North Columbia

Low alpine danger does not mean no danger, it may still be possible to trigger a wind slab avalanche in extreme terrain. Small avalanches continue to be a concern at and below treeline where buried surface hoar remains preserved.

Moderate - Timing, track, or intensity of incoming weather system is uncertain on Thursday

The ridge that has delivered beautiful cold and clear conditions the last few days begins to collapse on Wednesday. High cloud invades the forecast region Wednesday and Thursday ahead of snow that is expected to begin Thursday night. Total storm amounts look to be on the light side for precipitable water, but, the cold nature of the flow should keep the snow very dry and light. This pattern continues with dribs and drabs of snow expected as we move through the weekend.TUESDAY NIGHT: Freezing level at valley bottom, light variable wind at most elevations with moderate northwest wind at ridgetop, no precipitation expected.WEDNESDAY: Overcast, freezing level near valley bottom, light variable wind at most elevations with strong northwest wind at ridgetop, 1 to 5 cm of snow possible.THURSDAY: Overcast, freezing level beginning at valley bottom creeping up towards 800 m in the afternoon, light west/southwest wind, 5 to 15 cm of snow possible during the day with another 15 to 30 cm possible Thursday night.FRIDAY: Overcast, freezing level at valley bottom, light southwest wind at most elevations with strong southwest wind at ridgetop, 10 to 25 cm of snow.

The mid-January Surface hoar seems to be most sensitive in this region. Two more reports came in Monday of skiers triggering persistent slab avalanches to size 1.5 on a variety of aspects between 1500 and 1800 m. This MIN post and this one (from the neighboring South Columbia region) do a great job of illustrating the nature of this problem which is most prevalent between 1400 and 1900 m. This MIN is a bit older, but it also offers a great visual of this mid and low elevation problem. This interface will likely wake up as storm snow begins to stack up over the weekend.On Sunday wind slabs failed naturally to size 2.5 on northwest, northeast, east, southeast and south facing aspects between 1900 and 2800 m. Persistent slab avalanches failing on the mid January Persistent Weak Layer (PWL) were widespread to size 1.5 on all aspects between 1500 and 1900 m.There were a few reports of remote triggering Friday on north, northwest and east facing slopes. One was triggered from a flat ridge 30 m away from the avalanche which ran on a moderately inclined north facing slope at 1900 m producing a size 2.5 avalanche.

Saturday delivered warmth and strong to extreme wind out of the west, south, southwest and northwest. This left a crust up to 3 cm thick on steep south and some west facing aspects and redistributed quite a bit of snow into wind slabs in lee features at and above treeline. These wind slabs are probably growing old and tired. In the absence of solar input Wednesday, these are probably only suspect in extreme terrain. The warmth also allowed 25 to 50 cm of snow to continue to consolidate above the mid-January persistent weak layer (PWL). This PWL is made up of large surface hoar that is most prevalent at and below treeline, with enhanced reactivity noted between 1400 and 1900 m. On steep solar aspects this interface presents as a crust, there may be locations where the surface hoar actually sits on the crust.The middle and lower portions of the snowpack are generally well-settled and strong.