Avalanche Forecast
Issued: Apr 16th, 2021 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wet Slabs, Cornices and Deep Persistent Slabs.
ldreier,
Avalanche Canada
With multiple warm days in a row and high overnight freezing levels it is important to start and finish your day early. Look for north facing objectives without overhead cornices. Plan your egress route carefully.
Check out the Forecaster's Blog on warming and how to stay safe.
Summary
Confidence
High - We are confident the likelihood of avalanche will increase with the arrival of the forecast weather.
Weather Forecast
A ridge of high pressure continues to bring sunny and dry weather with high freezing levels even at night. On Sunday the ridge is forecast to break down bringing light precipitation and lowering freezing levels.
Friday night: Clear light northeast wind, alpine low +2 C, freezing level 2900 m.Â
Saturday: Sunny, light to moderate northwest wind, alpine high +8 C, freezing level 2900 m.
Sunday: Mainly cloudy with isolated flurries, 10 cm new snow and rain below treeline, moderate to strong northeast wind, alpine high +4 C, freezing level lowering to 1700 m during the day.
Monday: Mainly sunny, light to moderate northwest wind, alpine high +3 C, freezing level 2000 m.
Avalanche Summary
No observations were submitted since Thursday by the time of writing. However, a continuing increase in avalanche activity and avalanche size of wet slab and wet loose avalanches and cornice falls is expected with the warm temperatures.Â
A large natural cornice fall of size 3 likely released a slab on the northeast facing slope below and was reported on Wednesday. Small wet loose avalanches to size 1 and a couple size 2 were observed on steep solar slopes. A few slab avalanches were reported that might have been triggered by cornices in the last few days.
Last week, a significant natural cornice failure triggered a persistent slab on the slope below and resulted in a size 3 avalanche. This occurred on a high alpine steep east-facing slope. This is an important reminder to stay well back from ridgetops that may be corniced and avoid slopes with overhead cornice exposure especially under the current conditions where recent rapid growth and warm temperatures have made them large and fragile.Â
Snowpack Summary
The snow surface consists of a melt-freeze crust on solar aspects and shaded aspects up to treeline which transitions into moist/wet snow during the day. Dry snow can still be found on northerly aspects in the alpine. 10-30 cm of recent snow has formed isolated pockets of wind slab on leeward slopes and behind terrain features. The new snow sits on a series of melt-freeze crusts on all aspects below 1900 m and southerly aspects to mountain top.Â
Cornices are large, fragile and primed to fail with warm temperatures. Many natural cornice falls were recently observed. Some triggered slabs on the slopes below.
Widespread crust layers formed over the last month in the mid to upper snowpack. One widespread crust from mid-March found 30-60 cm deep saw isolated instances of large solar-triggered slab avalanches during the last sunny period. The early November crust is buried down 180-200 cm with faceted crystals above and below it. With each day of warm weather the likelihood of deep persistent slab avalanches will increase.Â
Terrain and Travel
- Use extra caution around cornices: they are large, fragile, and can trigger slabs on slopes below.
- Avoid slopes when the solar radiation is strong, especially if they have large cornices overhead.
- Avoid sun exposed slopes, especially if snow surface is moist or wet.
- The likelihood of deep persistent slab avalanches will increase with each day of warm weather.
Problems
Wet Slabs
The likelihood of wet slab avalanches will increase with yet another hot and sunny day. Freezing levels are forecast to stay around 3000 m overnight. Clear skies during the night might allow for a shallow refreeze but the snow will likely become moist/wet in the early morning hours. The upper snowpack can quickly lose cohesion and become unstable with strong solar radiation and warm temperatures.
Wet loose avalanche activity can be expected, especially during the heat of the day.
Back off slopes when the snow becomes moist or wet. Avalanches might grow larger than expected by entraining snow along the way and reach lower elevations. Avoid exposing yourself to slopes that are exposed to the sun, especially in the afternoon. Be aware what is above you when leaving your skiing or riding area.
Aspects: South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Cornices
Cornices are large and fragile. Warm temperatures and sun exposure will lead to cornice falls. Cornice falls are dangerous on their own and they can trigger slabs on slopes below. There is a chance that the heavy load of a failed cornice triggers wet slabs or deep persistent slabs.
Cornices often break further back than expected. Give them a wide berth when travelling on ridgetops. Avoid slopes with cornices overhead, especially if they are exposed to the sun and during the heat of the day. Take into account long runout distances when travelling underneath slopes with cornices overhead.
Aspects: North, North East, East, South East, South.
Elevations: Alpine.
Likelihood
Expected Size
Deep Persistent Slabs
Each day of warm weather increases the likelihood of reactivating buried weak layers. It is uncertain if and when large slab avalanches may release but the possibility remains during this warm period.
Deeper releases are most likely in shallow snowpack areas on the eastern slopes of the Purcells. Heavy cornice falls might trigger large slab avalanches. Skiers and riders might be able to trigger buried weak layers in rocky and shallow snowpack areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 17th, 2021 4:00PM
