Biblical floods… no longer unusual

A new study published Wednesday in Nature finds that tropical cyclones — which is a catch-all term for hurricanes and typhoons — move more slowly than they used to. This, combined with the increase in rainfall already expected to occur from such storms as the seas and air temperatures warm, paints a troubling picture of future storms. Slower storms have more time to dump greater amounts of rainfall, and this, combined with the capacity of a warmer atmosphere to hold more moisture, means we should expect large storms to pose more water dangers than ever before.

Those of you who were paying attention may recall Houston had “500 year storms” three (3) years in a row.  A 500 year storm is one with a probability of 2 tenths of one percent – in other words, on average you’d get one every 500 years.  A 100 year storm would have a probability of 1%.  The odds of three 500 year storms in a row are: 8*10^-9, or 0.0000008%, or 0.000000008 in regular numbers.   Obviously, the probability of those storms is NOT the same as they used to be.  Those 500 year storms are now kinda normal.  As the Gov. of NY, Andrew Cuomo said: “New York has a 100-year flood every two years now.”  Similarly, get ready for California.

Further, there are weaknesses in our understanding of storm risk probability, as explained well here.  Over the course of a 30-year mortgage, a house in a 100-year floodplain has a 26 percent chance of being inundated at least once.1  Sorry, but we think you’re better off knowing.  Don’t build there, don’t live there.

HOUSTON, TEXAS — TUESDAY, AUGUST 29, 2017: Residential neighborhoods near the Interstate 10 sit in floodwater in the wake of Hurricane Harvey on August 29, 2017 in Houston, Texas. (Marcus Yam / Los Angeles Times)

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