Good evening all! There’s a lot of buzz regarding Friday’s storm, so I thought I’d write a quick update clarifying some of the potential impacts of the storm and how the forecast has changed since yesterday.
Models continue to show a very strong but weakening cyclone arriving from the west on Friday, bringing heavy rain in the lowlands, heavy snow in the mountains, very strong winds for the coast, and extremely tall waves offshore. The strongest winds will be Friday night or early Saturday morning as this cyclone moves ashore somewhere between Central BC and the Southern Washington/Northern Oregon Coast.
“Cone of uncertainty” for the potential windstorm on Friday (based on ensembles). Note that there is considerable uncertainty on timing, location, and minimum pressure. Most likely track is over the northern end of Vancouver Island. #wawx pic.twitter.com/90JNMYovXh
— Joe Zagrodnik (@joejoezz) November 10, 2020
But while there is high confidence in heavy rain/mountain snow throughout the region and strong winds/high seas for the coast, confidence is much lower with wind speeds inland. Models are still all over the place with the projected strength and location of the low, and this will have huge impacts on winds for the I-5 Corridor.
The track of the low – due westerly and weakening as it moves ashore – is an unfavorable one for high winds over the Portland metro area. Our strongest windstorms scrape the coast and arrive from the SSW, bringing strong pressure gradients to a wide swath of the Willamette Valley. Additionally, the SSW track of the storm means that winds at upper/mid-levels are southerly, and these have an additive effect when they mix down to the SSW in the Willamette Valley (the Coast Range/Cascades help channel SW or WSW winds off the coast to S or SSW winds in the Willamette Valley). For a weakening low arriving from the west to give us from winds, it has to be very deep/intense to overcome the fact that the Willamette Valley is not as favored in west-moving systems, and it has to be just to the north of our area (a landfall along the southern Washington coast would be ideal). There is less “margin for error” in track with a westerly system vs a SSW system when talking about high winds, though small deviations in track with systems arriving from the SSW (as we saw during the “Ides of October” forecast bust of 2016) can also have huge effects on wind speed, particularly if the storms are relatively compact.
The image shows all the low-pressure centers at 10 am Friday from each of the 50 ensemble members from today’s 18Z Euro run. Most of the centers are off Vancouver Island and the North/Central Washington Coast, though there are a few off the Southern Washington/Northern Oregon Coast. And although there are a few exceptions (namely, a 965 mb behemoth west of Grays Harbor), the ones further to the north are generally deeper – approximately 965-970mb – while those to the south are closer to 980mb. Ignore the 995-1000mb low-pressure centers – these are likely just “localized” low pressure centers and don’t accurately capture the depth of the main system.
All of these solutions would make Portland breezy/windy, but only a few would bring a moderate/major windstorm to the Portland metro area. Western Washington has a better chance of reaching high-wind criteria due to the larger number of ensemble solutions further north, but like the Willamette Valley, the westerly track still doesn’t favor strong winds for the Puget Sound area, particularly if a localized area of low pressure forms downwind of the Olympics over Central Puget Sound and causes relatively lower winds there.
To summarize, here’s my *very* vague forecast for the probability of gusts reaching high wind warning criteria for the area. I’ll need to dig into the ensembles a bit more tomorrow before giving specific probabilities.
- highly likely for the coast (especially north)
- likely for Washington’s Northern Interior
- possible for Western Washington
- unlikely, but still possible for Portland and the Willamette Valley.
As mentioned above, there are still a handful of ensemble members that bring a deep enough low close enough to the Willamette Valley to give a significant windstorm in spite of the storm’s westerly track. I’ll continue to monitor the models and keep you posted with any changes.
Charlie
Featured image: Wave breaking over the islet of Rockall in the North Atlantic. Credit: James Fisher/Royal Air Force
1 Comment
Well, since they closed razor clam digging, we won’t have to worry about clam diggers out in the storm.