Watch out for increasing reactivity on the buried surface hoar. Once a threshold amount of snow forms slab properties, slab avalanches will become more easily triggered and larger at all elevations and aspects. Choose terrain to limit your exposure & assess every slope.
Friday night's snow and recent South west and Westerly winds will also contribute windslab formation in the Alpine and at exposed Treeline locations. Buried and fresh windslabs and wind effect have been reported in Alpine and on exposed features at treeline. Use added caution in lee terrain features, particularly near ridges.
A widespread layer of surface hoar found at all elevations and on all aspects has been reactive to human traffic. The layer is buried around 35 to 50 cm deep. On south aspects, the surface hoar may be found overlying a melt-freeze crust, which is a particularly nasty combination. However, recent reported reactivity has been on northwest to easterly aspects. To date, the layer has been most reactive between around 1700 and 2300 m and where the overlying snow has gained slab properties from snow load and wind affect. This elevation band of reactivity may increase on Saturday and Sunday with forecast new snow, strong wind, and recent warming trend, which all may increase slab properties.
FRIDAY NIGHT: Cloudy with snowfall, accumulation 5 to 10 cm. Moderate Southwest winds with occasional Strong Gusts. Alpine temperature lows around -7 C with freezing level dropping to 700 m as the cold front passes.
SATURDAY: Cloudy with snowfall, accumulation 5-10 cm. Light to moderate West winds. Alpine highs temperature around -7 C with freezing level of 1000 m.
SUNDAY: Cloudy with Sunny periods and isolated flurries. Trace accumulations. Light west wind increasing to Strong by evening. Alpine low temperature around -11C and High of -8. Freezing Level 800m.
MONDAY: Cloudy with isolated flurries. Trace accumulation up to 5 cm. Moderate west winds. Alpine temperature Lows around -11C and High of -5. Freezing Level 900m
Many small (size 1 to 1.5) and a few large (size 2) storm/persistent slab avalanches were triggered by humans Tuesday, Wednesday and Thursday. Early reports from Friday suggest a continuation of human triggered small and large avalanches, including an increase in the number of large remote triggered avalanches on Surface Hoar. They occurred at all elevation bands, between 1600 m and 2350 m, and generally on northwest to east aspects but a couple released on south aspects. They were commonly 20 to 40 cm deep and released on the surface hoar layer described in the Snow pack Summary.
There is uncertainty as to how this persistent weak layer will respond to the continued loading of snow, but we expect avalanches to increase in size as the slab properties of the overlying snow pack increase.
Check out this MIN for an example of a recently reported avalanche on the Surface Hoar.
Friday night's forecast 5-15cm of new snow will result in a total of 35-50cm overlying the widespread layer of surface hoar that was reported to be between 10 and 20 mm in size and at all elevations. This Surface Hoar layer has been reported at all elevation bands - but is most prominent near treeline and in sheltered alpine locations. Recent warm temperatures have built slab like properties in this snow, particularly at mid and low elevations, making the persistent layer more reactive as evidence in the avalanche discussion.
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 with more snow, wind, and Friday's warming trend, 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.
