First Snow of the Season for the Lower Passes

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.

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

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!

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.

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.

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.

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