Avalanche Forecast
Regions: Cascades - West.
Modest amounts of fresh snow and decreasing wind will create generally small slabs that you can trigger on steep wind-loaded slopes. On the highest northerly slopes, new or older wind slabs could be large. Check small test slopes on similar aspects for firm or punchy layers beneath the snow surface before committing to larger, steeper terrain.
Discussion
We expect snow to taper during the morning hours on Friday and sunshine may materialize during the mid-day and afternoon hours.Â
When the sun comes out, change your aspect to avoid slopes receiving direct, strong sunshine as the surface snow will quickly destabilize to form small natural or human-triggered loose wet avalanches. Rollerballs and shedding trees portend this rapidly developing, but simple to manage, problem.
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
Wind Slabs
We’re expecting a weakening frontal system, but strong upper-elevation winds early in the storm should rapidly re-distribute a portion of the 2-6” of expected snowfall. This wind-affected snow may be covered by less wind-affected snow that falls toward the end of the storm. If visual clues don’t indicate where wind slabs have formed, you’ll need to use feel: probing the snow to the most recent interface for firmer layers with less dense layers underneath. Or use small slope tests to check the reactivity of the slabs. As you ascend higher in the terrain where the winds were stronger, slabs will be larger. In isolated areas, there may be a large older wind slab from earlier in the week that you could trigger. Steer around convex slopes steeper than 35 degrees, particularly under recently formed cornices, where you are most likely to trigger an avalanche. And tread more cautiously at the highest elevations.
This new snow will fall on a variety of snow surfaces. Observations from neighboring zones report that most slopes have crusts below 4000 ft and on sunny aspects. Higher shaded terrain still harbored cold dry settled snow. This could create a variable bond between the crust and the new snow.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1