First Snow of the Season for the Lower Passes

Paradise Ranger Station, Looking East from Jackson Visitor Center. Elevation: 5,400

We got a taste of winter in mid-September, when an unseasonably cool upper-level trough dropped several inches of the white stuff to Timberline, Paradise Ranger Station, and many other locations in the Cascades above 5,500 feet. But an even chillier upper-level trough is currently over the area and will direct cool, moist and unstable air into the region as it moves east, bringing up to a foot of snow to Snoqualmie Pass and much, much more in the higher elevations of the Central Oregon Cascades.

We currently have a textbook upper-level pattern for mountain snow, with a large ridge near 155W and a trough over the Pacific Northwest.

500 hPa height anomalies at 5 am PDT, October 11 2017
500 hPa height anomalies at 5 am PDT, October 11 2017

Notice how much cooler the 850 hPa (approx. 5,000 feet) temperatures are compared to average.

850 hPa temperature anomalies at 5 am PDT, October 11 2017

Whenever one of these upper-level troughs comes over our area, the temperatures at the surface decrease far less than the temperatures in the upper atmosphere due to the moderating effect of the Pacific Ocean. This results in a large decrease in temperature with height, giving us instability and the “showers and sunbreaks” pattern Pacific Northwesterners know and love.

The famous Puget Sound Convergence Zone commonly forms during this pattern, particularly in the hours immediately following a frontal passage. As the satellite image below shows, the PSCZ was out in full force yesterday!

Visible satellite Tuesday afternoon (10/10/2017) as seen from MODIS sensor aboard polar-orbiting AQUA satellite.
Visible satellite Tuesday afternoon (10/10/2017) as seen from MODIS sensor aboard polar-orbiting AQUA satellite showing the tail end of a cold front, a Puget Sound Convergence Zone, and scattered showers. I must admit – I got a little sentimental looking at this image, as I now live in Portland and miss my beloved PSCZ dearly.
Credit: NASA

This instability can be quantified using CAPE (convective available potential energy), which is the integral of the buoyancy of a parcel between the level of free convection (LFC) and equilibrium level (EL). For more on CAPE and all those other fancy words, click here.

Surface-based CAPE valid 11 AM PDT 10/11/2017
Surface-based CAPE valid 11 AM PDT 10/11/2017

With marginal CAPE around (200-300 Joules/kilogram), there’s the potential for some weak thundershowers in Western Oregon today, especially on the coast. Because the air aloft is so cold, some of these storms could contain ice pellets or small hail. And finally, since this airmass is unstable, any source of lift (such as uplift from the Cascades) greatly enhances clouds and snow there. 😉

You can bet your bonnet that we’ll see buckets of snow in the mountains over the next 72 hours. The UW WRF-GFS shows some locations in the north/central Oregon Cascades getting over 4 feet between now and Saturday morning! Even Snoqualmie Pass at 3,022 feet will get in on the action, with up to a foot possible there.

72-hour snow accumulation from 5 am Tuesday 10/11/2017 to 5 am Friday 10/14/2017
72-hour snow accumulation from 5 am Tuesday 10/11/2017 to 5 am Friday 10/14/2017
Credit: UW Atmospheric Sciences

The Extended:

We’ll dry out and warm up for the weekend and Monday as a weak ridge builds over the area, but next week looks downright stormy as a strong jet stream races across the Pacific and plows directly into our area.

500 mb heights, wind, and vorticity at 5 pm Wednesday October 18 2017
500 mb heights, wind, and vorticity at 5 pm Wednesday October 18 2017

I don’t see any particularly notable storms from this setup – just a series of healthy low pressure systems that promise to bring the trifecta of winter weather we all love: winds, rain, and mountain snow, with snow levels again lower-than-usual for this time of the year.

Thanks for reading, and have a great Wednesday!
Charlie

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4 Comments

  1. Great post Charlie! Great weather up there in the Pac NW mountains now. Can’t wait to see how big a dump key places get. I have a friend who has been doing extensive traveling with his lovely wife and just passed through Crater Lake. He was fascinated by the long term snowfall record at the COOP station at Park HQ there. There has been a general trend downward through the 20th century and into the 21st He has a theory to explain this drop, and it isn’t due to global warming. Fascinating! Thanks for this great post Charlie!

  2. Thanks Kevin! Yup, tons of snowfall for this early in the season. Timberline at Mt. Hood even opened a chair for one day!

    I’d like to hear his theory! I believe there’s been a general downward trend throughout the Washington Cascades as well. I wonder if El Nino/La Nina play a role… there were far more La Ninas (good for snow here) in the 1950s-1970s and there were far fewer to end the 20th century.

    http://appinsys.com/globalwarming/ENSO_files/image008.jpg

  3. Thanks Charlie. My friend is still driving out west with his wife so he won’t be able to shed any light about his theory on the decline of Pac NW snowfall (Particularly focus of study are the Crater Lake OR long time snow records). One thing I have been looking for over the past few years are Pac NW climate records for January 1950. As I understand it, it was a really cold and snowy month. NWS/NCEI doesn’t have any records for the month for Washington State sights, and I believe Oregon as well. The limited amount of data I have seen of low, normally milder spots like Portland and Seattle even had several very cold days and significant snows. Meanwhile here in Maryland and through much of the Eastern US we had a record warm month, with temps reaching 80 on a few days near the end of the month at many sites. Must have been quite an amplified pattern of trough in the NW and ridge over parts of the Eastern US. Fascinating!
    Stay well Charlie,
    Kevin

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