Avalanche Forecast
Issued: Dec 28th, 2019 4:30PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
astclair,
Avalanche Canada
The persistent slab is evolving to a low probability/high consequence scenario where you may not observe any signs of unstable snow before making a dangerous decision. Managing isolated wind slabs adds further complexity to choosing safe terrain on Sunday.
Summary
Confidence
Moderate - We are confident about the possible sizes of avalanches, should one release; what is less certain is the likelihood of triggering. Uncertainty is due to the extreme variability of wind effect on the snowpack.
Weather Forecast
Saturday night: Partly cloudy, light northwest winds, alpine temperatures around -7 C.Â
Sunday: Mix of sun and cloud, isolated flurries with a trace of accumulation, light north winds, alpine high temperatures around -3 C, with freezing level near 800 m.
Monday: Mostly clear, light west winds, alpine high temperatures around -3 C, with freezing level near 700 m.
Tuesday: Mostly cloudy, 10-15 cm of snow possible during the day and overnight, moderate southwest winds, alpine high temperatures near -5 C, with freezing level around 600 m.
Avalanche Summary
Several large (size 2-3) persistent slab avalanches, both human and explosive-triggered, were reported on Saturday. These avalanches released on both the December surface hoar and November crust layers across a variety aspects. A recent large skier-triggered persistent slab avalanche was observed in the Kelowna area on Wednesday. It featured a 30-70 cm crown fracture, scrubbed into the lower snowpack, and was noted for being triggered on a shallow, rocky, convex slope.
These avalanches offer a much needed reminder that large, destructive avalanches are still possible even a week out from the storm and that this persistent problem still needs time to heal.
Snowpack Summary
15-20 cm of new snow has fallen on a weak interface that may be reactive on lee features in the alpine where winds have been strong enough to form slabs.Â
The stormy period that ended early this week saw around 70 to 100 cm of snow deposited in the region. This snow brought a significant load to multiple weak layers, including a feathery surface hoar layer (down 70 to 110 cm), an older surface hoar layer associated with a melt-freeze crust on steep south aspects (down 90 to 130 cm), and
a layer of sugary faceted snow, surface hoar, and melt-freeze crusts buried in late November found in the bottom half of the snowpack. Snowpack tests continue to produce sudden results on these layers (check out this MIN report from Friday).
Although signs of instability are becoming less obvious, it remains prudent to make terrain decisions with the understanding that one or more of these deeply buried layers are present and could produce large and destructive avalanches with human triggers.
Terrain and Travel
- Uncertainty is best managed through conservative terrain choices at this time.
- Persistent slabs have potential to pull back to lower angle terrain.
- Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
- Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
Problems
Persistent Slabs
Several different deeply buried weak layers can be found in the snowpack that have recently produced large avalanches. These persistent slabs are gradually becoming less likely to trigger, but it remains prudent to assume these layers are present, reactive to human triggers, and capable of forming very large, destructive avalanches.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Wind Slabs
Although forecast winds have not fully materialized, there may still be areas at higher elevations where wind has drifted the 15-20 cm of new snow into deeper, more reactive slabs that will need to be managed. Expect this problem to increase with elevation and to be most pronounced in the immediate lee of wind-exposed terrain features. Wind slabs perched above shallow, rocky areas have potential to trigger a deeper weak layer to create a large avalanche.
Aspects: North, North East, East, South East, South.
Elevations: Alpine.
Likelihood
Expected Size
Valid until: Dec 29th, 2019 5:00PM
