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December 2015

Relentless Rain

Posted: December 9th, 2015




Last Monday forecast discussions started using the old, reliable word “active” to describe the then developing pattern. When we see that term it usually means that the jet stream is in our hemispheric vicinity and it will deliver us a series of systems. Also last Monday medium-range forecast models suggested that an atmospheric river (AR) would strike somewhere along the west coast over the weekend. By Thursday or Friday, “active” had changed to a “parade of systems,” and “vigorous” proliferated. Those descriptions weren’t bad, but now that the parade is over I think “relentless” should have been employed. One of our longest lasting on record, this storm was memorable.


Between about 10:00 AM Saturday and about 10:00 AM Tuesday, SPU in-city rain gages recorded between 3.73 and 5.24 inches of rain, generally north to south. Over SPU’s watersheds between 7 and 8 inches fell, according to USGS and SNOTEL data.


There aren’t many 96-hour ARs on record to which this can be compared, at least with respect to rainfall. Previous city-flooding deluges (notably during Decembers 2007 and 2010) didn’t really exceed 48 hours. November 2006 or January 2009 may compare regionally but the former struck further south and the latter featured a rain shadow over Seattle. (I’m calling this a single continuous event but clearly visible in the “precipitable water” loop above are four separate low pressure systems that induce waves in the AR—that there were 3- to 6-hour breaks certainly provided SPU’s drainage system with some relief.)


A closer look at preliminary SPU rain gage data reveal a handful significant events embedded within the long storm. Generally, rainfall was most intense at 48- and 72-hour durations ending early Tuesday morning. At least 11 sites recorded 2- to 5-year recurrences, with Georgetown and Hillman City toward the top. But the most rain fell at West Seattle’s Lafayette Elementary School, which recorded a total of 5.24 inches in 96 hours. It took the lead early Tuesday when a fast-moving post-frontal thundershower dropped 0.25 inches (and some hail) in just 5 minutes. That downburst was the only significant short-duration event recorded anywhere in the city during the entire event, which to me is pretty amazing.


Records fell at Sea-Tac, too. At 2.13 inches, it’s the new wettest December 8th. That’s the second time this year Sea-Tac has reached 2 inches in a day (the other was on March 14). That has only happened 8 other years out of the past 120. It was also the 13th day of 2015 in which at least an inch has fallen—that’s a new all-time high, analogous perhaps to our record consecutive 80- and 90-degree warm streaks from this past summer.


Speaking of temperatures, yesterday tied the daily record maximum at 60 degrees. That came five days after another 60-degree record maximum. For what it’s worth, no two consecutive Decembers have reached 60 before until now. Yesterday also set a new record high minimum temperature of 50 degrees.


In terms of impacts to Seattle, over the 96-hour rainy period SPU received 308 calls from customers, three-quarters of which were related to street flooding. Unfortunately nearly all of SPU’s sewer overflow sites spilled for at least a minute or two if not longer. As for SPU’s watersheds and the rivers they protect, the Cedar River’s flow reached 5,910 cubic feet per second at Renton on Wednesday afternoon, which put this event near the top five all-time. And what little mountain snow we had accumulated was decimated by the warm rain. Fortunately freezing levels are dropping fast and the forecast looks decent for rebuilding.



NWS Area Forecast Discussion
NWS Forecast Table Interface—Seattle
NWS Forecast Table Interface—Tolt Reservoir
NWS Forecast Table Interface—Chester Morse Lake
SPU Water Supply Outlook