Bollettino valanghe

Purcell Mountains

Idaho Panhandle Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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Non aggiornato — questo bollettino è scaduto last year. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Snow doesn't like rapid change and the snowpack just received a heavy load hit and needs time. Dangerous avalanche conditions persist from wet, dense snowfall at upper elevations, and moderate to heavy rain saturating the snowpack below about 5,000 feet.  With lots of uncertainty around how buried weak layers will handle the new load or the presence of liquid water, conservative terrain choices are crucial. Avoidance remains the best choice for today.

Problemi valanghivi

  • Wet Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2

    Rain saturated snow in the upper snowpack may keep a lingering threat to trigger a wet snow avalanche on slopes steeper than about 30 degrees. Loose snow slides that travel long distances can gouge deeper and leave a dangerous pile of snow at the bottom of a slope, or in gullies and creek bottoms. Some wet avalanches may fail as a more dangerous slab of wet snow, and those types of avalanches will be more difficult to escape. Despite the rain coming to an end, water will likely continue to percolate toward weaker interfaces or buried crusts as temperatures stay mild around 5,000 feet and lower. If you see fresh glide cracks or older ones that look wider, avoid traveling on or below those slopes.

    (4/4/2024) Photo: Mikey Church

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2

    Wet, heavy snowfall has built denser slabs of snow above softer, lower-density snow creating an inverted, “upside-down” layering at the surface. Knowing "true" snowfall is tough with these warm storms, but storm totals likely range 10 to 15 inches of new snow and 1.2 to 1.8 inches of Snow Water Equivalent. The significant hardness difference between the storm snow and what’s below makes them more reactive than usual, and these dense slabs have a higher potential to break wider than you expect, even in wind-sheltered areas. Heed the warning signs of unstable snow like cracking or collapsing, give yourself or your group an extra wide buffer to avalanche terrain above the rain-snow line - which likely fluctuates throughout our forecast zones.

    (1/1/2025)

    A shallow, soft slab avalanche failing in the recent storm snow. Bitterroot Mountains. January 1, 2025.

    Photo: Izzy Davis

  • Persistent Slabs

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

    Prior to this weekend's storm, lingering weak layers - surface hoar, faceted grains, and a faceted crust - two to three feet deep were the culprit of the atypical, lingering instability in the upper snowpack. Snowpack tests on slopes near treeline down to about 5,000 feet continued to produce ongoing failures in these weak grains. Preserved surface hoar lingered on north to east-facing slopes, while southeast through south to west-facing slopes harbored a layer of facets or a breakable crust with facets lurking above and below. What is not known is how deep recent rainfall percolated into the snowpack and whether it made deep enough to weaken bonds in these persistent grains, or alter their change completely. The added water load to the overlying slab is enough to flex the sensitivity needle even if these grains did not see liquid water.

    (2/16/2025)

    Surface hoar feathers are still 10 to 12 mm in size below a stiffening slab. While becoming harder to impact, they still produce propagating failures in tests. Selkirk Mountains. February 16, 2025.

    Photo: Chris Bilbrey

Avalanche Discussion

Wild weather often breeds unusual avalanche behavior. 36-hour snowfall totals around 5,000 feet and higher range 6 to 12 inches of very wet, dense snow - 17 to 25 percent snow densities. Liquid water input so far is 1 to 2 inches over the same timeframe and most elevations around 4,000 feet and lower have received all rain. Another potent slug of warm air arrives Sunday afternoon and drives the snow line higher in the sky. An above mountain top rain event will likely pan out for the Bitterroot Mountains while the highest terrain across the Northern Mountains has the best odds of being spared the major snow wetting event. 

By Monday morning, the snowpack in some places could hit the 3 to 4 inch watermark adding an insurmountable load to persistent grains that have not disappointed us in terms of their lingering sensitivity since burial on January 31. 

Storm slab densities have significantly increased from the 5–7 percent storm snow that accumulated over the past 10 days. This inverted layering makes newly formed slabs touchy, allowing avalanches to be triggered on lower-angle slopes than usual. Near and above the treeline, deeper and harder slabs can produce large, terrain-wide avalanches. Slides may travel long distances into mature forests, causing timber damage—especially if they initially fail in the upper snowpack and then step down into deeper, persistent weak layers.

Weak layers- Lingering instability in the upper snowpack comes from surface hoar, faceted grains, and a faceted crust two to three feet deep. Snowpack tests near the treeline and down to 5,000 feet continue to show failures in these weak grains. North- to east-facing slopes hold well-preserved surface hoar, while southeast- to west-facing slopes have a breakable crust with facets above and below. If prolonged rain occurs, water may penetrate deeper, reaching the January Drought Layer (JDL). If this happens, Wet Slab avalanches become a serious concern with unpredictable failure timing.

If you decide to head into the mountains today, be prepared for anything. Keep it simple and avoid pushing the limits. Avoidance is the best strategy is to steer clear of risky terrain altogether.

Regioni coperte