Avalanche Forecast
Issued: Jan 20th, 2021 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
astclair,
Avalanche Canada
Conditions are more complicated than meets the eye. Wind-drifted snow and a buried weak layer warrant careful evaluation in specific areas.
Summary
Confidence
Weather Forecast
Wednesday night: Partly cloudy, light variable winds, alpine temperatures dropping to -15 C.
Thursday: Mainly sunny, light northeast winds, alpine high temperatures near -14 C.
Friday: Mainly sunny, light northeast winds, alpine high temperatures near -15 C.
Saturday: Mainly sunny, light west winds, alpine high temperatures near -16 C.
Avalanche Summary
Recent strong west winds have formed wind slabs in lee features at upper elevations that may be possible to human trigger. There have been no recent avalanche observations in the region. Operators in neighboring Glacier National Park reported several large (size 2-2.5) wind slabs releasing naturally on a variety of aspects over the past 48 hours. Cornices may be reaching their breaking point and can act as triggers on slopes below. Avalanches breaking in the recent snow have the potential to step down to deeper layers, creating larger and more destructive avalanches.
This MIN report from the Quartz zone on Thursday shows a reactive layer of Jan 11th surface hoar on north-facing slopes near treeline. It may be possible to trigger this layer in areas where the surface hoar has been preserved.
In the aftermath of the storm last Tuesday, we received reports of avalanches reaching size 3 in the central part of the region, lending support to the idea that some larger releases may have involved more deeply buried persistent weak layers. Although avalanche activity on these layers from early December and November has been isolated, the possibility exists for large triggers to reactivate these deeper instabilities.
Snowpack Summary
Recent strong winds have scoured terrain and built wind slabs in lee features at upper elevations over a variety of surfaces, including isolated surface hoar, sun crusts, and settling storm snow. Snow and wind from the past week have contributed to notable cornice growth. A new sun crust may be forming on steep solar aspects.
30-60 cm of snow from the past week has buried a weak of layer of surface hoar from Jan 11th. This persistent weak layer is most suspect in sheltered, open slopes at treeline. This layer has produced sudden results in snowpack tests at treeline elevations in areas north in the region near KHMR and warrants careful evaluation. Limited observations across the region make it difficult to get a sense of the distribution and reactivity of this problem.
Two deeper layers of surface hoar from December 13th and December 7th may still be found down 100-150 cm. The Dec 7th weak layer consists of a combination of decomposing surface hoar with a crust and faceted snow. Avalanche activity on these persistent weak layers has dwindled in the last week, with snowpack tests results trending to resistant and unreactive.
The lower snowpack is characterized by a notable rain crust from early November that is surrounded by a weak layer of sugary facets. It may still be possible to trigger with large loads in steep, shallow, rocky areas with a thin to thick snowpack.
Terrain and Travel
- Stay off recently wind loaded slopes until they have had a chance to stabilize.
- Carefully assess open slopes and convex rolls where buried surface hoar may be preserved.
- Avoid shallow, rocky areas where the snowpack transitions from thick to thin.
- If triggered, wind slabs avalanches may step down to deeper layers resulting in larger avalanches.
Problems
Wind Slabs
Strong winds have redistributed recent snow into wind slabs on lee features at upper elevations that may be possible trigger. Given the recent variability in wind direction, anticipate wind slabs across a wide range of aspects. Ongoing wind and snow over the past week have contributed to cornice growth. Cornice falls may trigger avalanches on the slopes below.
Aspects: North, North East, East, South East, South, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
A weak layer of surface hoar buried Jan 11 buried 20-50 cm deep may be found on sheltered, open slopes near treeline. There is uncertainty as to the distribution and sensitivity to triggering of this persistent slab problem across the region. Deeper layers from early December and November can still be found in the snowpack and remain a cause for concern in thick to thin areas at upper elevations. The possibility exists for avalanches in the surface snow to step down to these layers.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 21st, 2021 4:00PM
