Avalanche Forecast
Regions: Snoqualmie Pass.
Pay attention as you begin to crest onto the ridgelines or if you travel to the tops of the higher peaks around the Pass. You could encounter a few pockets of wind deposited snow in more wind prone locations. Steer around smooth pillow-like drifts on steep or 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
Wind Slabs
We keep adding a few inches here and a few inches there to the upper snowpack. None of these storms has been or is expected to be very impactful and that’s allowed the snowpack to gain strength and the danger to drop. The avalanche hazard should be limited to a few specific locations where the wind drifted the new snow into new small wind slabs. You could find these in windy locations such as higher ridgelines, near the summits of prominent peaks, or in complex exposed terrain. Take time to carefully evaluate steep lines for signs of wind affected snow before committing to the slope. Smooth pillow-like drifts and sharp cornices could be your most reliable clues that a wind slab may exist in the terrain.
As of Friday afternoon, only a few inches of new snow had accumulated at mid and upper elevations around the Pass. Areas below treeline experienced very light rain. The storm snow accumulated on an aspect dependent snowpack above 4000’. On shaded slopes, the snow added to a layer of dry settled powder over a prominent ice crust formed early last week. Sunny aspects consisted of a firm melt-freeze crust over the same firm icy lense. This could result in the new snow bonding differently to the old surface by aspect and elevation. If the sun comes out on Saturday afternoon, you could see new rollerballs or a few loose avalanches running on the old surface, especially in the near treeline band. The avalanche hazard should be limited to the upper snowpack above the first prominent crust you find.
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