Early season cold air outbreak to bring widespread snow to the PNW this weekend

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The calendar reads October 20 but the upcoming weather pattern in the PNW is more typical of mid-winter. In fact, it looks like an unusual widespread October snowstorm will occur Friday into Saturday in the mountains and eastern Washington, with the chance of at least some snowflakes in the lowlands west of the Cascades. Here’s the details.

The 500 hPa height model forecast for Friday afternoon shows a setup that is sure to excite winter weather fans. The most identifiable feature in the image below is a sideways-tilted coastal trough over British Columbia. As is typical, this pattern is also associated with a high amplitude ridge over Alaska.

500 hPa anomaly map for Friday October 23, 5 PM PDT, map by Alicia M Bentley

The position of the Alaska ridge/BC trough allows arctic air to flow southward into interior portions of British Columbia and eastern Washington. The presence of arctic air on Friday can easily be seen on the 850 hPa temperature anomaly forecast map below–anomalies of up to three standard deviations below the climatological normal.

850 hPa anomaly map for Friday October 23, 5 PM PDT, map by Alicia M Bentley

This is very cold, but not record-breaking cold air. We can look at plots from NOAA’s National Centers for Environmental Prediction (NCEP) to see how frequently such cold air occurs this time of year. The percentile plots below suggest that at lower levels the airmass will be in the 0.5th percentile for this time of year (i.e. 99.5% of periods are warmer). I chose Saturday night (one day after the above plots) to show the time period after the cold air has more time to move farther south into Washington.

Forecast temperature climatological percentile (%) at various pressure levels for 5 PM Saturday Oct 24.

In terms of return interval, this is a 10+ year event at lower levels (look closely at 850 hPa in eastern Washington).

Forecast temperature return interval at various pressure levels for 5 PM Saturday Oct 24.

So there’s clearly no shortage of cold air on the way, and these type of anomalies in late October are plenty cold for mountain snow.

Of course to get snow we also need a weather disturbance to bring in some moisture and lift. This is where the models really show an exciting setup. The UW-WRF run from Tuesday morning (12Z) has a surface low dropping southeast into the region and deepening to 1010 hPa as it approaches the coast between Hoquiam and Astoria.

UW WRF 12km sea-level pressure and temperature forecast for 2 PM PDT Friday Oct 23, 2020

The reason this low position is so great for snow is that it creates a strong near-surface pressure gradient between the cold, dense arctic air to the east of the Cascades and the warmer, less dense maritime airmass that is usually west of the Cascades. So cold air is fed through the gaps through the Cascades and most famously the Fraser Gap in southern BC.

Sometimes these type of storms lack moisture to bring significant snowfall, but that is not at all the case with this one. Plenty of moisture will be advected anticyclonically around a surface high offshore.

GFS model forecast precipitable water for 2 PM PDT Friday 10/23/2020

Everything we’ve seen so far suggests this is a slam dunk for a decent snowfall in the mountains, as well as in eastern Washington. The UW-WRF snowfall forecast (through 5 PM Friday) does indeed show a widespread 6″+ snowfall in the Cascades above 2,000 ft.

UW-WRF accumulated snowfall forecast through 5 PM Friday. More snow can be expected after 5 PM Friday, especially in far eastern WA.

Another notable feature of the above map is the lack of a snow “shadow” on the east side of the Cascades in the Yakima/Ellensburg/Wenatchee areas. As seen in the surface map two images above, the easterly flow induced by the low pressure system will favor upsloping on the east side of the mountains. So I expect there will be considerable snow and travel impacts Friday into Saturday in those areas. Snow will also fall along a large stretch of the I-90 corridor from the Cascades to Idaho.

What about the western Washington lowlands? As snow fans know, it’s very difficult to bring the snow level down to sea level in the Seattle area, even in the middle of winter. So to get accumulating snow in Seattle on October 23-24 would be quite an achievement. In fact, accumulating snow has only ever been observed once in Seattle in October, 2.0 inches on 10/27/1971.

I highly doubt that record will be challenged this year, as even the ‘goldilocks’ 12Z WRF run on Tuesday morning generally keeps the snow level above 1,000-2,000 ft, except possibly near Bellingham.

What about the ensembles? About a quarter of the ECMWF ensembles currently show a trace of snow at Sea-Tac on Friday evening. So we can’t entirely rule out a few snowflakes near or below 500 ft elevation on Friday night. Enough to dust the Issaquah Alps, perhaps. Snowfall odds are somewhat higher closer to the Canadian border.

ECMWF ensemble snow accumulation for Sea-Tac airport

It would be no fun to end this blog with such a disappointing snow map. So here’s the ensemble forecast for Wenatchee, where the ensemble mean is currently around 6 inches.

ECMWF ensemble snowfall accumulation for Wenatchee (Pangborn airport)

Best to avoid travel through the passes on Friday!

2 comments

  1. So, if I walked out on our back deck Monday morning, in the dark before dawn, and heard a hissing sound (not rain, not hail, not corn snow), and walking on the deck made a distinct crunch, crunch– was that sleet or frozen rain? (About 4 mm diameter, and air temp was about 36 degF) The frozen balls were not white, and would melt quickly in the air, so I couldn’t make a visual determination between hail and frozen rain. The overall impression made me think of miniature glass tree ornaments, filled with water!

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