Avalanche Forecast
Issued: Mar 10th, 2020 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Very strong winds and snow showers should be enough to increase the avalanche danger in the West South on Wednesday. Investigate slopes for wind-drifting before stepping into consequential terrain and be prepared for changing conditions if the sun appears and is able to warm snow surfaces.
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
Wind slabs may be present at upper elevations or on the volcanoes due to strong winds and new snow Tuesday night into Wednesday morning. If the storm continues to linger during the daytime hours, snow may start to pile up and increase the size of potential slabs. Look for these slabs to develop on specific terrain features such as leeward slopes, at the base of cliffs, or the sides of gullies. You can look for textured snow surfaces or pillow-like piles of snow to help you determine wind-loaded features that you should avoid. Hand-pits and small test slopes can give an indication of how the new snow is bonding to the old snow surface. If you find a poor bond or experience clues of unstable snow such as shooting cracks or whumpfing collapses as you travel, steer around wind-loaded terrain.
If the sun is able to make it through the clouds tomorrow and warms freshly fallen snow, Loose Wet avalanches may follow. If you notice the snow surface getting moist, fresh rollerballs, or snow falling off trees and rocks, head for slopes out of direct sunshine.
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
Expected Size
Valid until: Mar 11th, 2020 11:00AM
