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Avalanche Forecast

Archived

Mar 31st, 2021–Apr 1st, 2021

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely.
Below Treeline
Natural avalanches unlikely.

Regions

North Rockies.

Flurries and strong wind feed fresh wind slab development at upper elevations, while sustained above-freezing temperatures make for moist snow below treeline on Thursday. Afterwards, easing winds and overnight refreeze will reduce avalanche hazard for the end of the week.

Confidence

Moderate - Confidence is due to a stable weather pattern with little change expected.

Weather Forecast

Wednesday night: Partly cloudy. Strong southwest wind. Freezing level 1500 m. Alpine temperature -7.

Thursday: Mostly cloudy with flurries up to 10 cm. Strong southwest wind. Freezing level 1200 m. Alpine temperature -10. 

Friday: Mostly cloudy. Light to moderate southwest wind. Freezing level dips to valley bottom overnight, up to 1200 m during the day. Alpine temperature -7. 

Saturday: Mostly cloudy. Light to moderate southwest wind. Freezing level dips to valley bottom overnight, up to 1400 m during the day. Alpine temperature -7.

Avalanche Summary

Evidence of large storm slab avalanches (size 3) was observed Monday and Tuesday between Sentinel Pass and Chetwynd, likely having released during the storm on the weekend.

On Sunday we received second-hand information about a size 2-2.5 (estimated) rider-triggered avalanche in the Hasler area.

This great MIN report from Saturday describes natural avalanche activity in Evanoff Provincial Park.

Last Thursday, there were numerous reports of size 2-2.5 explosive triggered avalanches in the Table River Valley.

Snowpack Summary

10-40 cm of recent snow has been redistributed by strong southwest to northwest wind, likely resulting in variable wind loading patterns, with wind slabs possible on many aspects and large, fragile cornices along ridgetops. This snow sits on melt freeze crusts at lower elevations and on solar aspects. Otherwise it adds to 30 to 50+ cm of settling dry snow from last week.

A weak layer of facets buried in mid-February may now be found around 120 to 150 cm deep, or shallower in the east of the region and in thin snowpack areas. The most likely place to trigger this layer is where the snowpack is shallow in alpine terrain. Large loads, such a cornice falls or multiple sleds on the same slope often trigger slopes that a single rider couldn't trigger on their own.

Terrain and Travel

  • Be careful with wind slabs, especially in steep, unsupported and/or convex terrain features.
  • Be aware of highly variable recent wind loading patterns.
  • In areas where deep persistent slabs may exist, avoid shallow or variable depth snowpacks and unsupported terrain features.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.