Avalanche Forecast

Issued: Mar 6th, 2020 10:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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On Saturday, you should see the avalanche hazard confined to more wind prone locations like higher ridgelines and the tops of peaks. If you travel near wind-exposed areas, keep your eyes open for signs of wind affected snow. Steer around smooth pillow-like drifts on steep slopes and in high consequence terrain.

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

An icon showing Wind Slabs

A weak storm on Friday and a few showers Saturday won’t bring much in the way of meaningful snow to the West-South zone. As a result, most avalanche concerns should be limited to windy locations where the snow may drift into slightly firmer and thicker slabs. If your travels take you higher in the terrain, near the tops of prominent peaks, or onto exposed ridgelines, be on the lookout for wind affected snow. Smooth pillow-like drifts and fresh cornices could clue you into where wind slabs may exist in the terrain. Steer around smooth lense-shaped deposits on steep slopes and in high consequence terrain. 

The new snow fell on a variety of snow surfaces Friday. As a result, you could see the new snow bond differently depending on aspect and elevation. On shaded slopes, the storm snow added a few more inches to a layer of soft settled snow. In locations that receive more direct sunshine and at lower elevations, the new snow accumulated on a firm melt-freeze crust formed earlier this week. If the sun comes out on Saturday afternoon, you could see the storm snow become moist and produce a few small avalanches running on this same melt-freeze crust. While there are several prominent layers and interfaces in the upper snowpack, most avalanche issues should be limited to the new snow and how it bonds to the old surface.

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

Valid until: Mar 7th, 2020 10:00AM