Monday March 11, 2019
Cornice Fall, Persistent Slab, Wind Slab
Travel & Terrain Advice
Avalanche terrain steeper than 30 degrees should be avoided until winds and snowfall subside, which is likely not until Thursday. Avoid lee loaded areas below ridges, in depressions and gulleys and wind slabs will be very likely to trigger.
No new avalanches reported however several persistent weak layers within the upper 50cm of the snowpack remain reactive to testing and ski cuts.
There is 30cm to 40cm of forecasted new snowfall for Monday March 11th, Extreme winds will likely create rapid cornice growth on a highly variable snowpack surface, Expect new cornices to grow and fail and potentially trigger avalanches on slopes below. Avoid terrain with overhead exposure to cornices until the wind speeds lessen and the new snow and wind slabs have an opportunity to bond to the pre existing snowpack. This bonding opportunity will likely not occur until much later in the week.
There are 3 persistent weak layers buried in the snowpack, two surface hoar layers are reactive and can be found within the upper 40cm of the snowpack. A 3rd persistent weak layer is composed of facets (dry sugary snow) and this layer can be found down 50cm from the surface of the snow. Increased snow load from new precipitation and winds in the forecast will likely overburden these persistent problems in the snowpack and increase their reactivity over the next week.
New precipitation and subsequent wind slab formation will rest on top of a widespread surface hoar hazard that exists throughout all aspects and elevation bands. This means human triggered avalanches will be very likely to occur on slopes steeper than 30 degrees. Be mindful of snow cracking underfoot or your snowmobile and seek out areas with dense vegetation (lots of dense trees) as these places may be less wind affected with zero exposure to steeper slopes (above 30 degrees) from above. Wind slabs will exist in areas below ridges, in terrain depressions and other areas that receive new snow as a result of wind redistribution.
Vancouver Island is experiencing a snowpack this winter that is much more in keeping with a rocky mountain (cold climate) snowpack. The upper 50cm of the snowpack contains highly preserved persistent weak layers (2 surface hoar layers) and 1 facet (dry sugary snow) layer. These 3 layers are providing results on ski cuts, compression tests and extended column tests. Until now however, the general unconsolidated (facet) nature of the upper 50cm of the snowpack was such that large avalanches was not likely. We could expect an increased likelihood of triggering within these persistent weak layers as a result of the incoming storm snow, extreme winds and warming,. The persistent weak layers are far from bonding (rounding) and will remain at the forefront of the avalanche hazard for weeks to come.
If we in fact receive 30cm to 40cm of new snow and extreme winds tomorrow, we will have a new surface hoar layer buried in the upper snowpack. Surface hoar growth is widespread throughout most aspects and all elevation bands throughout most of the island. Once buried, this new layer will become a extremely dangerous avalanche hazard.
|Surface||Surface Hoar size 7mm to 1cm+|
|Upper||Upper 50 cm consists of 2 buried surface hoar layers within top 40cm and a facet layer down 50cm (all are reactive to tests)|
|Mid||Well bonded mid pack that consists of several melt freeze crusts from January that are well settled|
Cold clear weather has promoted upper snowpack faceting (dry sugary snow). Strong daytime sun has created (as of Sunday March 10) a breakable sun crust at tree-line and below tree-line elevation bands.
If forecast models are accurate, the new snow and extreme winds will dramatically increase the avalanche hazard over the course of Monday March 11th.
Monday - 30cm to 40 cm of new snow, Extreme winds from the South, temps -6 to 0, freezing level 1,200M.
Tuesday- 1 to 3 cm of new snow, Strong winds from WNW, temps -7 to -3, freezing level 1000 m.
Wednesday - trace to 1cm of new snow, Moderate winds from WNW, . temps -7 to -4, Freezing level 900 m.
Posted on Monday March 11, 2019 by Ryan Shelly