Avalanche Forecast

Lizard Range and Flathead

Date issued

Feb 20, 2018 - 4:06 PM

Valid until

Feb 21, 2018 - 2:00 PM

Prepared by

mconlan

Wind slabs still linger in lee features at higher elevations. Watch for signs of avalanche activity and locally unstable snow, such as whumpfing and shooting cracks.

Regular season
Tuesday - February 20, 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.

Wednesday - February 21, 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.

Thursday - February 22, 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?

Wind slabs formed during last weekends storm still linger in lee features. Be cautious of overhead exposure on solar aspects when the sun is out.

Travel and Terrain Advice
  • If triggered, the wind slabs may step down to deeper layers, resulting in large avalanches.
  • Choose shallow-angled and sheltered terrain where new snow hasn't been wind-affected.

Avalanche Problem 2: Deep Persistent Slabs

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

Several weak layers are lurking in our snowpack. Although the probability of triggering them is relatively low, they would produce avalanches with high consequences. Watch out for thin spots in the snowpack, where it is more likely to trigger them.

Travel and Terrain Advice
  • Be aware of the potential for wide propagation.
  • Avalanches could run full-path: avoid runout zones of paths with large cornices or on solar aspects.

Avalanche Summary

On Monday, a few small slab avalanches (size 1 to 1.5) were reported on northerly to westerly aspects, triggered by small cornice chunks and skiers.  They were 10 to 30 cm deep and in treeline and alpine terrain.

Avalanche activity may slow down with a period of cold, clear weather that is expected to persist this week.  However, the sun is packing more of a punch as the days get longer and it will warm the snow surface on solar aspects if clear skies prevail. Watch for steep sunny slopes due to the sun’s influence, lee features for reactive wind slabs, and be extra cautious near thin spots and shallow snow pack areas where triggering a deeper weak layer is more likely.  There is a lower probability of triggering a deeper layer than in previous weeks, but the consequence would be very high if one was triggered.

Snowpack Summary

Around 50-80 cm of recent snow with strong winds have created wind slabs in direct lee features. On steep solar aspects, a thin sun crust exists.  These overly a layer of surface hoar, graupel, and a melt-freeze crust buried mid-February.  The melt-freeze crust exists up to 1700 m in elevation.

Deeper in the snowpack, a widespread weak layer from mid-December composed of facets, crusts, and surface hoar is 120-160 cm deep. Also, a rain crust with sugary facets buried in late November is near the base of the snowpack. If triggered, these weak layers could produce large, destructive avalanches with high consequences.

Weather Forecast

WEDNESDAY: Mostly sunny, moderate westerly winds, alpine temperature near -13 C, freezing level below valley bottom.

THURSDAY: Partly cloudy, light easterly winds, alpine temperature near -12 C, freezing level below valley bottom.

FRIDAY: Mostly sunny with increasing cloudiness, light to moderate southwesterly winds, alpine temperature near -11 C, freezing level below 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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