Fall is the most rapidly-changing season here in the Pacific Northwest. September generally starts off warm and sunny, with some of our highest fire danger of the entire year. But by late September/early October, temperatures cool substantially as our days rapidly become shorter, and typically, by the 3rd or 4th week of October, we start seeing a steady parade of systems racing along a strong Pacific jet stream right into the Pacific Northwest.
Consider these stats. For Seattle, the average high/low/precip on October 1 is 66/51/0.08″, and on October 31, it is 56/44/0.17″. That’s a pretty big monthly change in temperature and precipitation for any climate, let alone the Pacific Northwest. During early October, you can schedule outdoor events far in advance with a relatively small chance of them getting cancelled due to rain. The same can’t be said for late October! October also features some of our most powerful storm systems; Seattle’s rainiest day on record was on 10/20/2003 when an incredible 5.02″ of rain fell over the airport, and a massive windstorm ravaged Western Washington on October 21, 1934 (see Wolf Read’s excellent writeup here for more details on this storm). And of course, who can forget about the Columbus Day Storm of 1962, arguably the most powerful and destructive extratropical cyclone to strike the United States in recorded history.
The transition out of winter into spring is much more gradual. Sea-Tac averages 49/38/0.17″ on January 1, and it isn’t until April 28 that average precipitation drops back to 0.08,” with 62/46 for an average high/low. We can still see windstorms and flooding rains in the winter and early spring, but they generally lack the ferocity of those in the autumn, and they become even more anemic as we head into April. For an explanation of why our pattern change in autumn is so much quicker and more dramatic than that in spring, read my blog on the topic here.
Forecast
Over the next week, a deep trough of low pressure will stall offshore, with systems rotating into our region from the SSW. However, this trough will be far enough offshore that the heaviest rain from these systems will be confined to the coast, the Coast Range, and Olympics, with Western WA/OR and the Cascades seeing near or even slightly below-average amounts.
The PacNW typically enters a much wetter pattern the 3rd or 4th week of October, & the 10-day precip forecast shows this well. Significant precip is forecast throughout the PacNW, but anomalies inland are below-avg due to storms stalling offshore, showing how wet we typically are pic.twitter.com/pbgAchKBaO
— Charlie Phillips (@GeoduckChuck) October 15, 2021
However, this trough is expected to weaken and move inland by next weekend and the following week, with the jet stream strengthening and taking a more direct aim towards the Pacific Northwest.
Benign weather is expected through next Fri, with weak storms stalling offshore and sending periods of light rain into the PacNW. But a stronger jet stream may arrive by next weekend and persist through the end of October, bringing a threat of heavier rain, wind, and mtn snow. pic.twitter.com/oAQolcnyMG
— Charlie Phillips (@GeoduckChuck) October 15, 2021
Here are the Euro ensembles at that time – note the uptick in rain around 10/23-24. It’s a long-range forecast, but it would align with climatology and the tendency for a more active pattern to take hold during mid/late October. Bottom line: if you are planning on doing any Halloween Festivities this year, you shouldn’t need snow boots, but you may need a poncho!
