Avalanche Forecast
North Columbia
Previously cold snow below treeline will begin to warm Tuesday which may increase the likelihood of human triggered avalanches. Avoid steep open features, convexities and terrain traps at low elevation as temperatures warm in the afternoon.
Avalanche Problem 1: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Conservative terrain selection remains prudent in this time of change.
- Numerous large avalanches have run on these layers in well supported, treed terrain.
- Avoid open slopes and convex rolls at and below treeline where buried surface hoar may be preserved.
Avalanche Summary
On Sunday there was a widespread natural avalanche cycle to size 3.5. These very large avalanches were running on southwest, south and southeast facing alpine features and were triggered by a combination of above freezing alpine temperatures and direct solar input. There was one report of a size 1.5 human triggered avalanche failing on the mid-December interface on a south facing slope at 1100 m. A second human triggered avalanche was reported from a gently inclined slope at 2100 m, aspect unknown. The size 2.5 avalanche ran on the early January weak layer.
On Saturday natural avalanches to size 3 were reported on northwest though south facing terrain between 1800 m and 2700 m.
Widespread avalanche activity occurred Thursday and Friday with numerous reports of slab avalanches reacting to both explosive control work and human triggering. All aspects were involved as avalanches ran to size 2.5 on the early January interface.
Snowpack Summary
A thin breakable crust has formed on the surface in many locations and fresh surface hoar has been reported too.
The last series of storms left 40 to 70 cm of snow in their wake. Moderate to strong southerly winds previously formed wind slabs in upper elevation terrain.
This snowpack is currently quite complex; there are three active Persistent Weak Layers (PWL) that we are monitoring.
The first PWL is known as the early January interface, it is 40 to 70 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 60 to 140 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 down 90 to 150 cm below the surface and has been less reactive in recent snowpack tests but it may still be susceptible to human triggering, especially in alpine terrain features with a thin or widely variable snowpack.
Weather Forecast
The ridge that setup the recent inversion will shift east as an offshore front makes landfall with the south coast Tuesday morning. The interior ranges will remain mostly dry with only some light precipitation on Tuesday. A stronger Pacific storm will hit the south coast on Wednesday morning with precipitation spilling into the interior on Wednesday afternoon. Light snowfall is expected to continue through Thursday.
TUESDAY: Increasing cloud cover, freezing level initially at valley bottom rising to around 1000 m in the afternoon. Moderate to strong southwest wind, 2 to 8 cm of snow in the afternoon, 1 to 5 cm of snow possible Tuesday night.
WEDNESDAY: Overcast, freezing level around 1500 m, moderate south/southwest wind, 2 to 6 cm of snow possible.
THURSDAY: Broken cloud cover, freezing level around 1500 m, moderate southwest wind, 1 to 5 cm of snow possible.
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Alpine temperatures are expected to remain warm Monday night and above freezing temperatures are expected between valley bottom and 1000 m Tuesday, which may increase the likelihood of human triggered avalanches in unusual locations at low elevation.