Avalanche Forecast

Issued: Apr 2nd, 2017 4:10PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Persistent Slabs, Wind Slabs and Loose Wet.

cgarritty, Avalanche Canada

Purcells.

The sun will be out in force on Monday. As it rapidly warms the snowpack, there will be an increasing likelihood of highly destructive persistent slab avalanches that run full path. Cornice falls are likely triggers.

Summary

Confidence

Moderate -

Weather Forecast

Monday: A mix of sun and cloud. Light northwest winds. Freezing level to 1600 metres with alpine temperatures around -5.Tuesday: A mix of sun and cloud. Light southwest winds. Freezing level to 1900 metres with alpine temperatures around -2.Wednesday: Mainly cloudy. Light to moderate southwest winds. Freezing level to 2300 metres with alpine temperatures around 0 to +1.

Avalanche Summary

One report from Saturday details a remotely triggered Size 1 wind slab avalanche that occurred on an east aspect at 2200 metres. This avalanche was noted to have released over a crust layer.Reports from Friday showed evidence of a natural, solar-triggered avalanche cycle in deeper snowpack areas of the region producing storm slabs releases to Size 2. (Two Size 2.5 and 3 slides were also reported.) One approximately 24 hour old Size 3 deep persistent slab release was also reported west of Invermere. This avalanche was triggered by a natural cornice fall and is suspected to have failed on the November crust.Reports from Thursday include observations of storm slabs releasing to Size 2.5 with explosives triggers. Explosives triggered cornices reached Size 3. Several slabs and/or cornice releases were noted triggering persistent slabs which then ran to ground. North aspects saw the majority of this activity.Looking forward, anticipate a stark change in the snowpack from morning to afternoon as solar warming breaks down surface crusts and promotes instability in a wide range of avalanche problems over the course of each day.

Snowpack Summary

10-15 cm of new 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

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.

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

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 4

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.Look for patterns of wind loading and avoid traveling near thinner edges of wind slabs.

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Loose Wet

Solar warming will progressively deteriorate the surface of the snowpack at lower elevations and on solar aspects. Snow that becomes moist will have the tendency to sluff from steep terrain, either naturally or with a human trigger.

Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.Avoid exposure to terrain traps where the consequences of a small avalanche could be serious.

Aspects: North, North East, East.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 2

Valid until: Apr 3rd, 2017 2:00PM