Avalanche Forecast
Purcells
A Special Public Avalanche Warning is in effect in this region.
Ongoing warm temperatures Monday are expected to initiate natural avalanche activity and further increase the likelihood of large human triggered avalanches.
Avalanche Problem 1: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Numerous large avalanches have run on these layers in well supported, treed terrain.
- Avoid exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.
- Only the most simple non-avalanche terrain free of overhead hazard is appropriate at this time.
Avalanche Problem 2: Loose Wet
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Loose wet avalanches are expected to be problematic again Monday as previously cold snow is subjected to above freezing temperatures and strong solar input.
Travel and Terrain Advice
- Avoid exposure to terrain traps where the consequences of any avalanche will be more serious.
- Avoid sun exposed slopes when the solar radiation is strong, especially if snow is moist or wet.
- Loose avalanches in motion may step down and initiate large destructive slab avalanches.
Avalanche Summary
We received observations Sunday of a natural avalanche cycle to size 3.5 in the neighboring Central Columbias. These very large avalanches were running naturally on southwest, south and southeast facing alpine features and are thought to have been triggered by warming alpine temperatures and direct solar input. Similar activity was reported from the southern Purcells where natural avalanches ran to size 2 on southeast facing alpine features.
On Saturday control work produced storm slab avalanches to size 2 on south and southeast facing alpine features. A cornice failure was also reported from a south/southeast facing alpine feature.
On Friday we received reports of a widespread natural avalanche cycle to size 3.0 that had occurred during the previous 48 hour period. It's suspected that these avalanches initially failed on the early January interface before stepping down to the mid-December and possibly late November weak layers.
Some themes that are emerging from recent activity in the Purcells and neighboring regions include accidental and remote triggering, 'step down' release types, releases on surprisingly low angle, supported terrain, as well as wide fracture propagations.
Snowpack Summary
The last series of storms left a variable 10 to 60 cm of snow in their wake. Moderate to strong southwest winds previously formed wind slabs in upper elevation terrain.
This snowpack is currently quite complex; there are three Persistent Weak Layers (PWL) that we are monitoring.
The first PWL is known as the early January interface, it is 10 to 60 cm below the surface. It is composed of surface hoar on sheltered slopes as well as sun crust on steep solar aspects. It is thought to be widespread at all elevations bands and has produced numerous recent large avalanches.
The next PWL is the mid-December facet/surface hoar/crust combination that is now buried 40 to 100 cm deep and is most problematic at and below treeline. This interface is not thought to be present in the alpine.
The most deeply buried PWL is the late November rain crust. It is found 90 to 150 cm below the surface. This layer is considered dormant but it may be triggerable in thin spots around variable snowpack areas in the alpine.
Weather Forecast
The normal decrease in temperature with height has once again been replaced by an inversion. This classical pattern allows for cooler air to remain in the valleys while alpine temperatures rise to a degree or two above freezing. This setup should stick with us until Tuesday when warm air is expected to begin mixing into the valleys as an offshore front impacts the coast. The interior should remain mostly dry until a more robust system hits the south coast late Wednesday with precipitation spilling into the interior on Thursday.
MONDAY: Scattered clouds in the morning, clearing throughout the day, freezing level at valley bottom with above freezing temperatures between 2200 m and 3000 m, light south wind, no snow expected.
TUESDAY: Scattered cloud cover, freezing level initially at valley bottom, possibly rising as high as 2000 m. There may be a slight inversion, stay tuned for details. Moderate to strong southwest wind, trace of snow possible.
WEDNESDAY: Scattered cloud cover, freezing level at valley bottom, rising to 1400 m in the afternoon, light to moderate southwest wind, trace of snow possible.
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Continued warm alpine temperatures and direct solar input Monday are likely to produce very large natural avalanches. Continue to avoid avalanche terrain, especially in the alpine, as the snowpack adjusts to recent loading and warmth.