The Arrival of the Aleutian Low Pressure System
Any tried and true Seattleite knows that November and December are the stormiest months of the year for the Pacific Northwest. There are exceptions of course – the Columbus Day Storm of 1962 was by far the most powerful windstorm on record to hit the Pacific Northwest, and we’ll occasionally even see a significant windstorm in April. But if mammoth waves on the coast, major flooding on our rivers, and giant puddles throughout the Arboretum are your thing, you will be in for a treat during the next two months.
When you think about it, it’s amazing how quickly we transition from the calm and sunny weather of early September to the downright nasty weather we see during these two infamous months. Our beautiful summers are caused by the North Pacific High, a large, semi-permanent ridge of high pressure between California and Hawaii that shunts the jet stream well north into Alaska. But as summer transitions into autumn and autumn transitions into winter, the North Pacific High weakens and moves to the south. In contrast, the Aleutian Low, a semi-permanent low-pressure system that splits its time between the Gulf of Alaska and the Bering Sea, awakes from its summer slumber and strengthens dramatically.
The rapid rise of the Aleutian Low creates a very strong north-south pressure gradient across the Pacific, and this gradient increases the strength of the jet stream and the storms that come with it. And since the North Pacific High has not only weakened but moved further south, the jet stream moves further south as well. More often than not during these two months, this jet stream is pointed directly at the Pacific Northwest.
And to make matters even more interesting, the Western Tropical Pacific is still very active into November and December. On occasion, moisture from typhoons originating over the Western Tropical Pacific can be entrained in this strong jet stream, adding some extra “oomph” to any storms that form along it. Some of the Pacific Northwest’s most memorable storms – the Great Coastal Gale of 2007, the Columbus Day Storm of 1962, and even the “Ides of October” forecast bust of 2016 – all had tropical origins.
Though the change to a more active regime is always fun for me as a meteorologist, it presents some dangers as well. First and foremost, the Seattle metropolitan area is highly susceptible to windstorms due to how many large trees reside in the city. Several deaths have occurred over the past few years due to falling trees and branches hitting both pedestrians and drivers, so please steer clear of trees when the wind starts howling. Additionally, here in the Leschi neighborhood, we also have to be cognizant of landslides when soils become saturated from all the rain, especially on East Alder St./Lake Dell Ave.
One of my favorite storm season traditions is a trip to see Snoqualmie Falls when the Snoqualmie River is overflowing its banks. You haven’t lived until you’ve seen half a million gallons of water flowing over a 269’ drop every second. I hope ya’ll are as excited for storm season as I am!
Charlie, a Madrona resident, received his B.S. in atmospheric sciences from the University of Washington and works in Portland as a meteorologist forecasting wind energy along the Columbia River Gorge. Check out his weather website at weathertogether.net.