It’s December… and Santa (Ana)’s coming to town.

Wednesday morning my wife went out to Los Angeles to visit her mother. When she called on Thursday, she said, “we’ve been having Santa Ana winds.”

Back on the east coast, I didn’t think anything of it. As readers of this blog know, Santa Ana winds are strong, extremely dry offshore winds that are a customary late fall/winter feature of southern California weather.  High-pressure sets up over the Great Basin, and the ensuing physics is inexorable. One major threat? They fan the flames of brush fires feeding on the chaparral that is such a prominent feature of the landscape. No fires? No worry.

But as all of us nationwide have quickly learned, the winds of the past few days have been anything but customary. Customary is wind speeds in excess of 40 miles per hour. This storm produced gusts approaching the 100 mph mark. Wind force? Maybe you know that it goes as the square of the wind speed. As a result, this event didn’t produce just 2.5 times the force of the more conventional events. Instead it subjected people, buildings and trees to more than 6 times the usual stress. Hundreds of trees were felled, blocking streets, and knocking out power to perhaps 400,000 customers. Much of that service has been restored, but some folks can expect their lives to be disrupted for weeks.  

You know you’ve been through the wringer when the forecast for this weekend is a “classic” Santa Ana event, and locals are breathing a sigh of relief.

This prompts a quick observation about natural extremes – not just Santa Ana winds, but also tornadoes, hurricanes, winter snows, earthquakes – you name it. Our historical record for these events is the merest instant compared with the geological period over which such events have occurred. For much of the United States, conventional data go back a couple of hundred years at the most. This is the twinkling of an eye from the standpoint of the planet. As a result, we’ve got only the sketchiest picture of what constitutes an extreme. Furthermore, most places they’ve looked, the climatologists who’ve studied proxy evidence for storm intensity, or cycles of flood and drought going back hundreds of thousands of years, or the seismologists and volcanologists who go back further yet, typically see evidence for substantially stronger events than anything recently recorded. We therefore should not expect the historical maxima or minima, or our community experience, even going back a lifetime, to be a reliable guide as to how bad things can get in our own city or neighborhood.

When it comes to building community resilience to extremes, there’s no substitute for being conservative when it comes to estimating the hazard.

Santa Ana (substitute our hazard here) coming to town? We’d better watch out.

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