Avalanche Forecast

Issued: Jan 30th, 2016 8:07AM

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

rbuhler, Avalanche Canada

Purcells.

Recently formed wind slabs and touchy persistent weak layers are making for tricky conditions. Conservative terrain selection is very important.Use extra caution on steep south-facing slopes if the sun is out in force strength in the afternoon.

Summary

Confidence

Moderate - Timing or intensity of solar radiation is uncertain

Weather Forecast

A mix of sun and cloud is expected for Sunday with light intermittent flurries possible. Freezing levels are expected to fall to valley bottom overnight and climb to around 1000m on Sunday afternoon. Alpine winds are forecast to be light from the northwest. Light snowfall is forecast for Monday but less than 5cm is expected. Freezing levels are expected to reach around 1000m on Monday and alpine winds are forecast to be moderate from the northwest. Similar conditions are expected for Tuesday with light snowfall, cool temperatures, and moderate west winds.

Avalanche Summary

Widespread natural avalanche activity up to size 3 was reported to have occurred during the storm Thursday overnight and Friday morning. Most of this natural activity was on wind loaded aspects (north through east) in the alpine and at treeline but a few avalanches were also reported on south and west aspects. Explosive use on Friday produced mainly size 2-3 storm slabs 20-50cm thick on a variety of aspects above 2300m. Some of the explosive results are assumed to be failing on persistent weak layers with slab depths up to 100cm. Also reported were a couple size 1-1.5 human-triggered storm slab avalanches. These were 25-30cm thick slabs. On Thursday, several natural and human triggered 50-60cm thick persistent slab avalanches up to Size 2 were reported running on surface hoar buried at the beginning of January. Some were remotely triggered by as much as 30m away. Natural activity is generally not expected on Sunday but could be possible on steep south facing slopes if the sun comes out in full strength in the afternoon. Recently formed wind slabs and a reactive persistent slab are both expected to be sensitive to human-triggering on Sunday.

Snowpack Summary

Weakness linger within and under the 20-30cm of rapidly settling recent storm snow with reports of moderate sudden results in snowpack tests. Southwesterly winds have continued to build fresh wind slabs on the leeward side of ridge crests and behind terrain features on cross-loaded slopes. The persistent weak layer of surface hoar buried early-January is now typically down 50-80 cm and appears to be quite touchy in some parts of the region. In general, the lower snowpack below this layer is well settled and strong.

Problems

Wind Slabs

Fresh wind slabs lurking below ridge crests and behind terrain features are sensitive to human triggers. Cornices are also large and fragile.

Avoid freshly wind loaded features. >Use ridges or ribs to avoid pockets of wind loaded snow. >Be cautious as you transition into wind affected terrain. >

Aspects: North, North East, East, South East, South.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 3

Persistent Slabs

Surface hoar buried at the beginning on January remains a concern for human triggers. The potential for widespread propagation and remote triggering makes this persistent slab particularly tricky to manage.

Avoid open slopes and convex rolls at and below treeline, and north-facing alpine slopes, where buried surface hoar may be preserved. >Be aware of the potential for remote triggers and wide propagations due to a layer of buried surface hoar. >

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

3 - 5

Valid until: Jan 31st, 2016 2:00PM