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Avalanche Forecast

Dec 17th, 2020–Dec 18th, 2020
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
4: High
The avalanche danger rating at treeline will be high
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Lizard-Flathead.

Slab avalanches may be primed for human triggers. Choose conservative terrain and back off slopes when you see signs of instability such as cracking, whumphing a fresh avalanche activity.

Confidence

Moderate - Uncertainty is due to whether buried persistent weak layers become active, triggering avalanches, with the arrival of the forecast weather. Uncertainty is due to the limited number of field observations.

Weather Forecast

Friday: Snow amounts 5 cm with alpine temperatures near -5. Ridgetop wind strong from the southwest and freezing levels valley bottom.

Saturday: Snow 5-10 cm with alpine temperatures near -7. Strong wind from the West and freezing levels 900 m.

Sunday: Snow amounts 10-20 cm. Alpine temperatures -1 and freezing levels 2000 m. Ridgetop wind 20-85Km/hr wind from the southwest. 

Avalanche Summary

On Thursday, a natural size 2 storm slab from NE alpine terrain was reported. As per the report, this slab likely failed on Wednesday. On Thursday, explosives were able to trigger slabs up to size 2 in the alpine and a size 1.5 at treeline.

Please consider sharing your observations on the Mountain Information Network. Thank you to those that have already submitted this winter.

Snowpack Summary

Strong wind from the southwest and up to 30 cm of new snow will likely build reactive storm slabs. The recent storm snow accumulations bring 40-60 cm above the early December crust which may have persistent weak, and facetted crystals above and below it. With slab cohesion adding stress and load on that interface, we may see this persistent slab avalanche problem come to life.

Deeper in the snowpack are two hard melt-freeze crusts that formed in November. The concern would be if there were weak faceted grains around the crust, but there is uncertainty if and where in the region this may be a problem. Without any recent avalanche activity, it appears that this layer is dormant for the time being.

Terrain and Travel

  • Avoid areas where the snow feels stiff and/or slabby.
  • Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
  • Choose relatively conservative lines and watch for clues of instability.

Avalanche Problems

Storm Slabs

The new storm snow is building reactive storm slabs. Strong southwest wind is forming deeper slabs on leeward slopes that could produce larger avalanches. Loose-dry sluffing will likely occur from steep terrain features. 

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Likely

Expected Size: 1 - 2

Persistent Slabs

40-60 cm of snow currently sits above a buried crust from early December. Sugary, facets above and/or below the crust may weaken the bond and the persistent slab may become reactive, especially to human triggers. 

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Possible

Expected Size: 1.5 - 2.5