Avalanche Forecast

Issued: Apr 13th, 2017 3:00PM

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

triley, Avalanche Canada

Purcells.

New storm slabs have developed at higher elevations. Watch for changing wind directions resulting in reverse loading.

Summary

Confidence

Moderate - Wind speed and direction is uncertain on Friday

Weather Forecast

Overnight: 5-10 cm of new snow with moderate northwest winds and freezing down to 1000 metres. Friday: Mix of flurries and broken skies with moderate-strong westerly winds and daytime freezing up to 1500 metres. Saturday: Mix of sun and cloud with moderate west winds and daytime freezing up to 1500 metres. Sunday: Mostly sunny with light winds and daytime freezing up to 2000 metres.

Avalanche Summary

No new reports of avalanches on Wednesday. Natural cornice falls to size 2.0 and explosives control to size 3.0 were reported from the central part of the region on Tuesday. Explosive control and natural triggers released cornices up to size 3.0 in the central part of the region on Monday. I suspect that wind slabs were touchy on Thursday on slopes lee to easterly winds. Forecast westerly winds may result in reverse loading on Friday.

Snowpack Summary

Recent moderate to strong winds have redistributed new snow into wind slabs on a range of aspects at higher elevations. Fragile new cornice growth also occurred along ridgelines over the course of the week. The new snow has buried melt-freeze crusts on all aspects below about 2200 metres and in the high alpine on solar aspects. Isolated surface hoar may be found below the new snow on shaded aspects at high elevations. Below the new snow interface, a number of storm snow and crust layers that formed over mid to late March appear to be well bonded. 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 early April, keeping these layers an ongoing concern.

Problems

Wind Slabs

New wind slabs have formed on slopes in the lee of easterly winds at higher elevations. A forecast shift to westerly winds may result in some reverse loading. Watch for pockets of wind transported snow on all aspects.

Keep an eye out for reverse loading created by shifting winds.Be especially careful with wind loaded pockets near ridge crests and roll-overs.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3

Cornices

Cornices continue to be reported as large and fragile. Natural cornice falls have been occurring on a regular basis. Due to changing wind directions, give cornices a wide margin of safety.

Pay attention to overhead hazards like cornices which could easily trigger persistent slabs.Avoid travel on slopes that are exposed to cornices overhead.

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

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

2 - 3

Persistent Slabs

A cornice fall or smaller slab avalanche has the potential to trigger large, destructive avalanches on deeply buried weak layers. This is more likely to occur in thin or variable snowpack areas at higher elevations, and when the sun is strong.

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

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

2 - 4

Valid until: Apr 14th, 2017 2:00PM