Avalanche Forecast
Regions: Cascades - North West.
Plan for deteriorating weather and avalanche conditions as small wind slabs build and become more reactive as the day progresses. As the winds and snow increase, note and avoid areas of active loading near and above treeline, particularly on slopes steeper than 35 degrees. Test the reactivity on an older, gradually healing mid-storm layer more than 1 ft down before committing to larger, very steep or poorly supported terrain features.
Discussion
Mt. Baker experienced loose wet avalanches running good distances on steep slopes, both on Sunday and Monday, with less reactivity on Monday due to cooler temperatures and increasing light winds.
Several slab avalanches were triggered on Saturday and Sunday in the West North zone (observation, observation). Most avalanches failed on density breaks within recent storm snow, and a couple broke deeper on the old/new snow interface buried Thursday. The majority of avalanches were triggered on convex slopes steeper than 35 degrees in northerly, upper elevation terrain. Several loose wet avalanches also ran naturally late in the day on south and west aspects.Â
Large cornices also loom on most high ridges. Use caution to not travel on or underneath overhung cornices.
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Snowpack Discussion
March 5, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
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
Increasing small wind slabs will gain size and strength over older storm snow or crust layers on Tuesday. As the storm intensifies, avoid actively loading slopes steeper than 35 degrees and those under fresh cornices where you can trigger wind slab avalanches. Punchy or styrofoam-textured snow tells you that you're moving through unstable snow. Routinely check the bond at the new/old snow interface. Small weak snow grains (surface hoar) have been observed at the surface prior to the storm and if preserved, they can produce more widely propagating avalanches.
Older storm layer instabilities have not produced avalanches since Sunday, but remain on our mind. If instabilities linger they remain at upper elevations. Continue to be leery of large convex or unsupported slopes greater than 35 degrees, looking for obvious signs of instability such as recent avalanches, shooting cracks, or whumphing collapses. Evaluate recent storm snow layers as you travel, using hand pits and small test slopes.
Loose-dry sluffs will continue running fast and far in wind-protected steep terrain. Even a small loose dry avalanche could have serious consequences if it were to carry you into trees or rocks or pile up debris in confined terrain.
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