Limit your exposure and be disciplined with your terrain choices as you enjoy fresh snow. A buried surface hoar layer lurks and is easily triggered, producing large avalanches, especially where wind affected. Check out this Fx blog on Surface Hoar.
Friday night's snow and recent South west and Westerly winds built windslab in the Alpine and at exposed Treeline locations. Use added caution in lee terrain features, particularly near ridges. Fresh Cornices have also been reported with recent snow and winds.
Thin windslabs have the potential to trigger deeper weak layers such as the Surface Hoar problem.
Recent storm snow adds another 10-25cm of snow load on top of a widespread layer of surface hoar found at all elevations and on all aspects. This layer has already been reactive to human traffic and avalanches will likely increase in size as slab properties increase. The layer is buried around 40 to 65 cm deep. On south aspects, the surface hoar may be found overlying a melt-freeze crust, which is a particularly nasty combination. However, recent skier triggered activity has been higher on northwest to easterly aspects.
SATURDAY NIGHT: Cloudy with clear periods and isolated flurries, Trace accumulation. Winds light to moderate Northwest. Alpine low temperatures -12 with freezing levels to valley bottom.
SUNDAY: A mix of sun and clouds with isolate flurries. Winds light to moderate West. Alpine High temperature -7 C with freezing levels near 800m
MONDAY: Cloudy with flurries, accumulation Trace to 10cm, with amounts diminishing to the south. Winds light to moderate Northwest gusting strong. Alpine high temperatures -5, lows to -11 and freezing levels rising to 1100m.
TUESDAY: Cloudy with localized Snow, accumulations Trace to 10cm, with amounts diminishing to the south. Winds Southwest Moderate to strong Southwest, gusting extreme. Alpine high temperatures -6 C and lows -9 with freezing levels rising to 1400m.
Early reports from Saturday suggest an increase in Natural and human triggered avalanche activity from recent snow and winds. Very Large (Size 3-3.5) natural and explosive controlled Avalanches were observed from extreme terrain in the higher mountains and along the highway corridor from North and South Aspects. There have also been numerous reports of large (Size 2) humanly triggered remote avalanches on the surface hoar issue at treeline and on North aspects .
On Friday there were widespread reports of human triggered small (size 1-1.5) and large (size 2-2.5) avalanches, mostly failing on the Persistent Surface Hoar weak layer buried 30-45cm. These avalanches occurred on all aspects (South to North) and from 1000m to 2400m, with the largest occurring at ridge crests where snow was wind affected. There were a few large avalanches up to size 2.5 reported from steep Solar and North facing Alpine features.
Check out this MIN for an example of a recently reported avalanche on the Surface Hoar.
Recent snowfall amounts of 10-30 cm from Friday into Saturday's storm bring total snow overlying the widespread layer of Surface Hoar layer to 40-60 cm. Not only may there be weakness within this recent storm snow, but a significant load has been added to an already problematic Surface Hoar layer that was was reported to be between 10 and 20 mm in size. This Surface Hoar layer has been reported on all aspects at all elevation bands - but is most prominent near treeline and in sheltered alpine locations. On Solar aspects there is also a suncrust associated with the surface hoar, which is a nasty combination. Recent warm temperatures have built slab like properties in this snow, particularly at mid and low elevations, making the persistent layer more reactive.
Across the region, this surface hoar layer is showing variability in its distribution and reactivity. This persistent weak layer has been most reactive to human traffic and natural trigger around treeline and alpine elevations where the overlying snow has slab properties and surface hoar is large. As this snow continues to gain slab properties activity on this layer is expected to increase and avalanches may grow in size.
The early-February melt-freeze crust down 50-100 cm is dormant but should be monitored. The remainder of the mid and lower snowpack is generally well-settled and strong.
