Between fresh wind slabs and reactive persistent weak layers, human triggered avalanches are likely. Maintain a conservative mindset and terrain use strategy on Friday.
Winds are forecast to change direction (northeast) and increase in a strong arctic outflow event. These winds are expected to drift the recent snow into fresh wind slabs that may be reactive to human triggering. Given recent variability in wind direction, few aspects will be spared from wind slab concerns.
Multiple surface hoar layers have been buried by successive storm events and have shown continued reactivity to human triggering. These layers can be found buried 35-50 cm, 70-120 cm, and 110-160 cm below the surface. These layers continue to produce human triggered avalanches, some of which have been remotely-triggered. These weak layers are most problematic at treeline elevations where the surface hoar is well preserved.
Thursday night: Decreasing cloud, strong outflow winds from the northeast, alpine temperature -12 C
Friday: Mostly clear, strong outflow winds from the northeast, alpine high temperature near -10 C
Saturday: Clear, strong outflow winds from the northeast, alpine high temperature near -10
Sunday: Clear, moderate northeast winds decreasing to light, alpine high temperature near 0 C
With increasing and shifting outflow winds, fresh wind slabs are expected to form and become reactive to human triggering.
In the aftermath of the storm earlier in the week, avalanches releasing on surface hoar layers have continued. Operators reported several large (size 2-3) avalanches breaking on both the March 9th and March 1st surface hoar layers from human and explosive triggers. Several of these avalanches were remotely-triggered. Check out this MIN from the Shames area for a helpful example.
The recent storm brought about a widespread natural avalanche cycle in the region over Monday and Tuesday. There were reports of numerous natural, explosives, and human triggered avalanches up to size 2.5.
The recent storm delivered 30-60 cm of new snow to the region with strong southwest winds. Winds have since shifted to the northwest and are forecast to shift again to the northeast and increase in a strong arctic outflow event on Friday. With this pattern, few aspects will be spared from wind slab concerns.
The new snow is sitting on a weak layer of surface hoar buried 30-60 cm deep in many areas, especially in sheltered areas at treeline and below treeline. See this MIN from the Shames area that shows this layer when it was on the surface.The new snow will also be sitting on a crust on solar aspects (south through west facing slopes).
There are several additional layers of surface hoar that are now buried 70-120 cm and 110-160 cm deep, and are likely the most prominent around treeline. On south through west facing slopes, this surface hoar may be sitting on a crust, which may increase the potential for triggering avalanches on these layers. Below about 1000m, the recent new snow is sitting on a melt-freeze crust.
A weak layer of facets that formed in January may be found about 150 to 200 cm deep, and an early season melt-freeze crust lingers at the base of the snowpack. These layers have produced a few very large natural and explosive triggered avalanches over the past two weeks.
