Avalanche Forecast
Regions: Cascades - North East.
While avalanches are unlikely, you should continue to use normal caution while traveling in the mountains. Large cornices hang over most ridges. Expect cooling temperatures and partly cloudy skies for Saturday.
Discussion
Saturday will bring cooling temperatures and light snow following a dusting of snow from Friday. There may be enough sun to warm the surface of sun-exposed slopes. You can find drifts near and above treeline that formed with about 10 inches of snow and strong westerly winds on Tuesday night into Wednesday. Avalanches will be difficult to trigger in these drifts. In isolated areas above treeline, you may still find a poor bond where a slick crust is found at the interface that is 1-1.5 feet below the surface. If you are venturing into remote terrain outside of the Highway 20 corridor, take time to assess the snowpack, as you may find different layering.Â
Snowpack Discussion
March 5, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
Â
Ridgetop wind transport near Washington Pass. 3/4/20 Photo: Josh Hirshberg
Â
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
Cornices
Large cornices loom on most high ridges from many westerly wind events this winter. While unlikely, a chance for cornice fall remains. Observers have reported sporadic cornice fall, with the most recent being a week old. Use caution to not travel on or under overhung cornices. Be aware of overhead cornices even when you are well-below ridge-tops. If direct sun shines on cornices for a significant portion of the day, consider avoiding underlying terrain.
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: All aspects.
Elevations: Alpine.
Likelihood: Unlikely
Expected Size: 1 - 1