Avalanche Forecast

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

A ridge of high pressure is bringing dry and sunny conditions for Christmas day. Mostly sunny conditions are expected on Sunday with light northerly winds in the alpine and treeline temperatures around -12C. On Monday, mostly cloudy conditions are forecast with treeline temperatures remaining around -12C. Alpine winds are expected to be light for most of the day but might increase in the afternoon with the approach of the next storm system which is forecast to arrive Monday evening. 10-20 cm of snowfall is currently forecast between Monday evening and Tuesday evening with treeline temperatures around -8C. Alpine winds are forecast to be strong from the southwest.

On Friday and Saturday, a few natural size 2-2.5 wind slab avalanches were observed in Glacier National Park in steep terrain features on north and northwest aspects at 2500-2700 m elevation. On Friday, a skier triggered a size 1 storm slab avalanche on a northwest at 1800 m elevation in the east of the region. This was a 40 cm thick slab which failed on the mid-December interface. On Thursday, a natural cornice release was reported on a north aspect at 2200 m but it did not trigger a slab on the slope below. On Sunday, lingering wind slabs which formed on Thursday are the main concern. These lingering wind slabs may be hidden under 10-20 cm of low density snow from Friday. There still remains some concern regarding the mid-December interface that was buried last week. It appears that in most places the recent storm snow remains relatively low density and has not formed a slab over this interface. The primary areas where this layer has been reactive over the last week is in wind loaded areas. However, I would continue to watch for signs that a more widespread slab problem is developing in areas that have not been affected by wind.

50-70 cm of recent storm snow typically overlies the mid-December interface which consists of a variety of old snow surfaces including wind affected snow, faceted (sugary) crystals, and/or surface hoar crystals. Recent tests suggest the the new snow is well bonded to the interface in many areas. However, where a spotty layer of surface hoar has been preserved, the interface is still reactive. In wind affected terrain, the recent storm snow has been redistributed into reactive wind slabs. These wind slabs may now be buried under 10-20 cm of low density snow which fell without much wind on Friday.The thick mid-November crust layer typically sits 1-2 m down in the snowpack. Recent test results on this layer suggest it is generally well bonded to the adjacent snow with the possible exception of shallow snowpack areas. This layer is considered dormant at this time but remains an isolated concern that we will likely be tracking through much of the season.