Hi all! Sorry for not writing anything anything this past week, I was vacationing with my folks in Costa Rica! My brother Henry has been teaching English abroad for the past year there, so we jumped at the chance to visit him for a week and haul him back home to Seattle for the holidays before he heads back to teach there for another several months. We visited different parts of the country and saw a variety of fascinating things; we started out in the town of Barva (where my brother lives) and then headed up to Monteverde in the mountains further inland, which is a lush tropical rainforest home to some incredible creatures. We went on several nature tours there, including one at night where we saw multiple tarantulas and came face-to-face with a giant “Side-Striped Palm Pit Viper.” I loved that hike, my mom did not!
We then traveled to Quepos on the Pacific Coast and spent a day at Manuel Antonio National Park, where we saw toucans, sloths, and even monkeys stealing sandwiches from tourists like us! We also spent a day deep sea fishing, and although we got skunked, we hooked two marlin and saw tons of flying fish and dolphins. The highlight of the day, and perhaps the entire trip, was saving a sea turtle that was ensnared in a derelict commercial fishing net. Besides all of our family adventures, it was great to hang out with my brother’s friends and roommate there. It was a very satisfying and fulfilling trip, and I hope I can visit other parts of the country some day.
But as privileged as I was to be with my family in Costa Rica, I was a little chagrined to learn that I missed the stormiest week of 2018 for the Pacific Northwest. Three strong storms barreled into the Pacific Northwest last week, with the middle one on 12/20 knocking out power to 300,000-400,000 people primarily in Washington (Dr. Wolf Read, the foremost source of knowledge on Pacific Northwest windstorms, has a great analysis of the storm here). But for me, the most notable weather event was far smaller in scope, and far more intense. It was the most powerful tornado to strike Washington in 32 years.
Credit: Logan Johnson/Seattle NWS
During the early afternoon of Tuesday 12/18, the most powerful tornado to strike Washington since 1986 slammed into southern Port Orchard on the Olympic Peninsula. And while this twister was a far cry 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 120-130 mph. The tornado left a trail of destruction 1.40 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 1:50 pm and dissipating by 1:55 pm.
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 mid-levels 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 pretty 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.
Thanks for reading, and have a very merry Christmas!
Charlie
