Tuesday February 19, 2019
Persistent Slab, Wind Slab
Travel & Terrain Advice
Dangerous Avalanche Conditions exist in the Alpine and near Treeline. Careful Snowpack evaluation, cautious route-finding and careful decision making essential. Beware of signs of instability such as cracking by your snowmobile or skis/snowboard/snowshoes etc. We could expect substantial wind loading of new snow on Leeward aspects. Be cautious of leeward terrain below cornices, below ridges and cross loaded terrain as the areas receiving wind transported snow will likely be areas to trigger avalanches.
A persistent weak layer (PWL) in the snowpack exists on all aspects and at all elevation bands (Below Treeline, Treeline and Alpine), human triggered avalanches are being initiated as a result of this PWL on steep alpine and Treeline slopes. We suggest selecting supportive terrain under 30 degrees while exposing 1 person at a time to hazardous areas and preferably staying in densely forested areas until the winds subside and this PWL manages to regain some moisture and round/bond with pre-existing snowpack.
A highly reactive facet layer rests on a very thick melt freeze crust. This facet layer has been responsible for triggering size 1 to size 1.5 avalanches. We expect the size and likelihood of triggering to increase this week (size 2 to 2.5) on this facet layer as a result of forecasted precipitation and strong winds. Strong winds and forecasted precipitation will create potential wind slabs and also increase the load on the already tenuous Facet/melt freeze interface.
There is a persistent weak layer (PWL) composed of facets sitting on top of a very dense melt freeze crust. These facets are producing human triggered avalanche activity at treeline and in the Alpine (size 1 to 1.5) however new precipitation/snow and strong winds in forecast will potentially increase the likelihood of triggering larger avalanches in the size 2 to size 2.5 range.
Wind slabs will become a concern late in the week as a result of consistent wind transport of cold dry snow available in most Alpine areas.
The persistent weak layer (PWL) responsible for a number of recent avalanches is a facet layer that sits on top of a very thick melt freeze crust. This facet/melt freeze crust interface can be found Below Treeline, at Treeline and in the Alpine on all aspects and is at a variable depth between 20cm to 40cm depending on aspect. As ‘day time high’ air temperatures are warming and consolidating the upper snowpack, the snowpack is becoming more firm, however the facets (PWL) below remain highly unconsolidated as the warmer temps have not YET managed to round/bond these facets. In the short term, this warming will serve to increase load of upper snowpack on this PWL and increase likelihood of triggering. More snow in the forecast for certain parts of the island as well as 40-50 km/hr winds. Leeward slopes may therefore receive excessive new snow load as a result of the wind transport. Wind Slabs will be a concern over the next several days and new wind load will deepen the facet (PWL) problem and likely increase the size and likelihood of avalanches.
|Surface||Alpine 30cm of dry cold snow, At Treeline snowpack is more dense as a result of warming|
|Upper||Well bonded upper snowpack with highly reactive facet layer down 20-40cm|
|Mid||Well bonded midpack that includes a weaker "dormant" Facet layer found 60cm down|
Light winds, light and limited precipitation and above zero degree day time high air temperatures followed by cold nights.
Tuesday: 6 to 10 cm of new snow, Temperature -8 to +1, Strong Winds from the WNW, Freezing Level 1,000M
Wednesday: 0 to 1 cm of new snow, Temp -5 to +1, Strong Winds from the North-West, Freezing Level 1,000M
Thursday: No new snow, Temperature -10 to +1, Moderate Winds from the North, Freezing Level 1,000M
Posted on Tuesday February 19, 2019 by Ryan Shelly