Avalanche Forecast

Issued: Nov 30th, 2018 4:05PM

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

swerner, Avalanche Canada

South Columbia.

There is a lot of uncertainty with the reactivity of the buried surface hoar layer down 40-80 cm. Human triggered avalanches are more likely at treeline and sheltered locations in the alpine where this weak layer exists. New forecaster blog HERE.

Summary

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Mostly cloudy skies, cooling temperatures and dry conditions moving into the weekend. SATURDAY: Mostly cloudy with a trace of new snow/ alpine temperatures near -7/ generally light winds from the northeast/ freezing level 900 mSUNDAY: Cloudy/ alpine temperatures near -9/ ridgetop winds light from the northeast/ freezing levels 800 mMONDAY: Mix of sun and cloud with isolated flurries/ alpine temperatures -12/ ridgetop winds light from the northeast/ freezing levels valley bottom

Avalanche Summary

Natural avalanche activity has tapered off but human triggered avalanches remain possible, especially in places where a buried weak layer exists. The most significant report we received on Friday was a persistent slab avalanche that was triggered by explosives. This avalanche was a size 2.5 from a northeast aspect in an upper alpine bowl. The avalanche initiated from a shallow, rocky spot with good propagation. We suspect this avalanche failed on the buried crust at the base of the snowpack. This avalanche character is very similar to the human triggered size 3 mentioned below.Last Saturday, a human triggered size 3 avalanche was reported in the region. This avalanche likely ran on the October crust, as it was reported as a full depth avalanche. Two reports on this avalanche can be found on the Mountain Information Network.

Snowpack Summary

The storm that arrived earlier this week distributed 40-80 cm of new snow by Wednesday morning with the higher accumulations in the Monashee Mountains. This recent snow sits on two layers of feathery surface hoar that are approximately buried 15 cm apart. One or both of these may be associated with a crust on south aspects. The surface hoar is mostly found at treeline, but it may be found in sheltered alpine areas.At the base of the snowpack is a crust that formed near the end of October. This crust is associated with sugary weak faceted crystals and has shown some reactivity in the South Columbia region. Suspect slopes and terrain are open bowls that host a rocky thin to thick snowpack in the alpine or upper treeline. Snowpack depths taper quickly with elevation, many areas have over 200 cm in the alpine, between 100-170 cm at treeline, and 10-100 cm below treeline.Check out the FORECASTER BLOG as it will express our uncertainties with the current snowpack in this region

Problems

Persistent Slabs

40 to 80 cm of recent storm snow overlies a weak layer of surface hoar (feathery crystals) and sun crust on steep south facing slopes. This problem may be more prevalent at treeline and in sheltered (northerly) alpine slopes.

If triggered the slab may step down to deeper layers resulting in large avalanches.Watch for signs of instability such as whumpfing, cracking, or recent avalanches.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2.5

Deep Persistent Slabs

A crust at the base of the snowpack will result in large avalanches if triggered. Smaller avalanches may have the potential to step down to the basal layer.

Avoid alpine bowls with a thin or variable snowpack.Pay attention to overhead hazards like cornices which could trigger the deep persistent slab.

Aspects: All aspects.

Elevations: Alpine.

Likelihood

Unlikely - Possible

Expected Size

2 - 3

Valid until: Dec 1st, 2018 2:00PM