The snowpack may take time to recover from the weekend warming and the buried surface hoar may remain reactive, especially in the north and west of the region where the overlying slab is thickest. Ongoing wind slab development should be expected in exposed high elevation terrain.
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.
Cornices may remain weak from the recent warming.
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, and is expected to remain reactive on Monday. Treat open slopes at and below treeline with extra caution. The weak layer from mid-January down 40-70 cm may also be a concern in some areas.
The warm air in the alpine is forecast to persist overnight and there is some uncertainty if it will linger into the morning. By mid-morning, the warm air will have exited and alpine temperature are expected to return to below freezing.
Sunday Night: Mainly clear, light to moderate SW wind, freezing level as high as 2500 m with a temperature inversion.
Monday: Increasing cloud cover in the afternoon, moderate to strong SW wind, afternoon freezing levels reaching around 1300 m.
Tuesday: Mainly sunny, light to moderate W wind, freezing levels reaching around 1200 m.
Wednesday: A mix of sun and cloud, moderate NW wind, freezing levels climbing to around 2000 m.
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 on Saturday 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.
20-40 cm of rapidly settling snow now typically overlies the January 30 interface which consists of a melt-freeze crust at lower elevations and on solar aspects extending into the alpine, widespread surface hoar in sheltered areas at and below treeline, and small facets in some areas. With the recent warming, the surface snow has settled into a cohesive slab over the weak layer and a new surface crust will likely form as temperatures drop.
Periods of strong wind over the weekend, mainly from the southwest through northwest, have formed reactive wind slabs in exposed terrain at higher elevations. Ongoing wind on Monday will likely continue to develop reactive wind slabs where there is still loose snow available for transport.
A crust/surface hoar/facet interface from mid-January is now down around 40-70 cm. It had been dormant prior to the warming but woke up this 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.
