Avalanche Forecast

Issued: Apr 8th, 2017 4:16PM

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

cgarritty, Avalanche Canada

Purcells.

The upper snowpack will gain strength overnight, but watch for breaks in cloud cover to undermine slab and cornice stability on Sunday. Spring sunshine packs a punch.

Summary

Confidence

High -

Weather Forecast

Sunday: Mainly cloudy with scattered flurries and a trace to 5 cm of new snow. Light southwest winds. Freezing level to 1600 metres with alpine temperatures of -5.Monday: A mix of sun and cloud. Light southwest winds. Freezing level to 1700 metres with alpine temperatures of -4.Tuesday: Mainly cloudy with isolated flurries and a trace of new snow. Light southwest winds. Freezing level to 1700 metres with alpine temperatures of - 4.

Avalanche Summary

Reports from Friday included observations of Size 1 wind slabs releasing with ski cutting in the Golden area. One report from the central Purcells detailed evidence of a natural avalanche cycle to Size 2 in steep terrain as a result of new snow and warm temperatures. Loose wet avalanches were observed running to Size 1 in steeper terrain below 1900 metres.Reports from Thursday showed loose wet avalanches releasing both naturally and with skier traffic in steep terrain from Size 1-2. Isolated natural Size 1-2 wind slab releases were observed occurring in steep, rocky terrain as a direct result of wind loading.On Tuesday there was a natural cornice fall near Golden that failed to release a slab on the slope below.

Snowpack Summary

Recent moderate to strong winds have been redistributing 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 these deeply buried weaknesses remain a serious concern as warming temperatures continue to penetrate the snowpack at increasingly higher elevations.

Problems

Wind Slabs

Recent moderate to strong winds have shifted between a variety of directions and new wind slabs now exist on a wide range of aspects. Slabs may be more reactive where they lie above recently formed crusts or more isolated surface hoar.

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

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

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 2

Cornices

Cornices have grown large and may break off naturally due to solar exposure or daytime warming. Cornice falls were responsible for triggering a number of large persistent slab avalanches last week.

Avoid 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

Possible

Expected Size

2 - 3

Persistent Slabs

A cornice fall or smaller slab avalanche currently 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.

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 9th, 2017 2:00PM