Avalanche Forecast
Regions: Cascades - North West.
A modest storm system should build new and reactive slabs Thursday night, especially in wind drifted snow and at higher elevations. Steer around convex rollovers, slopes below cornices, and steeper rocky terrain where you are more likely to trigger an avalanche.
Snowpack Discussion
March 5, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
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Ridgetop wind transport near Washington Pass. 3/4/20 Photo: Josh Hirshberg
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The beginning of March brought the first spring-like weather to the region. The days are getting longer and the sun (when it shines) has a noticeable effect on the snow. Alternating sun breaks and snowstorms is the theme of the week and the avalanche danger fluctuated just as fast as the weather. The southern zones and the east slopes have seen periods of light snowfall and significant sunshine while the northern zones and west slopes have had significant snowfall and occasional sun breaks. Above freezing temperatures were common at lower elevations throughout the region.Â
Last week in review: Temperatures were above freezing at most trailhead locations and concerns about the previous week’s buried surface hoar and weak layers were fading. Saturday temperatures plummeted as strong winds and new snow built wind slabs in most areas and drove the avalanche danger to considerable in almost every zone. Sunday’s calm weather and clearing in most areas helped stabilize wind slabs, but our attention turned to the strength of the sun and daytime warming. Lingering wind slabs and loose wet avalanches were forecast in every zone. For the remainder of the week, the avalanche problems in every zone were confined to the recent snow and each brief storm cycle had us thinking about wind slabs at upper elevations and loose wet avalanches from daytime warming and sunshine.
Spring isn’t actually here -- another round of cold temps and snow showers are on the way this weekend, but this past week’s oscillating weather is a good reminder to begin to shift our mindset. The sun is gaining strength, the days are getting longer, and we begin daylight savings this week. Even a brief period of sunshine can dramatically alter fresh, new snow. Check the mountain weather and avalanche forecasts for the most up to date info, enjoy the backcountry and let us know what you see out there!
--Peter Moore
Wet loose avalanche debris near Snow Lake. 3/5/20 Photo: Dallas Glass
Fresh cornice development from strong winds in the Stevens Pass backcountry. 3/4/20 Photo: Tom Whipple
Avalanche Problems
Storm Slabs
This isn’t expected to be a major storm, but snow combined with a little wind should be enough to create new and reactive slabs. Since most of this storm occurs Thursday night and early Friday, you could encounter the highest avalanche danger in the morning. Identify and avoid features where you are more likely to trigger an avalanche, such as steep wind affected slopes, areas near convex rollovers, and in more complex terrain. Gather information as you ascend by digging with your hand and using small slopes to test the snow. If you begin to see evidence of the snow failing as a slab, you found the avalanche problem, and it will only get larger and harder to avoid as you go higher. We’re expecting about 8 to 10” of new snow for the West North zone. If the storm exceeds expectations, or the area where you are traveling receives more snow, you could encounter slightly higher avalanche hazard.
This new snow will fall on a variety of snow surfaces. Observations from the past several days report crusts on most slopes below 4000’ and on sunny aspects at upper elevations. Higher shaded terrain still harbors cold dry settled snow. Because of this variety on the surface, you could see the snow bond differently as you change aspect and elevation. However, the primary avalanche concerns should be limited to the new snow and how it bonds to the old surface.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1