The largest flood was that of 1605 (± 5 years). As noted in a 2017 Quarternary Research article, this event may have even produced a large lake in the Mojave Desert that lasted for several decades. The Quarternary Research article claims that this flood may have been linked to a global cooling cycle associated with this so-called Little Ice Age. As the authors note, “This cooling was probably accompanied by an equatorward shift of prevailing wind patterns and associated storm tracks.”

The current concern is that global warming will lead to increased flood risks in California – along with increased drought risks. As Xingying Huang and Daniel Swain wrote in an August 2022 Science Advances article:

Despite the recent prevalence of severe drought, California faces a broadly underappreciated risk of severe floods. Here, we investigate the physical characteristics of “plausible worst case scenario” extreme storm sequences capable of giving rise to “megaflood” conditions using a combination of climate model data and high-resolution weather modeling. Using the data from the Community Earth System Model Large Ensemble, we find that climate change has already doubled the likelihood of an event capable of producing catastrophic flooding, but larger future increases are likely due to continued warming. We further find that runoff in the future extreme storm scenario is 200 to 400% greater than historical values in the Sierra Nevada because of increased precipitation rates and decreased snow fraction. These findings have direct implications for flood and emergency management, as well as broader implications for hazard mitigation and climate adaptation activities.

(See the map made by Xingying Huang and Daniel Swain posted blow to compare historical ARkStorms and those predicted for the future.)


Daniel Swain’s avid followers at Weather West have noted how the current situation follows Swain’s recent ARkStorm article:

Many researchers are concerned that California is not doing enough to prepare for the possibility of devastating floods. One proposal for dealing with extreme precipitation events is to allow rivers to occupy more of their natural floodplains, as outlined in a New York Times article published today. Such an approach would also help recharge California’s aquifers, many of which are severely depleted. But as the author observes, this would be an expensive solution that would generate pronounced opposition. From an environmental perspective, however, it makes a lot of sense.