In the wake of the investigation into the 2006 "Hanukkah Eve" rain storm that hit Seattle and drowned a Madison Valley woman in her flooded basement office, I questioned the terminology of "100-year storms." In doing a quick survey of media stories, I found that central Puget Sound had experienced a minimum of six "100-year storms" in 20 years. I concluded that the term was misleading. Now, with the terrible flooding in the Midwest this year, it's becoming clear that the terminology is hurting some flood victims.
MSNBC reports that weather experts are saying that the "100-year" metric for storms and floods is misunderstood by the public:
Now, with the [Midwest] region struck by a supposedly once-in-a-lifetime flood for the second time since 1993, some scientists and disaster officials say the use of terms like "100-year flood" should be re-evaluated because they are often misunderstood and can give the public a false sense of security.
A "100-year" flood is one that has a one percent chance of occurring in any given year, and that means it's possible, statistically, to have back-to-back 100-year floods. In addition, weather data and trends can be outdated due to climate change or even increased development (more paving means more runoff means worse floods). On top of that, the term is geographically imprecise: a 100-year storm in north Seattle is a distinct event from a 100-year storm in south Seattle. Just because you have one in one place doesn't mean you can't have another one in the other. One big storm in Seattle could produce multiple 100-year storms, from a meteorological perspective.
But apparently some Midwesterners believed that having one 100-year flood 15 years ago meant that they were safe from floods for the next century, and so they dropped their flood insurance. Oops.
The term is easy media shorthand for describing a storm or flood's severity, and it tends to dramatize weather events, something the media are always looking to do. Another temptation is for policy-makers to hype the term to get public attention for climate-related spending programs. But weather experts are rethinking whether the term is doing more harm than good:
"We, the United States Geological Survey, almost need to quit using the term '100-year flood,'" said hydrologist Gary Wilson with the USGS Missouri Water Science Center in Rolla, Mo.
Villanova University professor Robert Traver, who specializes in storm water management, was more succinct: "Whoever invented that term should be shot."
Is it time to convene a 100-year firing squad for flood experts, weather forecasters, and media storm exaggerators? And what about wishful thinkers who live in flood zones without insurance?