Weekly Update
January 9th-15th
This week was a there-and-back-again story. It started with a dry, high-pressure system and ended the same way, but there was a lot of action in between. At the beginning of the week, the avalanche danger was moderate. The main concerns were wind slabs that developed last week and persistent slabs surface hoar about Dec.30. Throughout the Swan and Flathead ranges, this buried surface hoar was reported to be 1 to 2 feet deep and reactive in tests and to people. In the Whitefish Range, its distribution was more spotty and the layer was not as reactive. As the week progressed, the wind slabs became more stubborn while he persistent slabs grew into the prominent concern. This calm winds and clear cold nights early in the week allowed more surface hoar to grow on top of the snowpack.
Several glide cracks were reported early in the week from the Flathead Range and Glacier National Park. It is very unusual for these cracks to open up this time of year. The best advice is to give these cracks a wide berth because they can fail unpredictably.
The weather changed drastically on the 12th when a powerful warm storm pushed into the region. Rain and warm temperatures brought a hazard of small loose wet slides at low elevations. Higher, storm slabs thickened as storm's intensity grew throughout the day. Temperatures continued to rise into the night, with the rain line reaching at least 6000 feet and likely higher. This storm brought with it a widespread but short-lived avalanche cycle. The natural avalanche activity peaked late on the night on January 12th and early on the morning of the 13th. While we are still gathering information about this cycle, several hundred natural D1-D2.5 avalanches failed with the rapid loading from the storm. Most reported slides have crowns less than 2 feet thick and ran in the new storm snow. There were, however, a few exceptions where the slides stepped down into deeper persistent layers, such as on Mt Aeneas and Mt Stanton.
On the morning of January 13th, a cold front blew in, and extreme winds mixed down from the jet stream overhead. Winds reached 100 mph in the Swan Range and in Glacier National Park. Rain-soaked snow refroze, creating a new crust for our growing but unwanted collection. The robust rain crust locked up the snowpack below the rain line, abruptly stopping the natural avalanche cycle there. The extreme winds that followed deposited wind slabs on leewards slopes, many in unusual locations or further down slope than typical.
Below the rain line, the hard Jan. 13th crust makes it unlikely a rider will trigger persistent weak layers buried before Christmas. However, slide for life conditions exist, and crampons could be a precious addition to your kit. Above the rain line, it’s still possible to affect deeply-buried persistent weak layers. That hazard, combined with a lingering hazard of triggering recently-formed wind slabs, leaves the avalanche danger at moderate. Looking forward, this new rain crust could develop into a persistent problem if it's buried under a few inches of snow that facets. This will be an important layer to keep track of as more snow falls.
