Avalanche Forecast
Issued: Dec 27th, 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 problem is evolving into a low probability/high consequence scenario where you may not observe any indication of unstable snow before making a dangerous decision. The formation of new wind slabs will add a layer of complexity to terrain selection on Saturday.
Summary
Confidence
Moderate - Uncertainty is due to the extreme variability of wind effect on the snowpack. We are confident about the possible sizes of avalanches, should one release; what is less certain is the likelihood of triggering.
Weather Forecast
Friday night: Mostly cloudy. Moderate northwest winds. Temperature around-10 C. A trace of accumulation possible.
Saturday: Mix of sun and cloud. Light to moderate west winds. Alpine high temperatures around -7 C. No precipitation expected.
Sunday: Mostly cloudy. Light north winds. Alpine high temperatures around -4 C. Scattered flurries with a trace of accumulation possible.
Monday: Mix of sun and cloud. Light northwest winds. Alpine high temperatures around -3 C. No precipitation expected.
Avalanche Summary
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.
Several more large persistent slabs were triggered with explosives control in the Nelson area on Monday. These slabs featured crown depths of 15-120 cm, giving some evidence of wind loading. These avalanches are suspected to have released on our early November facet/crust layer, which exists below two other persistent weak layers of concern.
A widespread avalanche cycle occurred last weekend on all aspects, generally above 2000 m. Numerous other large to very large avalanches were also triggered by explosives on the late-November layer described in our Snowpack Summary.
Snowpack Summary
10-15 cm of new snow has fallen on a weak interface with moderate west winds forming shallow, touchy wind slabs on lee features in the alpine.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 now buried around 70 to 110 cm.
- an older surface hoar layer buried about 90 to 130 cm, associated with a melt-freeze crust on steep south aspects.
- a complex layer of weak and sugary faceted snow, surface hoar, and melt-freeze crusts in the bottom half of the snowpack, buried in late November.
Although avalanche activity on these layers has been declining since the storm, they each continue to produce concerning snowpack test results and are expected to heal slowly. 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 snowpack areas, rock outcroppings and steep convex terrain where triggering is most likely.
- 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: All elevations.
Likelihood
Expected Size
Wind Slabs
Although forecast winds have not fully materialized, moderate west winds expected overnight and into Saturday have the potential to drift the recent 10-15 cm of new snow into small touchy wind slabs in the alpine 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 28th, 2019 5:00PM
