Avalanche Forecast

Issued: Mar 5th, 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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Look for pockets of wind affected snow near ridgelines and on other exposed features. Steer around steep slopes below cornices and smooth pillow-like drifts where you are most likely to trigger an avalanche.

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

Light snow Thursday night and Friday should combine with moderate winds to form small new slabs in more wind prone areas such as near ridgecrest, in passes, and on exposed features. Because the storm totals for this system are so modest, we expect the highest danger to be near the volcanoes and at upper elevations where you could encounter larger avalanches. Keep your eyes peeled for signs the wind may have drifted the snow into slightly deeper and firmer slabs. Fresh cornices and drifts behind trees can point out potentially wind-loaded areas. Steer around steep slopes below cornices, smooth pillow-like drifts, and more complex terrain where you are most likely to trigger a wind slab. As more snow falls through the day, it could hide some of the obvious visual clues of wind affect snow making it harder to spot and avoid. 

The new snow will fall on a variety of surfaces that developed over the past few days. Lower elevation and sunny aspects formed firm crust during several days of sunshine and warm temperatures. Shaded slopes continued to harbor cold soft snow or a firm freezing-rain crust depending on location. Expect the new snow to adhere differently to the old surface depending on aspect and elevation. Avalanche concerns 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 6th, 2020 10:00AM