Avalanche Forecast

Issued: Mar 26th, 2013 10:13AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Cornices, Persistent Slabs and Storm Slabs.

ccampbell, Avalanche Canada

Purcells.

Summary

Confidence

Good

Weather Forecast

The ridge of High pressure is expected to continue to influence the interior ranges for the next few days. As the ridge weakens we may see high cloud reducing the exposure to strong solar radiation. Nights are expected to continue to be clear and cool. Freezing levels in the North should rise during the day to about 1800 metres, and to about 2000 metres in the South.

Avalanche Summary

Reports from Monday are limited to small natural wet loose snow avalanches on steep rocky sun-exposed slopes. However, the potential for large natural cornice releases, or other heavy human triggers, triggering very large slabs remains. A size 3 avalanche taking out the whole basin was triggered by snowmobilers Saturday in Hell Roaring Creek, just south of the region resulting in one fatality. Details are limited at this time; it may be indicative of the persistent slab problem (surface hoar and/or crust buried about 70 - 120 cm) we continue to describe.

Snowpack Summary

Surface faceting and surface hoar growth has been prevalent on shady slopes and at upper elevations, while the surface snow on sun-exposed slopes and at lower elevations has been subject to daily melt-freeze cycles. Cornices are reported to be very large and weakly bonded. Wind and storm slabs are reported to be strengthening, but may continue to be sensitive to human-triggers. The weak layer of buried surface hoar sitting on a crust down around 70-120cm, and has been producing varialbe but sudden results in snowpack tests. This deep persistent weakness remains a concern because of the potential for very large avalanches particularly with heavy triggers such as cornice falls. Old deep persistent weaknesses, including basal depth hoar, are a concern in shallow snowpack areas.

Problems

Cornices

Natural cornice falls are a concern during daytime warming. Cornices may also be triggered easily by traveling near them. Slopes with corniced entrances, or exposed to cornice falls should be avoided with these conditions.

Cornices become weak with daytime heating, so travel early on exposed slopes.>Extra caution needed around cornices with current conditions.>Give cornices a wide berth when travelling on or below ridges.>

Aspects: North, North East, East, South East.

Elevations: Alpine.

Likelihood

Possible - Likely

Expected Size

2 - 5

Persistent Slabs

Small solar triggered slides, cornice fall, or thin-spot triggering may overload a persistent weakness buried near 100 cm deep. This could lead to surprisingly large avalanches.

Be aware of the potential for large, deep avalanches due to the presence of buried surface hoar.>Be aware of thin areas that may propagate to deeper instabilites.>If your sled is bogging down, don’t spin the track and trigger the weak layer below.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

3 - 6

Storm Slabs

Wind slabs linger on lee and cross-loaded alpine and treeline slopes, while down-flowing katabatic winds may have created weak wind slabs in unusual places. Expect to see wet loose snow avalanches out of steep rocky sun-exposed slopes.

Avoid freshly wind loaded features.>Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 4

Valid until: Mar 27th, 2013 2:00PM