Be on guard for signs of instability in the snowpack, especially in immediate lee of ridge features and where snowfall amounts exceeded 15cm. A buried surface hoar layer lurks and is showing signs of reactivity. Check out this Fx blog on Surface Hoar.
Fresh snow will be touchiest where wind affected. New snow with be easily transported by moderate to strong winds overnight building fresh windslabs.
These windslabs will overlay a variety of surfaces including buried windslabs, meltfreeze crusts, or buried surface hoar.
Additional new snow continues to load a widespread layer of surface hoar, with about 25 to 50 cm overlying it across the region. So far, the layer has been most reactive at treeline and alpine elevations in the Western (and deeper snowpack) regions of the forecast area and where the recent snow has been redistributed by the wind and formed a slab above the surface hoar.
Weak snow at the bottom of the snowpack continues to pose a deep persistent slab avalanche problem. Treat wind scoured, rocky, shallow terrain as suspect. Also avoid big alpine slopes that are threatened by cornices overhead.
SATURDAY NIGHT: Cloudy with clear periods and isolated flurries, Trace accumulation. Winds light to moderate Northwest. Alpine low temperatures -12 C with freezing levels at valley bottom.
SUNDAY: A mix of sun and cloud. Winds light to moderate West. Alpine high temperatures near -9 C and lows of -12 C, with freezing levels 1200m.
MONDAY: Cloudy with sunny periods and scattered flurries, accumulations Trace to 5cm. Winds moderate West /Northwest gusting strong. Alpine temperatures high of -8 and lows -13 C with freezing levels near 1300m.
TUESDAY: Snow with accumulations of 5-15cm. Winds moderate to Strong Southwest gusting extreme. Alpine high temperatures -8 and low temperatures -10 C, freezing levels 1600m.
There were a few early reports of human triggered avalanches on Saturday, including a snowmobile triggered small avalanche (size 1) at a wind affected NE treeline feature. Additionally, cornices have grown large with recent snow and wind.
Reports from Friday of several small (size 1-1.5) skier triggered avalanches, one large (size 2) Skier remotely triggered avalanche and another natural avalanche on the widespread layer of Surface hoar. Depths ranged from 30-45cm and were reported from 1600 - 2100m predominantly on NW to Eastern aspects.
A few small (size 1 to 1.5) wind and storm slab avalanches were triggered by humans each day Monday thru Wednesday with activity tapering on Thursday. They occurred on all aspects, were from 2100 to 2700m and were 15 to 40cm thick. In the Western (& deeper) portion of the region a few of the avalanches released on the surface hoar layer described in the Snowpack Summary.
5-20cm of snow fell Friday into Saturday, with highest amounts reported in the North and Western mountainous areas of the forecast region. Winds were moderate to strong from the Southwest as new snow fell. This new snow likely formed widespread wind affect and wind slab in Alpine and exposed treeline locations, particularly near ridges.
30-45 cm of recent snow now overlays the widespread Surface Hoar (size 5 to 15 mm) that was found at all elevations and on all aspects. Recent warm temperatures have built slab like properties in this recent snow, particularly at mid and low elevations.
The middle of the snowpack is strong. The base of the snowpack contains basal facets, which have been responsible for occasional and hard-to-predict Deep Persistent Slab avalanches in shallow, rocky start zones.
