Avalanche Forecast

Kootenay Boundary

Date issued

Dec 24, 2017 - 4:42 PM

Valid until

Dec 25, 2017 - 2:00 PM

Prepared by

jmcbride

Watch for areas of stiffer snow, where a cohesive soft slab has formed above the buried crust/surface hoar layer. A prime combination for human triggered avalanches.

Regular season
Sunday - December 24, 2017
Alpine

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.

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

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.

Monday - December 25, 2017
Alpine

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.

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

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.

Tuesday - December 26, 2017
Alpine

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.

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

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.

Avalanche Problem 1: Wind Slabs

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

Winds at upper elevations have created soft slabs on alpine leeward and cross-loaded slopes.

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

Avalanche Problem 2: Persistent Slabs

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

Recent storm snow has settled into cohesive soft slabs above the December 15th crust/surface hoar layer. Expect these slabs to be easily triggered especially on steep or convex slopes and sheltered areas where surface hoar has been buried.

Travel and Terrain Advice
  • Use conservative route selection, choose moderate angled and supported terrain with low consequence.
  • Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.
  • Watch for signs of unstable snow or slab formation such as whumpfing, or shooting cracks.

Avalanche Summary

Saturday several explosives triggered slab avalanches to Size 2 were reported on northwesterly to southeasterly aspects in the alpine as well as sloughs to Size 1 in steep terrain.

On Friday storm storm was sloughing naturally in steep terrain with loose snow releases to Size 1, while skier traffic produced small slab releases to Size 1 in areas where the storm snow had formed into a slightly stiffer, cohesive slab above the December 15th layer.

Earlier in the week numerous Size 1-2 natural, skier, remote, and explosives triggered slab avalanches were reported on all aspects, releasing both within the storm snow and on the December 15th crust/surface hoar layer; including a Size 1.5 skier accidental.

Also several classic signs of instability (whumpfing, shooting cracks, small slab releases) have been reported since Monday. See the Mountain Information Network (MIN) for great recent reports here, here and here, as well as a video here.

Check out the Forecasters' Blog here for some great terrain advice.

Snowpack Summary

Storm snow totals over the past week average between 50-90 cm across the region. This new snow has now settled to 30-80 cm and overlies the "December 15th crust/surface hoar layer", a variety of old snow surfaces depending on aspect and elevation. On solar aspects a thin melt-freeze crust has been buried. On northerly aspects, feathery, surface hoar crystals (10-60 mm in length) have been buried below treeline as well as protected areas at treeline. In the alpine the old surface consisted of sugary snow (facets) and hard, wind scoured snow.

Recent snowpack tests have produced very easy to hard, sudden and resistant results on the December 15th crust/surface hoar layer indicating a high degree of variability and uncertainty in this layer. Below the December 15th layer, the late November crust is now buried 70-100 cm and has been producing hard, resistant results in recent snowpack tests. Beneath this crust the lower snowpack is well settled.

.

Weather Forecast

Trace new snow expected overnight Sunday and Monday morning.

MONDAY: Mix of sun and cloud. Ridge wind light from the northwest. Temperature -13. Freezing level surface.

TUESDAY: Mix of sun and cloud. Ridge wind light from the southwest. Temperature -14. Freezing level surface.

WEDNESDAY: Cloudy, light flurries. Accumulation trace. Ridge wind light from the southwest. Temperature -12. Freezing level surface

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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