Colder Weather Arrives Friday, First Mountain Snows, and Critical California Wildfire Danger

The Pacific Northwest isn’t known for its rapidly changing seasons. We don’t exactly plunge into winter like the Northern Plains, and our slow trudge towards summer out of the depths of January is even slower. June is the hottest month of the year for the Desert SW, but many years, while Death Valley is sweltering in 120 degree heat, the Pacific Northwest is still stuck in “Juneuary.”

But if there’s one time of the year our weather does change substantially, it’s the month-and-a-half period from early October through mid-November. On October 1, Portland’s record high is 92. Our weather cools and often becomes noticeably wetter the 3rd/4th week of October – in fact, today is the 17th anniversary of Seattle’s wettest day on record, when a staggering 5.02 inches of rain fell at Sea-Tac Airport. By mid-November, Mother Nature can even hurl major, early-season lowland snowstorms our way. Many of the coldest days in Pacific Northwest history occurred during an extreme, mid-November arctic outbreak in 1955. And one of my favorite weather memories is the surprise snowstorm of November 22, 2010, which occurred just 19 days after Seattle reached an astonishing 74 degrees under bluebird skies.

In the spirit of 2020, some weather models showed some truly outlandish October snowstorm solutions for Western Washington and NW Oregon this past weekend. Inevitably, such wacky model solutions naturally made their way through social media.

When the GFS shows Seattle snow in October 😉 #wawx #orwx #wxtwitter pic.twitter.com/KGctlPBIiR

— Charlie Phillips (@GeoduckChuck) October 20, 2020

If there’s one thing I’ve learned in my years as a weather forecaster, it’s that forecast communication is far more important than the forecast itself. Most weather forecasts inside 120 hours are quite good, and though there are still the occasional forecast busts, more of these can be attributed to a failure of meteorologists to communicate the impacts of the storms rather than a failure of the models themselves. This is especially true with our varied terrain, where several inches of heavy, wet snow can fall within a quarter-mile of the Columbia River Gorge while locations a few city blocks outside the Gorge see 33-degree rain. Such an event happened on February 9, 2019, when Portland Airport received 4.9 inches of snow but most areas in the metro area (including NE Portland just south of the airport/Columbia River Gorge) saw rain.

As expected, these nutty model solutions soon fizzled, but we’re still expecting a pattern change to much cooler weather this weekend, with heavy early-season snow likely in the Cascades. Let’s take a look.

Current Conditions:

The current national temperature map looks very typical of mid-late October, with chilly weather over the Northern Plains and Rockies as an early-season Canadian trough brings cold, arctic air into the Lower 48. The southern tier of the continental US is warm, but with the winter solstice only 2 months away, there simply isn’t enough sunshine to bring temperatures to the triple digits

The upper-level pattern shows the broad trough over Canada and the Northern US. If you look carefully, you can also see the beginnings of a shortwave trough forming in the NE Gulf of Alaska near the northern Alaskan Panhandle.

Nestled between this broad, inland trough and a flat ridge of high pressure in the Northeast Pacific is a powerful, 140+ knot jet stream centered over Vancouver Island and extending ESE into SE Montana.

Forecast:

Tomorrow, the developing trough currently near the Alaskan Panhandle will strengthen further and ride into Western Washington along this powerful northwesterly jet. This should give the first significant snow of the season for the Washington Cascades.

The charts below show the predicted precipitation and snowfall from 5pm today to 5pm tomorrow (Wednesday). With strong WNW flow, both the Cascades and Inland Rockies will see significantly higher precipitation on their windward slopes and strong rainshadowing over their lee slopes. The WNW flow may also create a Puget Sound convergence zone, enhancing precipitation in Northern King/Southern Snohomish Counties and bringing heavier snowfall to Stevens and Snoqualmie Passes.

My guess is that the Washington Cascades above 2,500 feet will see 2-4″ of snow out of this, with locally higher amounts over the volcanoes and in the central Cascades where a convergence zone may form.

Thursday will be a chilly “day between storms,” with Portland likely only reaching the low 50s. Some patchy frost is even possible in outlying areas Thursday morning.

More impactful weather arrives Friday. A robust, deepening upper-level trough will move south from BC and an associated surface low will develop offshore and make landfall along the Central WA/Northern OR Coast.  The upper-level trough will bring a reinforcing shot of cold, continental air into the Pacific NW, and the surface low will spread precipitation inland.

Locations to the south of the surface low will see warm southerly winds, but locations to the north will see even stronger northerly winds/cooler temperatures than if there was just this upper-level feature without a surface low to enhance low-level pressure gradients.

If this event was occurring a month later, we’d be talking about a serious chance of snow north of Seattle. Our temperatures aren’t nearly cold enough for lowland snow this time around, but snow levels in the Cascades could drop to 2,500 feet in Oregon and 1,500 feet in Washington Friday night/Saturday morning as this system comes through, resulting in heavier and more widespread snowfall than tomorrow’s event. And as the image below shows, most of the Columbia Basin is expected to pick up an inch or two as well.

Saturday evening through Monday will be chilly and dry. Sunday and Monday mornings will feature widespread sub-freezing temperatures and frost and will be our coldest mornings of the year thus far. You will notice the cold Sunday and Monday mornings!

Extreme California Fire Danger Sun/Mon:

Unfortunately, the same pattern that will give us our early-autumn cold spell may also give extreme fire danger to California. Much of Northern California is under Red Flag Warnings through tomorrow (Wednesday) morning for warm/dry/windy offshore flow, and I suspect that additional Red Flag Warnings will be issued for the same areas from Wednesday night-Friday afternoon as offshore flow picks up once again.

The events this week are just “garden-variety” autumn events – fire danger is commonly critical in California in the autumn. Sunday/Monday’s setup is more of a classic setup for strong Santa Ana/Diablo Winds, with a deep trough of low pressure over the Intermountain West. This causes cold, dry air to build up over the Great Basin and form a dome of high pressure there, creating extremely strong offshore gradients between the Great Basin and the California Lowlands.

When this air races through small gaps in the Sierra Nevada and descends from the 5000-6000 foot Great Basin all the way to sea level, it can warm by as much as 25-30 degrees while maintaining dewpoints in the single digits, sending relative humidities to extraordinarily low values… as low as 1 (yes, one!) – 5% in extreme cases. Couple this with prolonged wind gusts 50-70+ mph and you have yourself a recipe for extraordinarily high fire danger.

The below graphics show the forecast dewpoints, temperatures, and pressure/winds at 11am Monday 10/26/2020. These are extremely critical conditions (widespread relative humidities below 10%, wind gusts to 40 mph in lowlands with double that in the favored mountains/passes). If this forecast holds, I would be surprised if PG&E, SoCal Edison, SDG&E, and other California utilities didn’t implement Public Safety Power Shutoffs to prevent the risk of energized lines experiencing damage due to high winds and subsequently starting wildfires.

To recap:

• A cool system Wednesday will bring the first significant snowfall of the year to the Washington Cascades.
• An even stronger system Friday/Saturday morning will bring heavier snowfall and even cooler temperatures. Again, most snow will be over Washington, but Government Camp could receive a few inches from this.
• Expect very cool temperatures this weekend, with sub-freezing temperatures Sunday/Monday mornings.
• Confidence is increasing in a major Diablo/Santa Ana wind event for California Sunday/Monday, but details still uncertain.

Thanks for reading, and have a great rest of your week!
Charlie