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Earth Sense: Weather probability based on long-term patterns

POSTED: September 22, 2013 12:04 a.m.

One may not have heard of Lyons, Colo., before, but recent news brought this town into the spotlight. Isolated from all its surroundings by raging flood waters, this normally pretty town of 1,600 people took the brunt of heavy rains during the first weeks of September. Steep slopes on surrounding mountainsides funneled water straight through residential neighborhoods, causing death and injury, and making roadways collapse.

Some locals expressed surprise at being hit by a hundred-year flood. The term is often misunderstood. It isn’t meant to predict when the next disaster will strike. Instead, it shows so-called “historic probability.”

If you take, say, 300 years of record, there will be three floods in them of the type just seen in Colorado. More importantly, it doesn’t mean that the next one isn’t due until 2113. It could happen again next month, or even next week. Terms like 100-year, or 1,000-year flood, only refer to probability based on historic record. Whether it’s going to happen again soon is determined by event probability.

This type, in turn, depends on meteorological conditions. The best example I know is from 1996, when hurricanes Bertha and Fran each had just visited Wilmington, N.C., with a bull’s-eye hit. As the National Guard was still distributing drinking water, and Georgia Power was graciously helping the local company restore electricity, Hurricane Hortense began to take aim at the same town.

“It can’t happen,” some said, “because the odds of getting three hurricanes in a row are so small.”

Hortense didn’t know that. Neither did Fran when she followed Bertha. Although Hortense turned away instead of reaching the coast, her probability of landfall had nothing to do with the previous hurricanes. Event probability is controlled by wind, the current energy budget of the atmosphere, ocean temperatures and related conditions. In the historic record, it all smooths out into the multiyear labels like the floods got.

Communities like Lyons and Drake, Colo., have continuing cause for worry because they are located on floodplains created by streams, confined by steep valley sides. In the Eastern states, a similar geographic layout prompted the largest civil engineering project in history when Pikeville, Ky. got its “Big Dig” (1973-1987), literally moving mountains to lessen the flood risk. This isn’t feasible, or even desirable, for other towns because of its enormous environmental impact.

For now, Colorado’s focus is on saving lives and restoring services.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.


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