Call it Spooky Moderate. The chance of triggering persistent slabs may be trending downward, but the consequences remain high - even before you factor in the cold. Check out our Forecasters' Blog for thoughts on managing current conditions.
Cold temperatures are gradually robbing them of cohesion, but some wind slabs may remain reactive to human triggering. More recent slab formations and places where wind slab may have formed over sun crust (south aspects) are the biggest concern.
60-80 cm of snow sits on a persistent weak layer that was buried near the end of January. This layer has been the most reactive at treeline, where it exists as surface hoar, but attention should also be paid to steep, south-facing alpine terrain where it may exist as a combination of facets and crust, and where avalanches have greater potential to step down to deep weak layers from the early season.
Thursday night: Sunny. Light northeast winds. Alpine high temperatures around -23.
Friday: Sunny. Light to moderate east or northeast winds. Alpine high temperatures around -19.
Saturday: Mainly sunny. Light to moderate northeast winds. Alpine high temperatures around -18.
Sunday: A mix of sun and cloud. Light south winds. Alpine high temperatures around -13.
We have many recent reports of persistent slab avalanches triggered by skiers at elevations around treeline (1800-2000 m) where our active persistent weak layer exists as surface hoar. Check out the MIN for more details - there's a lot to see!
Another explosives-triggered persistent slab in the Bonningtons on Tuesday stepped down to our early December crust layer to produce a 200 cm-deep, size 3.5 (very large!) avalanche. A similar avalanche (size 3.5, step down to December crust) occurred naturally in the same area in the first week of February.
Observations have been dwindling over the past couple of days, but this may owe more to bitter cold temperatures keeping folks indoors than to the idea of a healing persistent slab problem. Cold may be diminishing cohesion and slab properties in the upper snowpack to a degree, but they also tend to further weaken persistent layers like we're dealing with.
Snow from the past week has seen some wind effect at upper elevations and may contain a freezing rain crust or surface hoar at lower elevations. It continues to settle over a weak layer buried 60-80 cm deep.
This persistent weak layer may exist as facets or a crust but it has been most reactive where it exists as large surface hoar crystals in sheltered areas treeline and below. Surface hoar has potential to surprise backcountry users with how widely the fracture can travel across slopes. This layer is slow to gain strength and requires careful terrain selection, even as its likelihood of triggering gradually diminishes.
There are potentially several more layers of surface hoar in the mid snowpack, with the most notable one down about 80-120 cm. This layer was buried in early January.
A crust surrounded by weak faceted grains is buried deep within the snowpack. It may be possible to trigger this layer with a large load in a shallow rocky start zone.
