‘Extreme rainstorms’ increased in Seattle over past 15 years

A sharp rise in severe rainstorms has officials scrambling to prepare Seattle infrastructure for a much wetter future as climate change progresses.

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“Over the past 15 years, a certain type of rainfall event has become more common,” said James Rufo-Hill, a climate adaption specialist with Seattle Public Utilities. “That type of rainfall event is one we are seeing right now — it’s called an atmospheric river. Basically, it’s the long duration, multi-day storm.”

A study commissioned by Seattle Public Utilities finds that the region has experienced a 30 percent increase in “extreme rainstorms” over the past 15 years. More recently, Seattle experienced the wettest four-year period on record. The study states:

Rainfall patterns across the city show that these extreme weather events have grown 30 percent stronger over the past 15 years, an expected impact of climate change. The new SPU study draws from data collected by 17 SPU-owned rain gauges and from regional observations taken by the National Weather Service (NWS).

SPU has monitored weather patterns for about 40 years. The last report on extreme rainfall patterns stated that 4 inches of rain in 24 hours qualified as an extreme event. Now, it’s 5.5 inches in that same time.

“So the extreme events are becoming more extreme,” Rufo-Hill said. “Downpours and other types of events haven’t changed, but these big storms that last anywhere from six hours to three days have grown even bigger and stronger.”

The study echoes the National Climate Assessment — that a rise in extreme precipitation is expected in all regions of the U.S. as climate change continues. It also is in line with what local climatologist Cliff Mass has predicted. Mass warns that climate change will alter conditions around the Northwest over the next 100 years — including more rain. But he has also come under fire for correcting how severe the signs of climate change have been.

Whether it’s national studies, the City of Seattle, or a local climate expert, all signs point to a much wetter and rainier Northwest in the years to come.

“Cities don’t have the luxury of climate denial and cannot wait for federal leaders to embrace science,” Mayor Jenny Durkan said. “The effect of inaction is already at our doorstep. Our city is preparing for our new reality while we work to cut emissions and prevent the catastrophic impacts of climate change.”

Seattle infrastructure

Local officials are pointing to the new study as an argument to accelerate Seattle infrastructure projects, such as the city’s move toward electric car facilities.

“Seattle will continue to lead on climate action and fostering our green energy economy,” Durkan said. “We are electrifying our fleet, implementing strong building efficiency standards, and partnering for improved resilience planning. But there is still more to do.”

Another important aspect of Seattle infrastructure is how the city handles rain. When Seattle designed its stormwater system more than 100 years ago, it combined its street drainage with sewage from buildings. That means toilets, showers, and street drains all feed the same pipes. During heavy rain, those pipes are often overwhelmed and overflow into the area’s bodies of water.

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This is what happens during modern rain storms around Seattle. Lake Washington, Lake Union, and Elliott Bay are where sewage overflows when pipes can’t handle the increased load. It happens so much that Seattle and King County have paid fines for how much they dump into the water.

Seattle has already started to address the issue by boring a massive tunnel underneath Ballard, Fremont and Wallingford. Those spaces will be used to divert overflow. But Seattle and King County — especially their wastewater treatment facilitates — still face challenges as more people move to the area and heavy rains increase.

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