Avalanche Forecast

Archived

Mar 27th, 2017–Mar 28th, 2017

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Purcells.

Convective spring-like weather may bring localized higher snowfall amounts. Moderate winds continue to build reactive wind slabs at higher elevations on the leeward side of terrain features.

Confidence

Moderate - Intensity of incoming weather systems is uncertain

Weather Forecast

Tuesday: Unsettled conditions and convective air could bring 5-10 cm of new snow with mostly cloudy skies. Ridgetop winds will be moderate from the SW and freezing levels near 1700 m. Wednesday: Snow amounts 10-20 cm. Ridgetop winds light-moderate from the South and freezing levels near 1700 m.Thursday: Cloudy with possible sunny breaks. Ridgetop winds light from the SW and freezing levels near 1500m.

Avalanche Summary

On Sunday, large natural and explosive controlled slab avalanches were observed. A natural cornice fall produced a size 3.5 avalanche from a SE aspect and control work continues to produce several size 3 and a size 4 avalanche that ran to ground from a NW aspect. On Tuesday, the recent storm snow is expected to be reactive at higher elevations, especially in wind loaded terrain and on steep convex features. Cornices are large and may become weak with daytime warming or during stormy periods and have proven to still trigger very large avalanches from the slopes below. We are in a low probability, high consequence scenario for persistent slab avalanches failing on deep buried weak layers. Click here for more details. Click here for the Forecasters' Blog.

Snowpack Summary

20-40 cm of snow has now accumulated over the past week which overlies a widespread crust below around 2100 m and higher on solar aspects. On steep solar aspects, a new sun crust is also being reported. Alpine wind has recently been strong mainly from south through west aspects and has loaded leeward slopes in exposed terrain at treeline and in the alpine. Large cornices are also reported in the alpine. At lower elevations, there may be multiple crust layers in the upper snowpack which are now generally well bonded and stable. At higher elevations, the February crust/facet layer is now down around 90-150 cm and has been reactive with several avalanches recently releasing on it. It is expected to be most reactive in the alpine where the snowpack remains dry. The deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack and a few avalanches and cornice falls have stepped down to these layers recently resulting in very large avalanches. Heavy triggers like cornices and explosives continue to sporadically trigger deep weaknesses in the snowpack as recently as Saturday.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.