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Avalanche Forecast

Archived

Mar 29th, 2021–Mar 31st, 2021

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

Regions

North Rockies.

Wind slabs may be found on many aspects due to recent variable winds. Back off if you encounter whumpfing, cracking, or hollow sounds.

At this time of year, even brief periods of direct sun can trigger avalanches, especially when there is a lot of fresh snow.

Confidence

Moderate - Uncertainty is due to how quickly the snowpack will recover and gain strength.

Weather Forecast

Monday night - Clear periods / moderate to strong northwest wind / alpine low temperature near -16 

Tuesday - Mainly cloudy with flurries / strong northwest wind / alpine high temperature near -5 

Wednesday - A mix of sun and cloud / strong southwest wind / alpine high temperature near -2 / freezing level 1700 m 

Thursday - Cloudy with flurries, 5 cm / strong southwest wind / alpine high temperature near -2 / freezing level 1700 m 

Avalanche Summary

With plenty of new snow to move around and moderate to strong winds in the forecast, wind slabs are likely to be found and may be reactive to human triggers. 

There have only been a few avalanche reports in recent days, though with up to 40 cm of new snow on the weekend, it is very likely that a natural avalanche cycle occurred, especially in the areas that saw the highest snowfall amounts.

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

On Saturday there was a report outlining natural avalanche activity in Evanoff Provincial Park. A great MIN report about it can be viewed here.

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

A widespread natural avalanche cycle with avalanches up to size 3 was reported last Wednesday.

Snowpack Summary

The North Rockies received anywhere from 10-40 cm of new snow over the weekend with strong south to southwest winds. Winds became strong out of the northwest on Monday, likely resulting in variable wind loading patterns, with wind slabs possible on many aspects.

The new snow has buried melt freeze crusts at lower elevations and on solar aspects and otherwise adds to 30 to 50+ cm of settling dry snow from last week. The new snow may be particularly reactive where sits on a crust.

Cornices have also become very large along ridgelines and always have the possibility of failing naturally or from the weight of a human. Direct sun and rising temperatures can significantly weaken cornices.

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

  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Be aware of highly variable recent wind loading patterns.
  • Avoid exposure to steep, sun exposed slopes, especially when the solar radiation is strong.
  • Be aware of the potential for large avalanches due to the presence of a persistent slab.

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.