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| Earth on Jan 26, 2011

Lucy Jones on California’s coming megastorm

Scientists believe a winter megastorm is inevitable for California. Such a storm might drop as much as 10 feet of rain on California over the course of a single month.

Scientists with the U.S. Geological Survey warned in early 2011 that California would inevitably experience a winter megastorm – a big one.

California floods in blue from simulated winter megastorm (USGS ARkstorm report).

It might not happen this year, or next. But, based on California’s history of storms in the past, another winter megastorm will happen sometime, these scientists say.

Such a storm might drop as much as 10 feet of rain on California over the course of a single month. Lucy Jones, Chief Scientist for the USGS Multihazards Demonstration Project, led a team that simulated the next possible megastorm. They called their simulation the ARkstorm. The purpose of the ARkstorm scenario – which ultimately involved about 120 scientists, lifeline operators, emergency planners and others – was to help agencies in California see and prepare for the impact of a catastrophic winter megastorm. Jones told EarthSky:

I want people to recognize that the storms of California are as big as hurricanes in the amount of rain that they drop, even if they don’t have as strong winds. And rain, and flooding, have the potential for extreme damage.

California flood of 1861-61, K street, Sacramento

California flood of 1861-61, K street, Sacramento. (USGS)

Jones said the last winter megastorms for California came in the winter of 1861-62. She said that six megastorms more severe than the ones seen in 1861-62 have been recorded in the past 1,800 years. She added that the USGS ARkstorm model suggested that a megastorm of very large magnitude would be likely to happen about once every 100 to 200 years.

When the megastorm comes to California again, she said, thousands of square miles of California – including cities – could realistically see floods.

There will be probably thousands of major landslides across California in an event like this. But it’s the flooding that really gets us. And with this model, we ended up flooding one-quarter of houses in California. So the economic impact is potentially devastating. This storm could end up costing a trillion dollars.

The USGS report on the ARkstorm estimated $725 billion in damages from the coming megastorm. The report also said the megastorm could require evacuation of 1.5 million people. In a future California winter megastorm, traffic from Los Angeles to the north could be cut off for weeks because of highway damage, according to these scientists. Dr. Jones said the point of the report isn’t to scare people.

This is not about being afraid, because in fact, we have good emergency management. The big issue here is to recognize that floods in fact are a risk to California. And when your community comes and talks about what we should be doing about our flood control dams, or other flood management systems, or our levees, people need to be taking it seriously. How do we, as a community, work together to protect against an ARkstorm coming through?

She said the megastorm phenomenon is based on what these scientists call atmospheric rivers. These rivers of air in the atmosphere have been better understood by meteorologists over about the past 15 years.

It would be an extremely wet storm, and the rain would lead to the highest level of damage. There are also high winds, but they tend to be in the desert areas of California and don’t lead to as much damage. (In the simulation) we also see coastal erosion and land sliding. There will be probably thousands of major landslides across California in an event like this. And part of the problem is that our flood control system has worked so well that we no longer feel the impact of the small or moderate events. Fifty years ago, people experienced major flooding across California pretty frequently. Now we’re doing well enough that many of the people in southern California, for instance, haven’t really experienced a major flood. But those flood control systems only function up to some level.

Jones said that her team used the geologic record in order to explore the question of how often winter megastorms can be expected to strike California. They used sediment deposits offshore from some of California’s big rivers.

And as we analyze those records – we’ve done one off of Ventura county and one in the Bay area – we’re able to see that in fact, there have been some very, very large storms that led to huge loads of sediment coming out of the rivers. It happened six times in the previous 1,800 years. The storm of 1861-62 doesn’t show up in that record. So that one was smaller than these very big events. This tells us that they’re very rare, but they are recurring. And the expectation is that the future will be like the past. In a time of global change, storms are going to be at least as frequent as the past, and the potential that they’ll actually increase in frequency, as we put more energy into the atmosphere.

She said she wanted to be clear is that the USGS report is not a prediction of a particular event. It’s a synthetic model, and every reality in the future will be different in some way, she said. But, based on the ARkstorm simulation, Jones and other USGS scientists believe a coming megastorm for California is extremely plausible.