Wind slabs are expected to remain reactive in exposed high elevation terrain on Tuesday. It may still be possible to trigger a buried weak layer in some areas at and below treeline but the likelihood is decreasing with the cooler weather.
Recent strong winds from the SW through NW have formed reactive wind slabs in exposed terrain at higher elevations. These slabs are particularly reactive where they overlie buried surface hoar.
20-40 cm of settling snow sits over the January 30 surface hoar. This layer was responsible for numerous natural and human-triggered avalanches over the weekend. Treat open slopes at and below treeline with extra caution where the surface hoar is most prominent.
The weak layer from mid-January down 40-70 cm remains a concern in isolated areas and avalanches may still have the potential to step-down.
Dry and mainly sunny conditions are expected for the week except for a weak disturbance Tuesday night which may bring light flurries. A major warming event is current forecast to arrive late Wednesday which may persist for several days.
Monday Night: Some cloud, light to moderate W wind, freezing level dropping to valley bottom.
Tuesday: A mix of sun and cloud with a chance of flurries overnight, light to moderate W wind, freezing levels reaching around 1400 m.
Wednesday: A mix of sun and cloud, moderate NW wind, freezing levels climbing to around 2000 m.
Thursday: Mainly sunny, light to moderate W wind, freezing levels reaching over 2500 m with an inversion.
On Sunday, several natural loose wet avalanches up to size 1.5 were reported. A skier triggered a size 1.5 slab on a NW aspect at 1900 m which failed down 30 cm. Just north of the region, a natural size 3.5 persistent slab was reported on a SE aspect at 2200 m which stepped down to a layer around 100 cm deep, propagated close to 1 km wide, and ran full path.
On Saturday, numerous natural and human-triggered slab avalanches were reported which were typically size 1-2 but as large size 2.5 with propagation as wide as 100 m. Most of these were failing on the January 30 surface hoar down 15-30 cm but at least two stepped down to the mid-January weak layer down 40-60 cm. The majority of the activity occurred between 1700 and 2100 m elevation. Many of the natural avalanches were on NE aspects and related to wind loading. A few were on SE-SW aspects which are expected to be related to solar triggering. The human-triggered avalanches were occurring on all aspects.
There are lots of great MIN posts from the region over the weekend describing instabilities such as whumphing and shooting cracks. This MIN post has some great photos of the type of wind slab avalanches we have been seeing recently and this MIN post shows the type of terrain where you might find reactive buried surface hoar below treeline. This MIN post shows what is likely a combination of wind loading and the persistent problem which is most likely around treeline elevations.
A new sun crust is being reported on solar aspects into the alpine and a temperature crust on all aspects at lower elevations. Ongoing periods of strong wind from the southwest through northwest have formed reactive wind slabs in exposed terrain at higher elevations.
The January 30 interface is now typically down 20-40 cm and consists of a melt-freeze crust at lower elevations and on solar aspects extending into the alpine, surface hoar in sheltered areas at and below treeline, and small facets in some areas.
The mid-January interface is now down around 40-70 cm and consists of a crust, facets, and/or surface hoar. It had been dormant prior to the warming but woke up over the weekend and at least a couple avalanches were reported to have stepped down to it.
The early-December crust/facet persistent weak layer is buried down 100-200 cm. It had been responsible for significant avalanche activity in this region during the first half of January but is now considered dormant. See this forecaster blog on how to manage this layer as it may come into play again later this season.
