Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Sun
Alta quota
1 · Debole
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Generally, stable conditions exist on most slopes. Buried weak layers are gaining strength, and crusts below the snow surface make triggering a large avalanche unlikely. Travel one rider at a time and keep an eye on your partner to reduce your risk. Rescue gear, including a beacon, shovel, and probe, are still essential when traveling in avalanche terrain. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    Weak layers of snow buried in late November and mid-December are strengthening, and stout melt-freeze crusts or ice lenses near the snow surface reduce the likelihood of a rider collapsing these layers. Slopes where the crusts are thinnest – and where the snowpack is generally shallow – are where you have the greatest chance of triggering a large avalanche. Take the time to dig down and assess the snowpack structure if you are traveling above 7000 feet, across wind scoured slopes, or thorugh rocky outcroppings.  Ride one at a time where this hazard lingers.

    (1/4/2023)

Avalanche Discussion

Too dry is the latest complaint in this season’s Goldilocks narrative. First, we were too cold, then too rainy, and now we’re looking dismally dry for the foreseeable future. I know I’m a complainer, but here’s why:

Periods of dry weather are the origin story for snowpack villains: persistent weak layers. When combined with cold temperatures, unconsolidated snow can rapidly facet into a weak, sugary layer. This “recycled powder” makes for decent riding when it’s on the surface. However, once it’s buried, the resulting structure can be unstable. We needn’t look any further than our familiar November Facet layer for an example of this phenomenon. 

Nearly half of this season’s SWE at Noisy Basin fell between October 21st and November 6th. The next notable snowfall started on November 22nd, a dry spell of roughly 16 days. Faceting was rampant during this time due to the early season, shallow snowpack, and unseasonably cold temperatures. Once covered, the old snow interface was immediately reactive, and we had a persistent slab problem that confined us to slopes less than 35 degrees until a rain crust capped it well into upper elevations. Now our attention turns to the snow above this crust. 

We’re on day seven of the current dry weather regime. This period also started with below seasonal temps. Unsurprisingly, observations have reported faceting above the crust, mainly where the surface snow layer is thinnest. Facet/crust combinations can be particularly troublesome. Although temperatures have since moderated – and are forecasted to remain at or above seasonal averages – we want to acknowledge the possibility of near-surface faceting and its future impact on stability. 

Also worth keeping in mind is the potential for surface hoar. These feathery crystals grow on the snow surface during nights with clear skies and calm wind. Surface hoar can support an impressive amount of weight, allowing large slabs to form on top of it. However,  once a shear load - like that of a skier or rider - is applied, the crystals fall over like dominos causing the snow above to avalanche. Surface hoar is easily destroyed by the sun or wind, so it will often lurk in sheltered terrain, though it can be found anywhere if it is quickly covered by new snow.

Keep an eye out for facets and surface hoar during these drying times, and let us know what you find. 

Regioni coperte