Active Pattern Next Two Weeks, Major Storm Possible on Friday

According to a 2005 Gallup poll, Americans think November is the 8th best month of the year. November has a few things going for it; the Macy’s Thanksgiving Day Parade, increasingly competitive football games, not-shaving – but it seems most Americans believe these attributes are overshadowed by how dark, cold, and depressing the month is as a whole.

But November is one of – if not THE – favorite month of the year for many Pacific Northwest weather geeks, and I believe our collective love for November has substantially buoyed its position in the month power rankings. November is the stormiest month on average for the Pacific Northwest, and there’s nothing that a bonafide weather nerd appreciates more than a classic Pacific Northwest windstorm.

The next two weeks will feature classic mid-November weather, with frequent storms bringing periods of heavy rain, strong wind, and copious mountain snow. And although the details are still coming into view, there is increasing confidence that a storm on Friday night could bring the strongest regionwide windstorm to the Pacific NW in several years, though it won’t be anywhere near as strong as the Hanukkah Eve Storm of 2006 or Great Coastal Gale of 2007. In this blog, I’ll give a quick overview of the weather expected through Thursday, and then I’ll dive into a little more detail about Friday’s storm and its potential impacts.

Today’s pattern was a real treat to view on satellite. A well-defined cold front arrived from the NW, and this front is now bringing rain/mountain snow to Western WA/OR. Behind this front, a deep trough (visible as the swirl of clouds off the Alaska Panhandle) is moving SE.

This trough will move into the Pacific NW tomorrow, bringing heavy showers to Western Washington and Western Oregon. Because flow will be from the WNW and nearly perpendicular to both the Coast Range and Cascades, both of these ranges will be very efficient in enhancing precipitation on their windward slopes. Additionally, this WNW flow means the Seattle area will see moderate rainshadowing from the Olympics and the Willamette Valley will see slightly weaker rainshadowing from the Coast Range.

The 48-hour precipitation chart from the UW WRF model below, which includes all the precipitation from tonight’s front along with the showers expected tomorrow, shows 0.75 – 1.5 inches in the Washington and North/Central Oregon Cascades, with the lowlands picking up 0.2 – 0.5 inches. Shower activity should increase around sunrise and gradually taper off in the evening.

Snow levels will be between 2000-3000 feet, so all the passes will see snow from this system. Both the Portland and Seattle NWS offices issued their first “Winter Weather Advisory” of the 2020-2021 storm season for the Cascades and are calling for up to a foot of snow from Snoqualmie Pass south to Willamette Pass, with lesser amounts to the north and south.

Wednesday will be relatively calm with mostly cloudy skies and a few light showers in the morning. Another front similar to the one that impacted us tonight will arrive Thursday afternoon and evening, bringing another round of rain in the lowlands and snow in the mountains.

Friday’s Storm

A much, much stronger and larger storm will arrive Friday morning and persist through early Saturday morning. This storm promises to bring heavy rain and mountain snow, but winds – and the potential for widespread power outages – are the main threat with this system. The strongest winds should occur Friday night/Saturday morning when the storm moves onshore and drags a very strong pressure gradient across the highly-populated I-5 corridor.

Friday’s storm will also bring extraordinarily large waves to the coast. The National Weather Service is currently forecasting 30-45′ seas off the Washington and Oregon Coasts Friday night. Friday and/or Saturday will be great days to go storm watching on the coast, but exercise extreme caution in doing so and stay far away from the shores so that you are not in danger of getting inundated from a sneaker wave. Additionally, make sure you aren’t driving through heavily forested areas when there are high winds. Many Pacific Northwest windstorm deaths occur when moving cars are crunched by falling trees.

Although confidence is high that Friday will be wet and windy, there is still low confidence in just how windy it will be. Some of the stronger solutions are indicating that this system could be one of the strongest to impact region in several winters, and it would easily be stronger than anything we experienced over the underwhelming 2018-2019 and 2019-2020 storm seasons (I am excluding the 2020 Labor Day windstorm/fires, as that was an atypical easterly wind event). However, it is currently NOT expected to be a memorable, historic windstorm like the Hanukkah Eve Storm of 2006, Great Coastal Gale of December 1-3, 2007, or the massive cyclone December 12, 1995 cyclone that ravaged the entire West Coast. We’ll get a much better idea of the potential impacts of Friday’s storm over the next 24-48 hours as model runs come into better agreement.

Formation and Evolution

An ‘open wave’ of low pressure that includes some of the remnants of Tropical Storm Atsani will undergo cyclogenesis well offshore on Thursday/early Friday as it rides along a 180 knot westerly jet. Although this low will reach peak intensity several hundred miles offshore, this strong jet will direct the very strong remnants of the low right towards the Pacific NW, bringing heavy rain through much of the day and very strong winds inland late Friday night/early Saturday morning as the storm’s center and the strong pressure gradients surrounding it move ashore.

The below charts show the Euro, GFS, and Canadian operational models at 1pm Friday. Note that there are still some significant differences in the timing, depth, and location of the storm, with the Canadian model being significantly faster and stronger than either the European or GFS. Despite these differences, all models are in agreement that this will be a very wet system with a very broad and expansive wind field.

The incoming 00Z WRF-GFS shows what the storm may look like at landfall. Note the wide swath of 50+ knot winds off the coast and extremely strong north-south pressure gradient across Western Washington and Oregon. This solution would bring damaging winds to the coast, with gusts of 70-75 mph possible in coastal communities and over 90 mph possible at exposed headlands. Winds would be less inland, but this solution could still bring 55-65 mph gusts and widespread power outages to Western Washington, with somewhat less of an impact for the Willamette Valley.

Despite the relatively good agreement in the deterministic models, there is still poor agreement in the ensembles with this system. Take a look at the sea-level-pressure charts at 1pm Friday from the 50 European ensemble members below, and note that many members lack this area of low pressure entirely. Given the agreement and consistency in the operational runs, I have very high confidence that we’ll see a strong system on Friday, but the variance in ensemble members underscores how confidence in the details is low at this time.

Extended Forecast

The pattern should remain active after the Friday/Saturday system, with plenty of rain, wind, and mountain snow as moderate/strong systems rush through the area.

The 15-day snow totals from the Euro ensemble are off the charts, and ski areas in the Cascades should have enough snow to open by Thanksgiving. After last year’s ski season getting cut short by COVID, I can’t possibly express how happy this map makes me!

I’ll keep you posted on the Friday storm with updates on my Facebook and Twitter pages. This is definitely a one to watch!
Charlie