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October 2015

On Seattle’s Rainiest Neighborhoods

Posted: October 14th, 2015


TWBS Rain Update


As a participant in Take Winter By Storm, an organization dedicated to preparing Western Washington for winter weather, Seattle Public Utilities recently featured its rain gauge network and some interesting information it has revealed: Rainier Beach is the wettest neighborhood in Seattle.


Southeast Seattle is wetter than all other areas primarily due to its distance from the Olympic Rain Shadow, and perhaps because of its proximity to the so-called Issaquah Alps, which extend from the Central Cascades to South Lake Washington and enhance precipitation through orographic uplift.


Beyond regional-scale precipitation climatology, SPU’s rain gauges also reveal that its many drumlins and hills create small-scale zones of orographic enhancement and shadowing. During most storms, prevailing winds are out of the southwest, which means downtown Ballard is often kept dry by Magnolia, SODO is kept dry by the ridge that is West Seattle, and so forth.


Some of Seattle’s precipitation microclimates can be seen in the widely distributed map above. One key point regarding the map is that while SPU’s rain gauge records stretch back more than 30 years, only the past few years were analyzed. A longer period of record, like those that belong to the National Weather Service, for example, would reveal a more gentle gradient. Ergo, Seatac’s 30-year annual average, not far from Rainier Beach, is 37.49 inches while downtown’s near-100-year record is 37.22 inches.


That said, from year to year, season to season, and storm to storm, there are significant differences across the City. While Rainier Beach most often ranks first annually, last year’s “winner” at 51.84 inches was SPU’s Sand Point rain gauge, a fact that was largely due to a strong Puget Sound Convergence Zone event in September 2014. This year, the Central District is leading the pack (and also revealing the on-going drought having collected only 19.04 inches through August).


This year’s key Take Winter By Storm message was that just because we’re experiencing El Niño, which portends anomalous warmth and decreased mountain snowpack, it doesn’t mean that we won’t see occasional heavy rain. Past El Niño events, which have been captured by SPU’s rain gauges, demonstrate that clearly. Look for updated statistics, analyses and maps as this season unfolds.


Update: if the numbers in the original rainfall analysis seemed low, you were on to something. A closer look at SPU rain gauge data revealed an error. Accustomed to reviewing water year data rather than calendar year data, I mistakenly added drier-than-normal 2015 rainfall totals to the mix. Still only utilizing 5 years of data rather than the full set, the updated analysis has more rainfall overall. Seattle’s shadowy pattern remains essentially unchanged, however, and Rainier Beach is still the wettest spot. Alee of West Seattle, the updated driest locale is SODO.