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Avalanche Forecast

Archived

Dec 24th, 2017–Dec 25th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Kootenay Boundary.

Watch for areas of stiffer snow, where a cohesive soft slab has formed above the buried crust/surface hoar layer. A prime combination for human triggered avalanches.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Trace new snow expected overnight Sunday and Monday morning.MONDAY: Mix of sun and cloud. Ridge wind light from the northwest. Temperature -13. Freezing level surface.TUESDAY: Mix of sun and cloud. Ridge wind light from the southwest. Temperature -14. Freezing level surface.WEDNESDAY: Cloudy, light flurries. Accumulation trace. Ridge wind light from the southwest. Temperature -12. Freezing level surface

Avalanche Summary

Saturday several explosives triggered slab avalanches to Size 2 were reported on northwesterly to southeasterly aspects in the alpine as well as sloughs to Size 1 in steep terrain. On Friday storm storm was sloughing naturally in steep terrain with loose snow releases to Size 1, while skier traffic produced small slab releases to Size 1 in areas where the storm snow had formed into a slightly stiffer, cohesive slab above the December 15th layer.Earlier in the week numerous Size 1-2 natural, skier, remote, and explosives triggered slab avalanches were reported on all aspects, releasing both within the storm snow and on the December 15th crust/surface hoar layer; including a Size 1.5 skier accidental.Also several classic signs of instability (whumpfing, shooting cracks, small slab releases) have been reported since Monday. See the Mountain Information Network (MIN) for great recent reports here, here and here, as well as a video here.Check out the Forecasters' Blog here for some great terrain advice.

Snowpack Summary

Storm snow totals over the past week average between 50-90 cm across the region. This new snow has now settled to 30-80 cm and overlies the "December 15th crust/surface hoar layer", a variety of old snow surfaces depending on aspect and elevation. On solar aspects a thin melt-freeze crust has been buried. On northerly aspects, feathery, surface hoar crystals (10-60 mm in length) have been buried below treeline as well as protected areas at treeline. In the alpine the old surface consisted of sugary snow (facets) and hard, wind scoured snow. Recent snowpack tests have produced very easy to hard, sudden and resistant results on the December 15th crust/surface hoar layer indicating a high degree of variability and uncertainty in this layer. Below the December 15th layer, the late November crust is now buried 70-100 cm and has been producing hard, resistant results in recent snowpack tests. Beneath this crust the lower snowpack is well settled..

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.