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March 2015

A Lot of Rain

Posted: March 16th, 2015




It’s been a few years since we’ve seen that much rain around here. Between the time that the first wave of the atmospheric river hit late Friday night, until the final bands of showers exited early Monday morning, over 3 inches fell across parts of the City.


That’s the most any single system has provided in Seattle since December 12th, 2010. Unlike that historic event and some others that have occurred since, rainfall was seldom very intense, which fortunately limited urban drainage impacts to a certain extent. In relatively low numbers, SPU customers primarily reported standing water and isolated flooding, though significant sewer overflows occurred, as did a few small landslides.


SPU’s Cascade Mountain watersheds received a tiny bit of high-elevation snow and up to 5 inches of rain, nearly all of which was or will be stored as runoff continues to fill our reservoirs. Given that our snowpack remains virtually non-existent, and that March started off quite dry, at least some of us at SPU were happy to see so much rain. Thanks to near-normal seasonal precipitation and on-going operational adjustments, our water supply outlook for this summer remains good.


You may have also noticed that Seattle set a daily record at 2.20 inches, which is good for its wettest day in 6 years, as well as the second wettest March day ever. If those facts seem to contradict statistics mentioned elsewhere, it’s because National Weather Service records are kept outside city limits at Seatac, and because they’re also kept according to calendar days, not by rolling periods.


According to preliminary SPU data, 12 neighborhood rain gauges recorded statistically significant rain. The highest overall total appears to belong to our West Seattle/Admiral gauge, which picked up 3.63 inches over a 48-hour period. The most intense rainfall also occurred over Admiral, and perhaps other areas across central Seattle, where 2.84 inches fell in a 24-hour period, and 1.96 inches fell in a 12-hour period, each good for almost 10-year recurrences. The least amount of rain, for what it’s worth, appears to have fallen across NE Seattle.


Looking at the bigger picture, we’ve been discussing quite a bit how this winter closely resembles what a normal year should look like by mid-century according to the latest climate change research. While that narrative has focused on our warmth and aforementioned lack of snow, it also applies to this weekend’s storm. Climate modeling also strongly suggests that increasing sea surface temperatures and water vapor will generate more frequent atmospheric rivers ’round these parts. The double-edged sword that is both the busted drought and the urban flood is so maddeningly sharp.



Gone for now is the ridiculously resilient ridge; the atmosphere firmly believes that Spring is upon us, and it is expected to keep sending systems our way throughout the latest forecast period.


After today’s sunny break, a weak trough will slip through the region tomorrow and kick off some light showers. It should be noted that the NWS mentioned the possibility of instability and thunderstorms—it’s that time of year again. Wednesday and Thursday look to be dry but kind of cloudy, or dirty in the parlance.


The next threat of rain comes late Friday. Forecast models disagree a bit on the details, and given how wet is just got—and how terribly said models just performed—we should watch the weekend closely, though at the moment there are no real concerns.


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