Watch for wind slabs in unusual places as winds shift again, this time to the south. Uncertainty about triggering large avalanches warrants careful terrain selection. Read about managing these conditions in this forecaster blog.
Shifting winds from north to east to south have formed a complex pattern of wind slabs at upper elevations that will likely remain sensitive to human triggers. Given the potential for cross-loading and reverse-loading, treat all aspects as suspect. Cornices may be reaching their breaking point.
Despite decreasing avalanche danger, there remains a lingering possibility to trigger persistent slab avalanches in isolated steep terrain features. The primary layer of concern is layer of surface hoar, facets, and/or crusts buried 60-100 cm deep throughout the region.
SATURDAY NIGHT: Mostly cloudy, light to moderate southeast wind, alpine temperatures around -18 C.
SUNDAY: Cloudy with sunny breaks, light south wind, alpine temperatures around -16 C.
MONDAY: Increasing cloud, light variable wind, alpine temperatures around -12 C.
TUESDAY: Cloudy, isolated flurries with trace accumulations, light southwest wind, alpine temperatures around -14 C.
Small to large (size 1-3) natural and human-triggered wind slab avalanches have been reported consistently throughout the week in the alpine, some of which were triggered by solar radiation or cornice falls. This MIN report from neighboring Glacier National Park on Thursday offers a helpful visual of this avalanche activity. Operators have also reported numerous recent dry loose avalanches (size 1-1.5).
Observations of avalanches on the late January persistent weak layer continue to trickle in. This MIN report from the Gorge on Friday is a helpful example of the lower elevation cutblocks where the surface hoar has been preserved. On Friday, operators reported a large (size 2.5) natural avalanche on a southeast aspect in the alpine, releasing on the facet layer. Similarly, on Wednesday, there were two notable large (size 2.5-3) human-triggered avalanches reported just outside of Glacier National Park on south and west facing alpine slopes (MIN report) also failing on a combination of facets with a crust.
Over the past week the persistent slab problem has produced fewer avalanches than in the first week of February, but is still showing signs of instability, particularly on south aspects in the alpine, and warrants careful assessment.
Winds are forecast to shift to a new direction overnight (southeast) and again tomorrow (south) and may form fresh wind slabs in lee features at upper elevations. Previous winds blew from the east/northeast with enough gusto to move snow around and build reactive wind slabs. Watch for these wind slabs in unusual places as tricky cross-loading and reverse-loading continues.
While wind may be forming slabs in lee features, the upper snowpack is becoming soft and faceted with the cold temperatures. 60-100 cm of snow from February is settling over a layer of surface hoar buried in late January. Reactivity on this layer has primarily been observed at treeline and in "treeline-like" features, like cutblocks, that are below treeline. Snowpack tests results continue to show the potential for propagation on this layer, like this MIN report from Clemina on Monday and this MIN report from the Gorge on Tuesday. On steep south-facing terrain, this layer may consist of facets on a sun crust.
The mid/lower snowpack is generally well settled and strong.
