During the early afternoon of Tuesday, December 18, the most powerful tornado to strike Washington since 1986 slammed into southern Port Orchard on the Olympic Peninsula. And while this twister was very different from the tempests that ravage the Great Plains each spring, it still damaged approximately 50 buildings and toppled many trees/power lines as it tracked through the area. Based on the observed damage, the NWS rated this tornado as an EF-2 with peak wind speeds of 120–130mph. The tornado left a trail of destruction 140 miles long and was quite wide, with a maximum width of 250–300 yards. Like many tornadoes, it only lasted for a couple minutes, with the tornado touching down by 1:50pm and dissipating by 1:55pm.
Tornadoes are rare in the Pacific Northwest because we lack the atmospheric instability to support deep convection. And even on those very rare occasions when the atmosphere over our area is extremely unstable and primed for thunderstorm formation, we almost never have the wind shear in the lower and midlevels of the atmosphere required to make these thunderstorms rotate and spin up tornadoes in the process. Tuesday was nothing more than a typical day of cool, post-frontal “showers and sunbreaks” for most folks, with a weakly unstable atmosphere and some isolated thundershowers for a lucky few. So, with such a marginally unstable atmosphere and weak wind shear, how did Western Washington end up seeing its strongest tornado in 32 years?
To find the answer, we need to highlight the differences between how the Plains and Pacific Northwest tornadoes form. The tornadoes over the Great Plains and other regions east of the Continental Divide are generally formed when strong wind shear causes an entire thunderstorm to rotate, and a tornado forms when this strong rotation extends all the way to the Earth’s surface. While fully-rotating Pacific Northwest thunderstorms do occur from time-to-time, most of our tornadoes are caused by particularly strong areas of local rotation in a thunderstorm that are relatively divorced from that thunderstorm’s overall structure. If a Plains tornadic thunderstorm rotates like a giant, single whirlpool, Pacific Northwest tornadoes spin up from turbulent eddies within a more disorganized thunderstorm.
This tornado likely formed because there was some weak wind shear in the lee of the Olympics, and as the thunderstorm that spawned this tornado moved into the Port Orchard region, it ingested some of this rotating air into its updraft and created a tornado in the process. A few tornadoes have formed in the lee of the Olympics before, so there may be tendency for the Olympics to induce weak shear—and the potential for relatively weak, short-lived tornadoes—over the Kitsap Peninsula during periods of unstable onshore flow.
Tornadoes have occurred over the Seattle metro area as well, and many of these are associated with Puget Sound Convergence Zones, where the contrasting northerly and southerly winds cause localized areas of circulation and weak tornadoes/funnel clouds as a result. Washington only averages a couple tornadoes a year and most are EF-0s, but if a strong shower or Puget Sound Convergence Zone is overhead, keep your eye out for any semblance of a funnel cloud or a weak tornado. While it will likely be a while before the next EF-2 strikes Western Washington, I wouldn’t be surprised if an EF-0 strikes Western Washington during 2019.
Charlie Phillips, a Madrona resident, received his B.S. in atmospheric sciences from the University of Washington and works in Portland as a meteorologist. Check out his weather website at to charlie.weathertogether.net.