Avalanche Forecast

Archived

Mar 25th, 2017–Mar 26th, 2017

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Purcells.

Remember snow can lose strength very quickly on sun exposed slopes. Watch for overhead hazards and stick to supported terrain with low consequence.

Confidence

Moderate - Timing or intensity of solar radiation is uncertain

Weather Forecast

SUNDAY: Sunny in the morning then clouding over in the afternoon, light south wind, alpine temperature around -5 C.MONDAY: Cloudy with sunny periods, light southwest wind, alpine temperature around -5 C.TUESDAY: Cloudy with sunny periods, light southwest wind, alpine temperature around -5 C.

Avalanche Summary

On Friday, cornice control produced 2 cornice chunks on northeast alpine ridges. One triggered a size 1 slab and the other entrained some loose snow. A natural size 1 wind slab and cracking under skies was reported in the central part of the region. A few larger avalanches were reported midweek, including a natural size 3 avalanche on a northeast aspect at 2500 m, an explosive triggered avalanche that failed on the ground down 100 cm, and a natural size 4 deep persistent slab that failed on a broad ridge feature on south through north aspects.On Sunday, morning solar radiation is a potential trigger for cornice falls and deeper weak layers. Read the forecasters blog (here) for advice on how to manage this low probability, high consequence scenario and check out the accompanying photos (here). Also expect wind slabs in the lee of exposed terrain at higher elevations.

Snowpack Summary

10-20 cm of new snow overlies a widespread crust below around 2100 m and higher on solar aspects. 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 and appear to be largest on northerly aspects. At lower elevations, there are a couple rain crusts in the upper snowpack. 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.

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.