Avalanche Forecast

Issued: Dec 13th, 2015 10:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Storm Slabs and Persistent Slabs.

ghelgeson, Avalanche Canada

Purcells.

The danger has come down, but large human triggered avalanches remain possible. Start with small test slopes and build up slowly to the bigger terrain features. There is still a lot of uncertainty associated with this early winter snowpack.

Summary

Confidence

Low - Due to the number of field observations

Weather Forecast

High pressure dominates the weather pattern for the next few days. The next major weather maker is not expected in the Purcells until at least Friday. MONDAY: No snow expected, freezing level rising from valley bottom to around 800m, light NW wind. TUESDAY: No snow expected, freezing level at valley bottom, light NW wind. WEDNESDAY: No snow expected, freezing level at valley bottom, light N/NW wind.

Avalanche Summary

No avalanche activity to report from Saturday. On Friday two significant observations were reported: The first was a size 3.5 natural avalanche that was observed on a SE facing feature at 2800m running to ground in some places. The second was a report of a skier triggered size 2.5 avalanche that was accidentally triggered on an east facing feature at 1500m in the northern Purcells. This avalanche failed on the early December Persistent Weak Layer. Control work in the central and southern Purcells Thursday produced avalanches to size 3.5 on a variety of aspects, some of which ran full path.

Snowpack Summary

A string of storms between December 2nd and December 13th produced 35 to 80cm of storm snow in the Purcells which is settling out at all elevations. You may find a brittle thin crust about 10 to 30cm below the snow surface as high as 1800m that was formed by rain and warm temperatures on December 8th. The early December persistent weak layer can be found down 30 to 70cm below the surface. This weak layer manifests as an old sun crust on due south facing features in the alpine, large grained surface hoar below 1800m and small facets in isolated pockets. The surface hoar has been the primary concern lately, especially below treeline between 1400m and 1800m. Recent test results on this interface have been widely variable. "Resistant planar," "sudden collapse" and even "no result" have been reported from test profiles as recently as Dec. 13th. All the variation in trigger sensitivity makes this layer tricky to manage. Recent winds out of the south, SW and most recently SE have formed wind slabs on lee features that are mainly confined to the alpine, but you may find the odd fresh wind slab at treeline too. The mid and lower portions of the snowpack are thought to be well settled.

Problems

Storm Slabs

The storm slab problem should be settling out quickly, but I would still be wary of steep unsupported features, slopes immediately lee of ridge crest, convexities, wind loaded chutes and rock outcroppings.

Avoid freshly wind loaded features and use ridges or ribs to sneak around these problem areas.>Start with simple terrain and take a curious/experimental approach to the snowpack. You may be able to step out into challenging and even complex terrain after learning more about the snowpack.>While unlikely, it is possible for storm slabs in motion on south facing alpine features to step down to a deeply buried crust resulting in very large destructive avalanches.>

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 4

Persistent Slabs

A layer of buried surface hoar can be found below treeline between 1400m and 1800m. Test profiles suggest this interface varies widely in reactivity and remains sensitive to human triggering.

Avoid open slopes, convex rolls, burns, cutblocks and terrain traps when traveling below treeline. This is where buried surface hoar is best preserved and most volatile.>

Aspects: All aspects.

Elevations: Alpine.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Dec 14th, 2015 2:00PM