The weather and snowpack conditions are changing, and so should our mindset!
The new snow will not likely bond well to previous weak surfaces. Expect fresh storm slab activity, and the possibility of avalanches stepping down to deeper weak layers, producing large destructive avalanches.
40-70cm of settled snow buries the Dec 23 facet interface. This layer is of particular concern in steep rocky areas, with thin coverage, where the majority of the snowpack is weak and facetted.
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 Nov 17 layer, down 70-130cm, consists of facets, a rotting crust in some locations, and decomposing surface hoar. Though becoming less likely to trigger, the resulting avalanche would be quite destructive.
There were about a dozen avalanches recorded in the Highway corridor January 10th-12th. Most of them were from extreme terrain on Mt MacDonald and Mt Tupper. Notably, there was a size 3.0 slab avalanche from MacDonald gully #10 on Thursday, which ran well on to the fan.
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.
No new avalanches were reported from the back country in Glacier National Park on January 10th-12th. However, there were several avalanches reported in the immediate vicinity of Rogers Pass. These were mostly wind slab and persistent slab avalanches, with many large enough to injure or kill a person.
Storm slabs will continue to build through until early Saturday morning, as continued new snow buries two weak layers of Surface Hoar (January 3rd and 12th) separated by 10-25cm of previous low density snow.
The December 23rd facet interface is down ~60cm and appears to be gaining strength. This layer remains a concern in shallow snowpack areas, where the mid and lower snowpack are made up primarily of weak facetted crystals.
The November 17th facet/crust/surface hoar layer is down ~100cm and has become less reactive in snowpack tests. When it does fail, there is a dramatic 'drop' of the entire snow column. Spooky!
On Friday the snowfall rates should peak, with an additional 10cm forecast to fall, along with moderate to strong SW ridgetop winds, and freezing levels possibly climbing as high as 1800m.
An additional 5cm is forecast for Saturday, with light winds and a 1600m freezing level.
On Sunday, temperatures cool and the sun should make an appearance later in the day, as the storm moves past Rogers Pass.
