Avalanche Forecast
Issued: Feb 24th, 2021 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Persistent Slabs.
ahanna,
Avalanche Canada
Watch for fresh and reactive storm slabs forming Thursday as new snow arrives amid strong wind. Keep in mind the potential for triggering deeper slabs persists even as natural activity from the previous storm tapers off. Recent rider triggered avalanches have been very large.
Summary
Confidence
Moderate - Uncertainty is due to the limited number of field observations.
Weather Forecast
Local enhancements in snowfall amounts possible near McBride.
WEDNESDAY NIGHT: Increasing cloud with flurries bringing up to 5 cm in most areas, strong southwest wind, freezing level valley bottom.
THURSDAY: 5-10 cm new snow in most areas, strong southwest ridgetop wind, freezing level 1100 m.
FRIDAY: 5-10 cm new snow, moderate westerly ridgetop wind, freezing level valley bottom.
SATURDAY: Up to 5 cm new snow, light to moderate southwest ridgetop wind, freezing level 800 m.
Avalanche Summary
Observations from McBride and Valemount areas show evidence of a widespread natural avalanche cycle that took place during the previous storm. Several natural and machine triggered storm slabs size 2-3.5 (!) were reported on Tuesday near Valemount at alpine and treeline elevations. At least one of the very large machine triggered avalanches was initiated from a wind loaded start zone at the top of a large alpine feature, propagated for hundreds of meters along a ridge and ran to valley bottom. Thankfully the rider was able to escape unharmed.
Snowpack Summary
Recent reports have been of excellent riding quality in the 40-80 cm of recent snow. Thick, reactive wind deposits up to 100 cm deep exist in lee features at upper elevations. The recent snow sits on old, facetted and/or extensively wind affected snow that sat on the surface during the extended cold, windy drought period. Some very large avalanches since the end of the storm have shown incredible propagation of hundreds of meters in alpine and treeline features, indicating that this layer is weak and may persist for some time. On the other hand, this MIN from Clemina describes very limited reactivity at this interface below treeline.
We've now got 80-180 cm of snow above the early February persistent weak layer which is composed of surface hoar. This layer is most prevalent at treeline and in "treeline-like" features like cutblocks. On steep south-facing terrain, this layer may consist of facets on a sun crust. We are awaiting reports of avalanche activity during the storm, specifically whether they indicate step- downs to this layer, to determine whether or not it continues to be a problem.
Terrain and Travel
- Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.
- Be aware of the potential for large avalanches due to the presence of a persistent slab.
- Keep in mind that human triggering potential persists as natural avalanching tapers off.
- Uncertainty is best managed through conservative terrain choices at this time.
- Be especially cautious as you transition into wind affected terrain.
Problems
Storm Slabs
10-20 cm of new snow fall amid strong wind. Storm slabs will be most reactive where they sit over a sun crust and in wind deposited pockets in lee terrain features at upper elevations.
Cornices are experiencing rapid growth with strong west to southwest winds. Fresh, unsupported tabs that break off can act as triggers on slopes below.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
40-80 cm of recent storm snow sits overtop a thick layer of weak, sugary facets. At upper elevations, the recent snow has been blown into thick, reactive slabs in lee features. Due to the weak interface, storm slabs have shown very wide propagation across slopes in recent avalanches that have surprised riders.
Another persistent weak layer is buried 80 to 120 cm deep. We don't yet have enough data to say for sure whether or not it continues to be a problem at this point. On the one hand, we have not yet heard any reports of recent large avalanches stepping down to this layer, which is encouraging, and the weight of the overlying snow is likely causing it to bond. On the other hand, this layer has been associated with a recent fatal avalanche in the neighboring North Rockies region, so we want to do our due diligence before we take it off our radar.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 25th, 2021 4:00PM
