Avalanche Forecast
Purcells
A highly complex snowpack is being loaded by new snow and strong winds. Three weak layers are reactive to natural and human triggers. The easy solution is to choose simple terrain free of overhead hazard 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 persistent weak layers are lurking in our snowpack, which remain capable of producing very large and destructive avalanches. Keep the terrain choices reined in as the snowpack adjusts to the new load.
Travel and Terrain Advice
- 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.
- Numerous large avalanches have run on these layers in well supported, treed terrain.
Avalanche Summary
On Saturday, small wind slabs were released by skiers and explosives on northerly lee slopes at treeline and in the alpine. One slab avalanche occurred on the early-January weak layer on a northerly aspect, 60 cm deep.
On Friday, there were several reports of small to large (size 1 to 2.5) storm slab, wind slab, and persistent slab avalanches, triggered naturally, by skiers, and by explosives. Similar avalanches were reported on Wednesday and Thursday. These avalanches have been releasing on all three persistent weak layers described in the Snowpack Discussion.
Snowpack Summary
The current snowpack is complex, with three active weak layers that we are monitoring.
20-50 cm of storm snow now sits on a newly formed crust and/or surface hoar layer (mid-January). Prior to the storm, the crust was reportedly widespread, except for possibly at high elevations on north aspects. The surface hoar is 10 to 30 mm in size and is at all elevations. The recent snow fell with strong southerly 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 40 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. Yet another persistent weak layer buried mid-December consisting of a facet/surface hoar/crust combination is buried 60 to 110 cm deep. It is most problematic at and below tree line. This layer is not thought to be present in the alpine.
A rain crust buried in November is 90 to 150 cm deep and is likely dormant for the time being.
Weather Forecast
MONDAY: Mostly cloudy with intermittent snowfall, accumulation 2-5 cm, moderate southwesterly winds, alpine temperature near -10 C, freezing level near 1000 m.
TUESDAY: Partly cloudy with intermittent snowfall, accumulation 1-3 cm, light to moderate southwesterly winds, alpine temperature near -10 C, freezing level near 800 m.
WEDNESDAY: Mostly cloudy with intermittent snowfall, accumulation 2-5 cm, light to moderate southwesterly winds, alpine temperature -6 C, freezing level near 1300 m.
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Up to 50 cm of recent snow has formed a slab that sits on a combination of weak, feathery surface hoar and a crust. Expect the slab to be deeper in lee features and to be sensitive to human triggers.