Recent snow and wind are likely to form reactive slabs. Avoid drifted areas and assess open slopes and rollovers where a weak layer may be preserved.
A weak layer of surface hoar buried Jan 11 can be found 50-70 cm deep. There is uncertainty with how this persistent slab problem will react to the load from the recent snow and wind. The possibility exists for avalanches in the storm snow to step down to these layers.
15-25 cm of recent snow and shifting winds have created storm slab problems that vary with elevation and wind exposure. In exposed areas, thick wind slabs should be expected in leeward features across a wide range of aspects. Ongoing wind and snow over the past week have contributed to cornice growth. Cornice falls may trigger avalanches on the slopes below.
Recent snow and wind has added a new storm slab problem, while also adding more load to the Jan 11 buried surface hoar. Cornices may be reaching their breaking point and can act as triggers on slopes below. On Sunday, operators in neighboring Glacier National Park reported a large (size 2) wind slab releasing naturally in the storm snow on a north aspect.
There is uncertainty as to how a weak layer of surface hoar buried Jan 11th will react to the recent snow and wind. This MIN report from Frisby on Thursday shows the reactive layer of surface hoar in a wind-loaded feature.
An extensive natural avalanche cycle occurred during last week's storm as up to 70 cm of new snow blanketed the region and was subsequently redistributed by strong winds. Several of these avalanches were thought to have stepped down to deeper layers (see this MIN reports from the Gorge area for an example). Avalanches breaking in the recent storm snow have the potential to step down to deeper layers, creating larger and more destructive avalanches.
Sunday night: Cloudy, continuing flurries bringing up to 5 cm of new snow, light northwest winds, freezing level dropping to valley bottom.
Monday: Mix of sun and cloud, light northwest winds, alpine high temperatures around -9 C.
Tuesday: Increasing cloud, scattered flurries moving in overnight with up to 5 cm of accumulation, winds shift to the southwest and increase to strong, alpine high temperatures around -7 C.
Wednesday: Partly cloudy, light west winds, alpine high temperature near -12 C.
Snow accumulations over the weekend ranged from 10-30 cm, with higher totals in the east of the region. Winds have switched direction from the south to the northwest, creating a tricky reverse wind-loading pattern at upper elevations. Recent new snow may be hiding wind slabs that were visible before the snow fell. Ongoing snow and wind over the past week have contributed to notable cornice growth.
50-70 cm of snow from the past week has buried a weak of layer of surface hoar that has shown reactivity in snowpack tests. This persistent weak layer is more likely to be preserved at elevations above where a thick crust exists and below more wind-affected terrain. It is most suspect in sheltered, open slopes at and below treeline.
A couple of older persistent weak layers exist in the mid to lower snowpack. The distribution of these layers is variable, but they follow a similar pattern to the layer discussed above. The upper layer from late December, down about 100-140 cm, consists of spotty surface hoar at shaded treeline and below treeline elevations. The lower layer from early December buried 150-200 cm deep consists of a combination of decomposing surface hoar with a crust and faceted snow. Snowpack tests on these layers show increasingly resistant results. The lower layer is now largely unreactive, but remains a prominent snowpack feature.
