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Avalanche Forecast

Dec 8th, 2011–Dec 10th, 2011
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
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
2: Moderate
The avalanche danger rating in the alpine will be moderate
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
2: Moderate
The avalanche danger rating in the alpine will be moderate
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

Regions: Purcells.

Confidence

Fair - Intensity of incoming weather is uncertain on Sunday

Weather Forecast

Continued high pressure across the interior mountain ranges on Friday and Saturday. The ridge should break down on Sunday when a weak low pressure moves across the interior. There may be some strong winds in the alpine when the light precipitation starts on Sunday.

Avalanche Summary

Reports of a few small explosive released avalanches in the Dogtooth. We have dropped the danger rating in the alpine to moderate. It is still likely that you could trigger a windslab in steep terrain. Triggering a full depth release is getting less likely, but weak spots are sure to exist that may propagate a large avalanche. The snow pack is still relatively young and needs some time to mature. I am having a hard time dropping the treeline danger level to LOW because I think the snowpack is weaker and more variable in this region than it is to the west.

Snowpack Summary

There is not much change taking place. Previous problems are healing slowly. The overnight temperatures have been colder than I thought on the east slope, and they have probably been causing some depth hoar development, and some surface facetting. We suspect that surface hoar may be growing during the clear cold weather that has dominated over the past few days. Some steep south facing slopes may have developed a thin sun-crust. Windslabs continue to be a concern in the Purcells. Strong winds last week developed unstable slabs of transported snow on steep terrain in the alpine and at treeline. Snow depths are quite variable through the region. In the alpine there is 170- 250cm. At treeline there is anywhere from 130-150cm.The upper snowpack is tightening up forming a stronger mid-pack. A highlighted concern for the Purcells was the mid-November storms that created a heavy slab over a weak basal faceted base, resulting in a significant cycle of large avalanches running full depth.

Avalanche Problems

Wind Slabs

Stiff wind slabs in the alpine and open areas at treeline may be triggered by skiers and riders. It may be possible to remotely trigger a slide from a weak spot nearby.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Possible - Likely

Expected Size: 1 - 3

Persistent Slabs

The weak facetted layer near the ground may still be triggered by human activity. Areas with a lot of variability in the snow cover may be the most suspect. Triggering may occur from weak spots like rocks or clumps of small trees.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Unlikely - Possible

Expected Size: 2 - 5