Silver Valley & Bitterroot Mountains

You could trigger a large avalanche that fails on weak layers buried 1 to 2 feet below the surface on slopes around 35 degrees. The likelihood of provoking a failure increases on west through north to east-facing slopes that have received less sun and warmth recently. Build in a wider margin of safety if traveling on, adjacent to, or below steeper slopes, and favor more southerly-facing slopes to help reduce your avalanche risk.

Friday acts as a transition day, marking the end of several days of uneventful weather and the beginning of a prolonged stormy period. Snow is forecasted to move into the region near sunset, so conditions aren’t expected to change much during the day. However, expect the danger to increase tomorrow as snowfall starts to accumulate overnight.

Field teams got into multiple zones this past week to document how the persistent slab problem continues to evolve. There are some differences between the more northern mountains (the Selkirks and Cabinets) and the southern end (Silver Valley-Bitterroots), but there are also some similarities. Let’s start with the similarities first:

Slopes that have preserved buried surface hoar continue to be the most reactive in snowpack tests. Aspects that are more sheltered did a better job of protecting these fragile grains, so you are most likely to encounter this layer buried on average 1 to 2 feet deep on slopes spanning from the west, to north, to east. Aspects that favor the southern side of the compass benefited from the sun and elements destroying the surface hoar prior to snowfall, and a melt freeze crust formed as a result of the solar input. Surface hoar isn’t the only weak layer that developed during the end of the January dry spell, at the same time under a series of cold and clear nights, faceting quietly evolved too.

Things diverge slightly in terms of the distribution of the weak layers. In the Silver Valley-Bitterroots, freezing levels worked higher up the mountains and temperatures increased in the first week of February. This helped moisten the snow and eroded the previously mentioned weak layers all the way up to above tree line on southerly facing slopes. The northern mountains stayed slightly cooler, which is why you might notice all aspects above tree line being included in the persistent slab distribution there.

Regardless of the zone you are recreating in, you should have this problem in the forefront of your mind as you move through the mountains and are evaluating slopes. We continue to get propagating results on these buried weak layers in snowpack tests but aren’t always seeing signs of instabilities on the surface or a lot of natural avalanches. A slight change in aspect could introduce a snowpack with a different structure than the previous slope you were on.

Another similarity we are finding is the conditions at the surface. We have had another crop of surface hoar developing the last few days, and with new snowfall just on the horizon now, conditions could change quickly if and where this new layer gets buried intact. Be mentally ready to really dial back your terrain selection and shift to a much more conservative mindset as snow returns to the mountains.      

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