Avalanche Forecast
Regions: Cascades - North East.
You can still trigger avalanches in areas of drifted snow above treeline near Washington Pass. if you see signs like cracks shooting through the snow or find areas of stiff snow that slide easily, avoid slopes 35 degrees and steeper. Cooling temperatures and cloud cover will reduce the chance for wet avalanches.
Discussion
For Friday, expect cooling temperatures, clouds with light snow, and southwest wind. The weather shouldn't bring an increase in avalanche danger. About 6-10" of snow fell above 5,000ft Tuesday night with periods of strong westerly wind. Since then, clear skies allowed the sun to form a crust on the snow surface at mid and low elevations, but wet loose avalanches have been kept to a minimum. Temperatures have stayed mostly below freezing at 6,000ft and higher.
Aside from recent storm interfaces, the snowpack near Washington Pass is strong and well-bonded. A rain event in early February formed a prominent crust up to at least 7500ft. You can still find it 3-5 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
Wind Slabs
Wind slabs are lingering at upper elevations near Washington Pass. Avalanches will be the biggest and easiest to trigger at the highest elevations. You may see some very shallow fresh drifts from ongoing wind and light snowfall. Use caution as you enter wind-exposed slopes, near ridgelines and open terrain. Steer around areas of recent snow that are thickly drifted or stiff feeling snow. Use small, inconsequential test slopes to check how these slabs are bonding. If you see signs of instability like shooting cracks or hear whumphing, avoid wind affected slopes 35 and steeper.
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.
Likelihood: Possible
Expected Size: 1 - 1