A rapid rise in temperature with intense sun will increase avalanche hazard. Be alert to signs that snow is warming and weakening, and be prepared to dial back terrain choices. Avalanches could potentially run full path.
Strong outflow winds have drifted recent snow into wind slabs in a reverse loading pattern that may remain possible to human trigger. Cornices may also reaching their breaking point.
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 weak layers are most problematic at treeline elevations where the surface hoar is well preserved. Rapid warming is expected to increase the likelihood of triggering deeper layers on Sunday.
Rising temperatures and intense solar radiation is expected to initiate wet loose avalanches. This activity is likely to be most prominent on steep slopes that receive direct day-time solar radiation (slopes that face south through west). Although with above freezing temperatures, the problem could become more widespread. Minimize exposure to steep, sun-exposed slopes and cornices.
Saturday night: Clear, strong outflow winds from the northeast, alpine temperature near -10 C.
Sunday: Clear, light northeast winds, alpine high temperature near 0 C, freezing level rising to 2300 m.
Monday: Clear, light north winds, alpine high temperature 2 C, freezing level near 2500 m.
Tuesday: Clear, light northeast winds, alpine high temperature 3 C, freezing level near 2200 m.
Avalanches are expected to become more reactive with a rapid and drastic warm-up.
During the arctic outflow winds, operators reported small wind slab avalanches releasing naturally in lee terrain features at treeline and alpine elevations.
Reports of avalanches releasing on persistent weak layers have continued through the week. Operators reported several large (size 2-3) avalanches breaking on both the March 9th and March 1st surface hoar layers from human, explosive, and cornice triggers. Several of these avalanches were remotely-triggered. Check out this MIN from the Shames area for a helpful example.
Two very large (size 2.5-3) natural glide slab avalanches were observed along the Terrace highway on Friday. Maintain conservative margins to avoid all areas with glide cracks.
Freezing level is expected to jump to 2300 m on Sunday with intense solar radiation. Alpine temperatures are forecast to rise from -20 C to 0 C in the span of 24 hours. This significant warm-up is expected to rapidly destabilize the surface snow and cornices as well as add significant strain to buried weak layers.
Over the past few days, strong arctic outflow winds formed wind slabs in a reverse-loading pattern. Last week's storm delivered 30-60 cm of new snow to the region with strong southwest winds creating ample snow available for wind transport. The recent 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. Observers have reported widespread whumpfing and propagation in snowpack tests on this March 9th surface hoar, confirming its propensity for human triggers.
There are several additional layers of surface hoar that are now buried 70-120 cm and 110-160 cm deep that may also be 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.
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. Sunday's warm-up may have the potential to re-awaken these deeper layers.
