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Avalanche Forecast

Archived

Mar 8th, 2020–Mar 9th, 2020

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

Regions

Purcells.

Intense sun may make recent snow and cornices more sensitive to failure. Adjust travel to give steep, sun-exposed slopes and cornices a wide berth. A buried weak layer warrants a conservative mindset and terrain use strategy.

Confidence

Moderate - Uncertainty is due to the fact that persistent slabs are particularly difficult to forecast.

Weather Forecast

Sunday night: Mostly clear, light west winds, alpine temperature -14 C.

Monday: Mostly clear, light west wind, alpine high temperature -5 C, freezing level 1000 m. 

Tuesday: Increasing cloud, isolated flurries with trace accumulations, moderate southwest winds with strong gusts, alpine high temperature -4 C, freezing level 1400 m.

Wednesday: Mostly cloudy, scattered flurries with up to 5 cm of snow, decreasing west winds, alpine high temperature -4 C, freezing level 1400 m. 

Avalanche Summary

On Saturday, several human-triggered and explosive-triggered storm slab avalanches were reported. These avalanches were small to large (size 1-2) and breaking 25-40 cm deep. Shallow storm slabs have the potential to step down to the buried surface hoar layer, creating very large avalanches. Use added caution on sun-exposed slopes during intense solar radiation, as rapidly warming snow and weakening cornices could trigger loose or slab avalanches.

Over the past week, human-triggered and remotely-triggered avalanches releasing on the February 22 surface hoar have been consistently reported. In many instances, slopes were ski cut with no results, only to have the third or fourth skier in the group trigger the slab. These avalanches have propagated widely across terrain features, warranting conservative terrain margins (see this MIN for a helpful example). Observations extend to all aspects and elevations, but north to east aspects near treeline have been a hot-spot for reactivity. 

On Thursday, professionals reported large (size 2.5) explosive-triggered avalanches releasing on weak layers near the ground. These avalanches occurred on north and east facing slopes above 2300 m, and they confirm that the deep persistent slab problem remains a concern. 

Snowpack Summary

Snowfall amounts from the weekend storm ranged from 20-40 cm. Slab formation and human triggering will likely be specific to where the wind has transported the snow into deeper drifts. Intense solar radiation may rapidly destabilize recent snow on steep, sun-exposed slopes and weaken cornices.

A weak layer of surface hoar from February 22 may be found 35-70 cm deep. It may sit over a crust on solar aspects. With successive storms, more areas have reached critical loading and developed a slab over this layer. Riders will be capable of triggering this layer for some time until there is a substantial change in the snowpack. Read more about surface hoar on our forecaster blog.

Deep basal facets lurk near the bottom of the snowpack. This layer has been responsible for sporadic deep persistent slab avalanches, usually triggered from shallow, rocky start zones.

Terrain and Travel

  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • Use conservative route selection and resist venturing out into complex terrain.
  • Minimize exposure to sun-exposed slopes when the solar radiation is strong.
  • Stay off recently wind loaded slopes until they have had a chance to stabilize.
  • Avoid shallow, rocky areas where the snowpack transitions from thick to thin.

Problems

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.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.