Avalanche Forecast
Purcells
New snow and warming are influencing a complex snowpack: touchy storm and wind slabs sit above three weak layers that are reactive to human triggers at all elevations. Perform cautious route-finding without overhead exposure as the snowpack adjusts.
Avalanche Problem 1: Storm Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- If triggered, the storm slabs may step down to deeper layers, resulting in large avalanches.
- Avoid exposure to terrain traps where the consequence of a small avalanche could be serious.
- Be very cautious as you transition into wind affected terrain, wind slabs may be deep and touchy.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Three weak layers are lurking in our snowpack, which have produced very large avalanches. Use extreme caution in openings below treeline (such as cutblocks) and sheltered areas in the alpine and treeline, where weak layers may be preserved.
Travel and Terrain Advice
- Be very cautious in open terrain features (e.g. cutblocks, gullies, cutbanks).
- Avoid open slopes and convex rolls where buried surface hoar may be preserved.
- Good day to make conservative terrain choices and watch for clues of instability.
Avalanche Summary
On Monday, small to large (size 1 to 2) storm and wind slabs were triggered by skiers, naturally, and by explosives on all aspects and elevation bands, generally 20 to 50 cm deep and often failing on the mid-January weak layer. Persistent slabs were also observed on the early-January weak layer, and were generally large (size 2 to 3), on polar aspects, at treeline and alpine elevations, and 40 to 100 cm deep.
Similar storm and persistent slab avalanches were reported since last Wednesday across the region. Expect to see storm slab avalanches in lee features and persistent slab avalanches where the weak layers are preserved.
Snowpack Summary
The current snowpack is complex, with three active weak layers that we are monitoring.
20-60 cm of storm snow sits on a crust and/or surface hoar layer (mid-January). The crust is reportedly widespread, except for possibly at high elevations on north aspects. The surface hoar is 10 to 30 mm in size and at all elevation bands. The recent snow fell with strong south winds, producing wind slabs and cornices in lee features.
Deeper in the snowpack, a persistent weak layer known as the early-January layer is buried 50 to 80 cm, present at all elevation bands, and composed of surface hoar on sheltered slopes and sun crust on steep solar aspects. Recent snowpack tests have shown sudden fracture characters with moderate loads and high propagation potential, as well as other signs of instability such as whumpfs and cracking. Another persistent weak layer that was buried mid-December is 60 to 100 cm deep and consists of a facet/surface hoar/crust combination. It is most problematic at and below tree line.
A rain crust buried in November is 100 to 150 cm deep and is likely dormant for the time being.
Weather Forecast
WEDNESDAY: Cloudy with snowfall, accumulation 10-20 cm, light to moderate southwesterly winds, alpine temperature -6 C, freezing level near 1300 m.
THURSDAY: Partly cloudy with intermittent snowfall, accumulation 3-7 cm, light to moderate southwesterly winds, alpine temperature near -9 C, freezing level near 700 m.
FRIDAY: Partly cloudy, light southwesterly winds, alpine temperature near -10 C, freezing level below valley bottom.
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Up to 60 cm of recent snow has formed a slab that sits on weak, feathery surface hoar and/or a crust. Expect the slab to be deep and touchy in lee features due to strong south winds. It will also be touchy below treeline with high freezing levels.