Archived

Avalanche Forecast

Dec 25th, 2016–Dec 26th, 2016

Alpine

4: High

The avalanche danger rating in the alpine will be high

Treeline

3: Considerable

The avalanche danger rating at treeline will be considerable

Below Treeline

3: Considerable

The avalanche danger rating below treeline will be considerable

Alpine

3: Considerable

The avalanche danger rating in the alpine will be considerable

Treeline

3: Considerable

The avalanche danger rating at treeline will be considerable

Below Treeline

2: Moderate

The avalanche danger rating below treeline will be moderate

Alpine

3: Considerable

The avalanche danger rating in the alpine will be considerable

Treeline

3: Considerable

The avalanche danger rating at treeline will be considerable

Below Treeline

2: Moderate

The avalanche danger rating below treeline will be moderate

Regions: Northwest Inland.

Expect avalanche danger to increase in areas that receive heavy loading from new snow and wind. Stick to mellow terrain and be extra cautious on Boxing Day.

Confidence

Moderate - Due to the number and quality of field observations

Weather Forecast

SUNDAY NIGHT: Flurries with accumulations of 5-10 cm with strong to extreme southwest winds.MONDAY: Stormy with accumulations of 10-20 cm, extreme southwest winds, temperatures around -10C.TUESDAY: Flurries with accumulations of 5 cm, strong west winds, temperatures around -8C.WEDNESDAY: Cloudy with isolated flurries, strong southwest winds, temperatures around -8C.

Avalanche Summary

Persistent slabs have been very touchy in areas with thin snowpacks. Over the past few days, several size 1.5-2 avalanches have been remotely triggered by skiers in the northern part of the region, and a size 3 persistent slab was triggered with explosives on Saturday. These avalanches have released on weak facets near the ground in steep shallow start zones. Remote triggering is a sign of a serious weakness deep in the snowpack and indicates that persistent slab avalanches may be triggered with very light loads.On Monday, the new snow will form reactive wind slabs. Wind slabs may be extra touchy in areas where they are burying a newly formed surface hoar layer. The additional load of the new snow will also make it possible to trigger weak layers deeper in the snowpack and produce large persistent slab avalanches.

Snowpack Summary

Expect 15-30 cm of fresh storm snow by Monday afternoon, with extreme winds forming extra deep pockets in the lee of ridgetops. The storm snow will likely bury a new layer of surface hoar that formed in sheltered areas, making wide propagations possible. A weak interface that formed during the early December cold snap can be found buried about 50 cm deep. The layer consists of weak faceted (sugary) snow, and preserved surface hoar in sheltered areas. The snow above this interface likely remains poorly bonded. The snowpack is still shallow throughout the region and widespread faceting of the entire snowpack is likely in most areas. A thick rain crust that formed in early November is probably near the bottom of the snowpack and is likely surrounded by sugary facets. Basal facets and depth hoar (more sugary crystals) right at the bottom of the snowpack are reportedly widespread.

Avalanche Problems

Wind Slabs

Expect fresh and touchy wind slabs to develop as the new snow arrives with extreme winds.

Avoid alpine terrain.Avoid freshly wind loaded features.Avoid open slopes and convex rolls at treeline where buried surface hoar may be preserved.

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

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 2

Persistent Slabs

The new snow will load several layers of weak faceted snow including a layer buried 50 cm deep and another layer near the base of the snowpack. Likely trigger points are unsupported (convex) slopes in shallower snowpack areas.

Be aware of the potential for full depth avalanches due to weak layers at the base of the snowpack.Minimize overhead exposure during periods of heavy loading from new snow, wind.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Possible - Likely

Expected Size: 2 - 3