The 2021-2022 storm season is knocking at our door. Take a look at the precipitable water imagery over the East Pacific and note the dramatic stream of subtropical moisture barreling towards the Pacific Northwest. This is technically an “atmospheric river,” a long, narrow filament of moisture from the subtropics (in this case, the East China Sea) to the mid-latitudes, but it’s a bit cooler than most of our so-called atmospheric rivers due to its northern origins and won’t cause any flooding due to river levels being so low. But it’s great news for the salmon, who could really use some cool, swift rivers to move upstream and spawn in!
There hasn’t been too much change to the forecast. Heavy rain is still expected to arrive Friday morning for Western Washington and persist throughout the day. The exception is for areas north and west of Seattle, which will see pronounced rain shadowing in the morning followed by steadier rain in the afternoon. In my opinion, Western WA forecasters have historically not emphasized rainshadows enough, particularly when they are further south than usual and over highly populated regions. To make matters more confusing, rainshadows during atmospheric rivers often tend to be further south than usual, so forecasters have the tricky task of highlighting a flooding threat while explaining that the majority of their viewers/readers will just see a light rain.
I think the WRF is overdoing precip, but I’d still expect 1-2.5″ for Western WA/OR lowlands and 2.5 – 5″ for the mountains from Fri am – Sun pm. Friday will be a washout for Western WA.
Higher flows will be great for salmon returning upstream. Skokomish could hit Action Stage! pic.twitter.com/1y9BEcWKId
— Charlie Phillips (@GeoduckChuck) September 17, 2021
HOWEVER, there’s a big caveat to the heavy rain tomorrow… strong rainshadowing over Seattle and Everett. Those locations will only see light/moderate rain for the day.
Western WA rainshadowing never gets the coverage it deserves, particularly when over highly populated areas. pic.twitter.com/HRLC0w2elc
— Charlie Phillips (@GeoduckChuck) September 17, 2021
We’ll see some gusty winds in addition to the rain. With dessicated branches from the ongoing drought that have accumulated throughout the entire summer and haven’t been trimmed by previous storms, I’d expect more power outages than usual for a storm of this strength. Wind advisories are in effect for the Washington Coast, San Juans, Whidbey Island, and coastal areas of Skagit and Whatcom counties from 5am – 8pm Friday, with gusts to 50mph possible. My folks live on South Whidbey Island and are very exposed to the wind, and I wouldn’t be surprised if they saw a brief outage tomorrow.
Saturday and Sunday will feature heavy post-frontal showers, with some showers potentially containing a rumble or two of thunder. Have a raincoat/umbrella handy this weekend and keep an eye on the weather radar if out and about… you don’t want to get caught unprepared under one of these showers!
