Snowpack conditions are changing, and so should our mindset!
New snow, warm temps and moderate winds are a perfect recipe for fresh storm slab activity. The possibility of avalanches stepping down to deeper weak layers persists, and would result in large destructive avalanches.
Storm slabs will build through early Saturday, with ongoing snowfall, Southwest winds and warming temps. These new slabs are building over a couple of weak Surface Hoar layers separated by ~15cm of low density snow.
The lower snowpack remains weak from cold temps in December, and includes the Nov 17 layer which consists of a facetted crust in some locations, and decomposing surface hoar near.
Be particularly cautious in steep rocky areas, with thin coverage, where the majority of the snowpack is weak and facetted.
The terrain above the highway corridor was active early Friday morning, with several large to very large avalanches from the extreme terrain on the North side of MacDonald and the South side of Tupper. Of note, MacDonald gullies #9 and #11 produced size 3.5 slab avalanches which ran into the creek in the valley bottom. There were also several smaller (size 1.5-2.0) slab avalanches from Mt Smart, Ross Peak, Mt Abbott and the Cougar Corner paths.
On Wednesday, explosive stability tests produced six size 1-2 avalanches. They were mostly very soft slabs failing on the Jan 3rd Surface Hoar which was down about 15cm. The 7th explosive scrubbed down to ground on a shallow rocky rib, and then stepped down to the Nov 17th PWL, 1m deep, resulting in a size 3.
Storm slabs (already up to 15cm thick by Friday afternoon) will continue to build through until early Saturday morning, as new snow buries two weak layers of Surface Hoar (January 3rd and 12th) separated by 10-25cm of previous low density snow.
The lower snowpack was facetted by the pre-Christmas cold spell. This weak base continues to be a concern, particularly in shallow snowpack areas, where the mid and lower snowpack are made up primarily of large facetted crystals.
Near the base of the snowpack, the November 17th facet/crust/surface hoar layer can still be found in snow profiles. It has become less reactive in snowpack tests, but when it does fail, there is a dramatic 'drop' of the entire snow column. Spooky!
Snowfall rates will peak overnight Friday, with an additional 10-15cm forecast, along with moderate Southwest winds, an alpine low of -3*C, and a 1500m freezing level.
Ongoing flurries on Saturday will give us an additional 5cm. Winds will be light from the SW. The alpine high will be -3*C, and the freezing level will bump back up to 1600m.
On Sunday, temperatures cool and the sun should make an appearance later in the day, as the storm moves past Rogers Pass.
Monday will be mainly cloudy, with light winds and an alpine high of -5*C.
