Avalanche Forecast

Issued: Mar 3rd, 2019 4:42PM

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

cgarritty, Avalanche Canada

Purcells.

The region holds a tricky mix of long-lived wind slab and persistent slab problems, with new snow instabilities to manage in the southwest of the region. Be mindful of how this mix changes by elevation and by location.

Summary

Confidence

High -

Weather Forecast

Sunday night: Mainly clear. Light east winds.Monday: Sunny. Light east winds. Alpine high temperatures around -12.Tuesday: Sunny with cloudy periods. Light east or southeast winds. Alpine high temperatures around -10.Wednesday: Mainly cloudy with isolated flurries and a trace of new snow. Light east or southeast winds. Alpine high temperatures around -9.

Avalanche Summary

Observations from the past couple of days included several storm slab releases to size 1.5 in an area of the southwest of the region that saw 25 cm of new snow from localized convective snowfall. Loose dry releases to size 1 were also observed, owing to the low density character of the new snow. Two 40-60 cm deep persistent slab avalanches were also observed on south and west aspects at around 2400 metres.On Wednesday, numerous natural wind slab avalanches up to size 2 and several human triggered size 1-1.5 releases were reported on primarily south aspects at treeline and above. Here is a link to a video of an avalanche that occurred last week in the Purcells. Great footage that illustrates the current wind slab problem in the region.Reports of persistent slab avalanches are becoming less frequent, suggesting it has evolved into a lower likelihood but still high consequence avalanche problem that requires discipline and careful terrain selection to manage effectively.

Snowpack Summary

Limited areas toward the southwest of the region received up to 25 cm of new snow on Thursday. This new snow likely overlies sun crust on sun-exposed aspects. Below any new snow, strong ridge-top winds (most recently from the northeast) have created wind slabs on all aspects at upper elevations. These wind slabs have shown prolonged reactivity because of the faceted (weak, sugary snow) they overlie.Lower down there are two layers of surface hoar (weak, feathery crystals) that were buried at the end of January and mid-January. These layers are around 30 to 80 cm deep and are most prominent at lower elevations - especially below treeline. The surface hoar may sit on a crust on south facing slopes in specific locations, which is a dangerous combination.The base of the snowpack is composed of weak and sugary faceted grains that sit on a crust. This weak layer has produced large and destructive avalanches that are sporadic in nature and difficult to predict. See the Forecasters' Blog here for more information on this problem. Continued cold temperatures have been progressively weakening the basal snowpack.

Problems

Wind Slabs

Recent wind slabs can be found on all aspects and new snow is available to redistribute into fresh slabs in the southwest of the region. Wind slabs have been showing prolonged reactivity due to the weak, faceted snow they sit on.

Steep and rocky terrain are likely places to trigger buried wind slabs.Forecast northeast winds will focus new slab formation on southwest aspects.Avoid freshly wind-loaded features, especially near ridge crests and in steep terrain.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

Persistent Slabs

Two weak layers of surface hoar are buried between 30 and 70 cm down in the snowpack. These layers have been slow to heal and have evolved into a low likelihood, high consequence avalanche problem focused at lower elevations.

Use conservative route selection, choose moderate angled terrain with low consequence.Avoid steep, open and/or sparsely treed slopes at and below treeline.Avoid low elevation cut-blocks where this layer is well preserved.

Aspects: All aspects.

Elevations: Treeline, Below Treeline.

Likelihood

Unlikely - Possible

Expected Size

1.5 - 2.5

Deep Persistent Slabs

The base of the snowpack is composed of weak faceted grains and a crust, particularly in thin snowpack areas. This layer has produced sporadic but very large avalanches. The probability of triggering this layer is low, but the consequences very high.

Minimize overhead exposure; avalanches triggered by cornice fall may be large and destructive.Use caution on alpine slopes, especially around thin snowpack areas.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Unlikely - Possible

Expected Size

2.5 - 3.5

Valid until: Mar 4th, 2019 2:00PM