Avalanche Forecast

Purcells

Date issued

Feb 23, 2018 - 3:52 PM

Valid until

Feb 24, 2018 - 2:00 PM

Prepared by

jmcbride

Not much change expected with only light snowfall amounts in the forecast. Watch for touchy wind slabs and cornices at upper elevations. And avoid shallow, rocky areas where triggering a deeper layer may still be possible.

Regular season
Friday - February 23, 2018
Alpine

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Treeline

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Below Treeline

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

Saturday - February 24, 2018
Alpine

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Treeline

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Below Treeline

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

Sunday - February 25, 2018
Alpine

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Treeline

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

Below Treeline

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

Avalanche Problem 1: Wind Slabs

What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?

New snow and wind are adding to reactive wind slabs that exist in many areas in the alpine as well as open areas at treeline. Cornices have also been touchy and should be given a wide berth when traveling on or below ridge lines.

Travel and Terrain Advice
  • If triggered the wind slabs may step down to deeper layers resulting in large avalanches.
  • Watch for whumpfing, hollow sounds, and shooting cracks.
  • Be careful with with loaded pockets, especially near ridge crests and roll-overs.

Avalanche Problem 2: Persistent Slabs

What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?

Buried weak layers exist in the snowpack and may be reactive to large triggers such as a cornice or step down from a wind slab release. Human triggering may also be possible in shallow or thin, rocky, unsupported terrain.

Travel and Terrain Advice
  • Avoid steep convexities or areas with a thin or variable snowpack.
  • Pay attention to overhead hazards like cornices which could easily trigger persistent slabs.

Avalanche Summary

Thursday there were reports of large (size 2.5 and 3) natural cornice triggered avalanches on northerly aspects above 2400 m. One of these, the cornice collapse initiated a slab failure at the base of the snowpack, 250 cm deep.

On Wednesday there were reports of several skier triggered slab avalanches up to size 2 on predominantly steep, south-facing features failing on the crust/surface hoar interface buried mid-February.

Tuesday there were reports of a skier triggered size 2.5 cornice failure that produced very large, bus-sized cornice chunks on an north aspect at 2400m. As well as a size 1.5 natural wind slab failure (50-70 cm deep) on a west aspect at 2050 m.

On Monday we received reports of a cornice triggered size 2.5 slab avalanche on a high north east alpine face near 2900 m, as well as natural wind slabs to size 2.5 (crowns 20-40cm thick) on a wide range of aspects above 1900m elevation.

Although deeper, persistent avalanche activity has become less frequent over the past week, light triggers in shallow rocky areas, as well as large triggers such as a cornice collapse or step down from a wind slab release, still have the potential to result in large destructive avalanches.

Snowpack Summary

A light dusting of new snow on Friday now covers storm snow totals from last week that range from15-30 cm, with closer to 50 cm in the very south of the region near Kimberley. Moderate south west through north west winds have redistributed these accumulations into deeper, reactive slabs in wind-exposed terrain.

These accumulations sit on an interface that was buried mid-February consisting of; a mix of older wind slabs in exposed higher elevation terrain, a sun crust on steep solar aspects, and surface hoar on sheltered slopes.

Below the snow surface several persistent weak layers make up a complex snowpack that is slowly beginning to show sign of improving but still remains suspect. In the top 80-120 cm of the snowpack, two surface hoar/ crust layers buried in January can be found. Expect to find one or other of these on all aspects and elevations.

Deeper in the snowpack (around 150 cm deep) is a facet/crust/surface hoar layer from December that is most prevalent at treeline and below.

Near the base of the snowpack is a crust/facet combo from late November.

Weather Forecast

SATURDAY: Mix of sun and cloud, light flurries. Accumulation trace. Ridge wind light, west. Temperature -8. Freezing level valley bottom.

SUNDAY: Flurries. Accumulation 5-10 cm. Ridge wind moderate to strong, west. Temperature -7. Freezing level valley bottom.

MONDAY: Mix of sun and cloudy, light flurries. Accumulation trace. Ridge wind light, west. Temperature -8. Freezing level valley bottom.

Danger Ratings Explained

1 - Low

Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

Natural and human-triggered avalanches unlikely.

Small avalanches in isolated areas or extreme terrain.

2 - Moderate

Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Natural avalanches unlikely; human-triggered avalanches possible.

Small avalanches in specific areas; or large avalanches in isolated areas.

3 - Considerable

Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

Natural avalanches possible; human-triggered avalanches likely.

Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas.

4 - High

Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

Natural avalanches likely; human-triggered avalanches very likely.

Large avalanches in many areas; or very large avalanches in specific areas.

5 - Extreme

Avoid all avalanche terrain.

Natural and human-triggered avalanches certain.

Large to very large avalanches in many areas.

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