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January 2017

Storm Summary: An Odd 24 Hours

Posted: January 18th, 2017
48-hour precipitation total

Wettest West Seattle, via Seattle RainWatch


That was one of the most intense 24-hour storm events Seattle has seen in a while. Post-frontal showers are still racing through the region, but they’re not likely to significantly add to storm totals.


Each of SPU’s 22 reporting rain gages recorded over two inches in 24 hours. Five gages recorded at least 2.40 inches, which is good for a 5-year return. The most intense rainfall, and the most overall fell at SPU’s new High Point gage, where 2.57 inches fell in 24 hours.


An interesting aspect to this storm (so far) is that it was never intense at short durations. A few gages picked up around a third of an inch in an hour, but that’s it.


Another oddity with the storm is that SPU’s Cascade Mountain watersheds picked up less than the City. The opposite was forecast. While the mountains will likely catch up as post-frontal showers progress, the slow speed of the storm and resultant emphasis on southerly flow likely cut-off access to moisture in west-east oriented watersheds.


Looking back at SPU records, two storms compare: November 19, 2012 and November 23, 2011. The former was extreme at slightly shorter durations, notably 6 hours, but like last night was significantly under-forecast. The latter, a bona fide atmospheric river that was among the first to receive full pre-storm incident planning treatment at SPU, lasted nearly twice as long.


Preliminary customer service data indicate that during the roughly 24-hour event, SPU’s operations response center received 107 calls, 65 of which were for “ponding,” or minor street flooding. SPU is also in the process analyzing at least 7.4 million gallons of sewage, monitored at 16 sites, that has spilled into area waterways.


Fortunately, forecasts call for only light showers for the rest of this week and possible dry conditions next week.


NWS Regional Storm Totals
NWS Area Forecast Discussion
NWS Forecast Tables
Extended Forecast (NAEFS, Select Seattle)