Avalanche Forecast

Issued: Apr 5th, 2017 3:36PM

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

triley, Avalanche Canada

Purcells.

Cornice falls have been responsible for most of the avalanches reported in the past few days. Wind slabs or cornices may trigger persistent weak layers resulting in large avalanches.

Summary

Confidence

High -

Weather Forecast

Overnight: A couple of cm of new snow with moderate southwest winds and freezing down to 1000 metres. Thursday: 3-5 cm of new snow with strong southwest winds and daytime freezing up to 2000 metres. Friday: Re-freeze down to 1200 metres, followed by a mix of sun and clouds with 3-5 cm of new snow. Saturday: 5-10 cm of new snow with light winds and a cooling trend that should start to bring freezing levels down.

Avalanche Summary

On Tuesday there was a natural cornice fall near Golden that did not release a slab on the slope below. On Monday there were several natural cornice falls up to size 2.5 that released storm or persistent slabs from the northerly aspect slopes below. On Sunday there was a natural cornice fall size 2.5 from extreme terrain.

Snowpack Summary

10-15 cm of recent snow now overlies temperature crusts below about 1900 metres and sun crust all the way into the alpine on solar aspects. This precipitation fell as rain below about 1600 metres. Below the new snow interface, storms over the past week brought 40-60 cm of snow to the region. Several other crusts as well as moist snow are likely to exist within this storm snow, mainly at lower elevations and on solar aspects. Moderate to strong southwest winds during and since the storm formed wind slabs on leeward slopes as well as fragile cornices along ridgelines. At higher elevations, the February crust/facet layer is now down around 130-150 cm and the deep mid-December facet layer and November rain crust both still linger near the bottom of the snowpack. These layers were active during a storm in mid-March and produced some very large avalanches. Occasional deep releases were also reported in late March and these deeply buried weaknesses remain a serious concern as solar radiation and warming temperatures begin to penetrate the snowpack at increasingly higher elevations.

Problems

Cornices

Cornices are large and may fall off naturally due to loading from new snow, wind, or from daytime warming. Cornice falls in motion may release persistent slabs from the slopes below, resulting in large avalanches.

Do not travel on slopes that are exposed to cornices overhead.Pay attention to overhead hazards like cornices which could easily trigger persistent slabs.

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

Elevations: Alpine.

Likelihood

Likely

Expected Size

2 - 3

Wind Slabs

Recent southwest winds have redistributed storm snow into wind slabs at higher elevations that may remain reactive to human triggering. These slabs weaken and become increasingly reactive with daytime warming.

Use ridges or ribs to avoid pockets of wind loaded snow.Be careful with wind loaded pockets, especially near ridge crests and roll-overs.

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

Elevations: Alpine.

Likelihood

Possible - Likely

Expected Size

1 - 2

Persistent Slabs

A cornice fall or smaller slab avalanche could trigger large, destructive avalanches on deeply buried weak layers. The likelihood of this happening will increase as solar warming weakens unstable cornices and wind slabs over the course of the day.

Recognize and avoid runout zones.If triggered, wind slabs or cornices may step down to deeper layers and result in large avalanches.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

2 - 4

Valid until: Apr 6th, 2017 2:00PM