Estimated rainfall trends around the Seattle area, 1890-2017. By-station estimates for trends in location parameter of maximum likelihood estimated GEV regression model using NWS data, 3-day duration. Graphic: Rath, et al., 2018 / Tetra Tech Inc. / Seattle Public Utilities

By Daniel Beekman
3 February 2018

(The Seattle Times) – Attention, Seattle residents: Have you noticed more steady, long-lasting rainstorms in a city better known for gray skies, short showers and drizzle? Turns out you’re on to something.

Over the last 15 years, the city’s had more extreme rain, according to a new study by Seattle Public Utilities (SPU) officials, who say the weather is a climate-change preview.

“This confirms our anecdotal evidence,” said James Rufo-Hill, an SPU meteorologist. “For years, people have been saying, ‘I think the rain is getting worse around here,’ and now the data shows that.”

SPU has been measuring rainfall for about four decades with gauges in 17 locations across the city, such as Magnuson Park, the Maple Leaf Reservoir, Fauntleroy Ferry Dock, and Aki Kurose Middle School.

The information from the gauges helps SPU plan and design drainage and sewage projects, Rufo-Hill said.

The last major rainfall study was in 2003, so SPU had some catching up to do. The sprinklings most common in Seattle aren’t going away, according to Rufo-Hill. And the city’s rare flash storms aren’t necessarily worsening.

Seattle has a longer-than-average rainy season, but is only 44th nationally in total accumulation, less than such cities as Atlanta, Houston and New York.

That’s partly because our typical rain comes from low, layered clouds rather than high, dense clouds. And that may be a reason why many of us are umbrella-averse.

“Most of the rain we get hasn’t changed,” Rufo-Hill said.

But according to the SPU study released in December, “extreme” rainstorms have become more common — and more extreme.

What exactly does that mean? SPU measures storms by how long they last and how much rain they dump, and then categorizes those storms based on how often they can be expected to occur.

What used to be considered a once-in-a-century storm for a 24-hour period — 4 inches — can now be expected once every 25 years, according to Rufo-Hill. The new standard for a once-in-a-century storm for a 24-hour period is more than 5 inches.

Another example based on the addition of 15 years of data: What used to be a once-in-a-century storm for a 6-hour period — 1.86 inches — can now be expected every 25 years, according to SPU’s modeling.

Also becoming more robust are rainstorms lasting six hours and those lasting three days with levels of precipitation expected every five or 10 years. Another, more poetic name for these superstorms: “atmospheric rivers.”

“Our definitions of extreme and normal have changed,” Rufo-Hill said. [more]

‘Extreme’ rainstorms becoming more common in Seattle, says city meteorologist

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