The California Super-storm (Ark Storm)

January 18, 2011

Worst Case Scenario - California Superstorm or Ark Storm

Dr. Greg Forbes, Severe Weather Expert

The United States Geological Survey (USGS) has come up with a worst case scenario winter storm for California that, if it happened, could bring a catastrophe to large parts of the state. You can see a summary of their study here or even the full report. They call it the Arkstorm, suggestive of Noah's Ark and a rain for 40 days and 40 nights.

This isn't a publicity stunt. A lot of scientific research went into the report and its findings. What they did was to try to replicate a storm today similar to the prolonged one that hit California for 45 days in winter 1861-62. That one turned the Sacramento Valley into an inland sea. There weren't many people living in California then. Needless to say, the impacts would be a lot worse today! Levees today protect the Sacramento Valley, but they are increasingly in disrepair. Once one or two fail, the whole valley could again become an inland sea -- not dissimilar to what happened in New Orleans from Hurricane Katrina.

There wasn't much weather data back in the 1860s, so to try to get a more quantitative handle on the worst-case scenario, they combined two big storms that have ample weather data - from January 1969 and February 1986. They combined these to make a prolonged 40-day storm in which an atmospheric river (see my previous blog) or "pineapple express" constantly blasted moisture from the tropics into California, as depicted below.

Their hypothetical prolonged storm gave rainfalls of the 500-year to 1000-year nature(having chance of occurrence 0.1 to 0.2% per year). Up to ten feet of rain fell. This resulted in widespread flooding of the Sacramento Valley and many other areas and hundreds of mudslides and landslides. Winds gusted over 60 mph in many locations and to 125 mph on some mountain peaks. Damage from flooding and landslides was estimated at $40 billion, with another $35 billion in losses to business (including agricultural losses). This is a substantial fraction of the damage from Hurricane Katrina.

Are the results reasonable? The most recent atmospheric river event in California on December 17-22, 2010 gave up to 26 inches of rain in 6 days. Rain at that rate persisting for 40 days could give more than ten feet of rain in places on west and south-facing mountain slope regions. There was a gust to 155 mph in the Sierra Nevada in the December 2010 storm, so gusts to 125 mph on mountain tops are not unexpected.

Could it happen? Unfortunately yes. The question is when. This kind of worst-case-scenario event with return period of 500 to 1000 years could be hundreds of years away, or it could happen this year. An atmospheric river that persistent isn't very likely, but it isn't impossible. In fact, The Weather Channel aired an episode of "It Could Happen Tomorrow" a while back essentially on this very type of flood event! You can see a snippet of that episode here.

What should be done? There's another great question. The bigger the event, the more it costs to build preventive measures against it. Man-made structures don't last 500 years in California, so there is a hesitancy to build for an event so extreme that it probably won't happen during the lifespan of the structure. But if you knew the flood would happen within a decade or so, that would probably change your opinion. The problem is, we can't be sure when it will happen!