Avalanche Forecast
Regions: Cascades - North West.
Use caution around open slopes greater than 35 degrees at upper elevations where you may still be able to trigger an avalanche within the recent storm snow. If the clouds do break, watch for snow surfaces to rapidly lose strength on sun-exposed slopes.
Discussion
Storm totals since Thursday are up to 2ft of new snow throughout the West North zone. On Saturday, a skier triggered a D1.5 avalanche in the Blueberry Chutes in the Baker Backcountry (N, 4800ft) that broke 14in deep within the recent storm snow. The skier was caught in the avalanche and carried 100ft downslope, partially buried, but uninjured. Observers also reported a handful of other small, triggered avalanches within new snow Saturday (observation), and numerous loose dry sluffs the past couple days. The most recent natural slab avalanches occurred Friday. Storm snow should continue to gradually settle and gain strength Sunday with mostly cloudy skies expected and a chance for another round of light snowfall. If Sunday's cloud cover forecast does not verify, and the strong March sun makes an appearance, be heads up. Whenever the next round of prolonged solar radiation arrives, recent cold storm snow will rapidly lose strength, and a wet loose avalanche cycle will likely occur on steep sun-exposed slopes.
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
You are most likely to encounter lingering storm slab instabilities at upper elevations. Continue to be leery of large convex or unsupported slopes greater than 35 degrees. If you witness obvious signs of unstable snow like recent avalanches, shooting cracks, or whumphing collapses, dial back your terrain to more conservative options. Loose dry sluffs have also been running far and fast in 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. Carefully evaluate recent storm snow layers as you travel, using hand pits and small test slopes. Gather plenty of information before venturing into bigger terrain.
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: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1