Avalanche Forecast
Issued: Mar 7th, 2020 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Pay attention as you travel into higher elevation and more wind-exposed terrain. Look for areas where the wind affected the snow creating a textured surface. You may find wind loaded areas below cornices and small pillow-like drifts that you can avoid.
Summary
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
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.
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
Problems
Wind Slabs
Snow showers the past few days kept adding a few inches here and a few inches there to the snowpack around the West-South. Most areas picked up between 4 and 6” of snow since early Friday morning. The new snow fell with cooling temperatures and light winds. As a result, unstable storm snow should be limited to windier locations such as higher peaks and exposed ridgelines. If you travel in more wind prone areas, look for smooth pillow-like drifts and steep slopes below fresh cornices that you’ll want to avoid. Observations Friday and Saturday found wind slabs were thin, small, and localized to terrain near ridgecrest.
You can feel the changing weather and the March date on the calendar reminds us that the snowpack is becoming more aspect and elevation dependent. Higher shadier slopes continue to hold a soft, dry, and layer snowpack. On sunnier aspects, frim crust form overnight and may soften during the day. The new snow seemed to bond well to these old surfaces. If you dig in the snow, you may find several interesting interfaces formed in February, however, our major avalanche concerns continue to be confined to the new snow. On Sunday, we think you will experience mostly cloudy skies, but if the sun comes for longer periods of time, you could encounter new rollerballs and small loose avalanches. If you notice the sun causing the surface snow to become wet and sticky, steer away from steep sunny slopes.
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
Expected Size
Valid until: Mar 8th, 2020 11:00AM
