Seattle Thinks It Knows Rain. Climate Change Begs to Differ.

By early spring, Seattleites are over the rain. Winter is a very long, very gray tunnel, during which the city sees most of its annual 150 wet days.

This is not surprising for anyone familiar with the Emerald City. What is surprising, though, is that a place synonymous with precipitation may not be prepared for the rain that’s coming.

Thunderstorms and downpours are historically rare in Seattle, which actually receives less annual rainfall than Miami. Most of its precipitation comes in the form of an inescapable drizzle. More of a mist, really. The city’s stormwater infrastructure is built with these steady, low volumes in mind.

However, a recent study found a significant rise in the number of heavy rains in recent decades, and climate models predict an increase in both the frequency and intensity of what officials call extreme weather events: deadly deluges that, within 24 hours, are capable of overwhelming water drainage infrastructure to cause flooding and send raw sewage into nearby waterways.

“We’ve got challenges including affordability and homelessness, but climate change and extreme weather events are right up there,” said James Rufo-Hill, a meteorologist with Seattle Public Utility who was involved in the study. The city has been collecting rainfall data through a network of rain gauges since the late 1970s. In the past 14 years, he said, “the stats have changed such that the definition of an extreme event has changed.” A storm that was previously expected only once in 100 years is now likely in 25.

One of the worst events on record occurred in early December 2007, when two storms flooded the Washington and Oregon coasts and brought wind gusts that topped 100 miles per hour. Seattle registered 5.61 inches of rain in only 24 hours. Just a year before, the city’s low-lying neighborhoods had flooded when a storm overwhelmed a drainage system clogged with debris, and claimed the life of a woman trapped in a windowless basement. The region again saw significant rain events in 2010, 2012, and 2014.

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