Avalanche Forecast

Archived

Dec 18th, 2019–Dec 19th, 2019

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

Kootenay Boundary.

Thursday gives us one more day of tiptoeing around our persistent slab problem before an incoming atmospheric river deals it a serious shakedown.

Confidence

High -

Weather Forecast

WEDNESDAY NIGHT: Mainly cloudy. Light to moderate southwest winds.

THURSDAY: Cloudy with scattered flurries bringing a trace to 10 cm of new snow, increasing overnight. Light to moderate southwest winds. Alpine high temperatures around -5.

FRIDAY: Cloudy with continuing snowfall bringing approximately 20 cm of new snow, with overnight amounts bringing new snow totals to 30-40 cm. Precipitation transitioning to rain as high as 1400 metres as temperatures warm over the day to reach around -1 in the alpine. Freezing level rising to about 1600 metres.

SATURDAY: Continuing snowfall bringing 20-25 cm of new snow and 48 hour snow totals to 70-90 cm. Precipitation falling as rain below around 1300 metres. Alpine high temperatures around -1 with freezing levels to 1500 metres.

Avalanche Summary

Observations from Monday showed one small new skier-triggered wind slab at 1950 metres in Ymir bowl. Elevated winds from varying directions have supported further wind slab formation since that time.

There were reports of a size 1 human triggered avalanche, and natural size 2 avalanche on Saturday that released on the persistent weak layer that is down about 50-60 cm.

Snowpack Summary

Light new snow amounts have been redistributed by southerly winds at higher elevations and have buried a thin new layer of surface hoar likely to be found in more sheltered areas.

Including the new snow, collectively 25-35 cm of recent snow sits on another weak layer of surface hoar in many sheltered areas, and on a crust on steep south facing slopes. This layer may be more reactive to human triggers where wind loading has formed a deeper, stiffer slab above it.

Another persistent weak layer found approximately 60-100 cm deep consists of a tightly-spaced series of crusts with sugary faceted snow as well as suface hoar found in between them. This complex layer is more likely to be reactive to human triggers in shallow, rocky areas.

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.