Avalanche Forecast

Lizard Range and Flathead

Date issued

Dec 28, 2017 - 4:49 PM

Valid until

Dec 29, 2017 - 2:00 PM

Prepared by

jmcbride

As much as 60 cm of low density storm snow could arrive by Friday morning. Expect both natural and human triggered avalanches in steep terrain and where wind loading has occurred.

Regular season
Thursday - December 28, 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

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.

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.

Friday - December 29, 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

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.

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.

Saturday - December 30, 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

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.

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: Storm Slabs

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

Expect sluffing in steep terrain and soft storm slabs in open areas near ridge lines where additional loading from wind has occurred.

Travel and Terrain Advice
  • Minimize exposure during periods of heavy loading from new snow and wind.
  • If triggered the storm/wind slabs may step down to deeper layers resulting in large avalanches.
  • Be cautious of sluffing in steep terrain.

Avalanche Problem 2: Persistent Slabs

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

A weak layer composed of surface hoar and/or crusts is buried 60-80 cm below the surface. Triggering an avalanche on this layer is possible in areas where the snow has settled into a cohesive slab.

Travel and Terrain Advice
  • Watch for whumpfing, hollow sounds, and shooting cracks.
  • Choose well supported terrain without convexities.
  • Use caution on open slopes and convex rolls at treeline where buried surface hoar may be preserved.

Avalanche Summary

Thursday's storm snow has produced numerous natural, storm slab and loose dry avalanches from Size 1-1.5 in steep terrain on all aspects.  Avalanche control work produced similar results with loose, dry and storm slab avalanches up to Size 1.

Earlier in the week on Monday, one large naturally-triggered avalanche (size 2) was reported on a north-facing alpine feature that likely failed on the mid-December surface hoar layer. There have been some isolated reports of whumpfs at small avalanches failing on the mid-December surface hoar layer, but in most areas the snow above this layer has not settled into a slab yet, this will change if winds are stronger than forecasted.

Snowpack Summary

Wednesday night and through the day on Thursday approximately 30 cm of low density snow fell with generally light winds.

This overlies roughly 50 cm of old storm snow from last week that formed harder wind slabs in exposed terrain near ridge top, but remains low density in sheltered areas.

The main question in the snowpack surrounds the mid-December layer now buried 80 cm below the surface. The layer includes crusts on south and west facing slopes and feathery surface hoar in sheltered areas at and below treeline, and has been producing moderate, sudden results in recent snowpack tests.

The lower snowpack is generally well settled with a thick prominent crust that was buried near the end of November.

Weather Forecast

THURSDAY NIGHT: Snow. Accumulation 15-25 cm. Ridge wind light northeast, possibly strong at times. Temperature -18. Freezing level surface.

FRIDAY: Mostly cloudy, snow developing and overnight. Accumulation 5-25 cm. Ridge wind light northeast possibly strong at times. Temperature -17. Freezing level surface.

SATURDAY: Mostly cloudy, flurries. Accumulation 5-10 cm. Ridge wind light west. Temperature -16. Freezing level surface.

SUNDAY: Mostly sunny. Ridge wind light southwest. Temperature -13. 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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