Bollettino valanghe

Mt Hood

Northwest Avalanche Center
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Pericolo valanghe

Thu
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 8 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

If surface crusts break down, you can trigger a generally small wet avalanche as the snow becomes moist. You might find the key ingredients on steep, sheltered, south-facing slopes harboring softer snow. Adjust your aspect and reduce your exposure to long, steep slopes where wet avalanches might initiate around warming rocks or cliffs.

Problemi valanghivi

  • Loose Wet

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1–1.5

    Although the mercury keeps rising through Wednesday, your risk of triggering a wet loose avalanche should decrease. That is because surface crusts are gradually thickening on most sun-drenched aspects. It should take most of the morning for the weak December sun to break down and expose dry (and often dense, wind-affected) snow underneath. Once crusts begin to weaken, start paying attention to steep slopes where you or snow shedding off rocks or cliffs might entrain moist surface snow. While we don't expect these to be large, a terrain trap such as a gulley, or shallowly submerged rocks will increase the risk associated with any avalanche.

    Example: Small loose wet avalanches on steep rocky slopes facing the sun.

    Photo: Andrew Kiefer

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Reactive hard slabs existed at all elevations as recently as Monday. While warm temperatures have reduced the distribution of these slabs, we expect some stubborn slabs linger on generally shaded or E-facing aspects at upper elevations into Wednesday. The structure to watch for is a firm slab sitting over angular grains just above the 12/24 crust. Use small wind-loaded test slopes to sample for this structure and the resultant reactivity. While an old slab is difficult to trigger, the consequence remain as slabs could be large. Thicker lense-shaped deposits of snow will be your greatest concern on a large, steep, or unsupported slope. When in doubt, keep your slope angle below 35 degrees on upper elevation wind-loaded slopes.

    Example: A large explosive triggered hard wind slab. Mt. Hood Meadows.

    Photo: Mt Hood Meadows Pro Patrol

Avalanche Discussion

Wind slabs returned to the avalanche problem set on Mt Hood after we missed seeing some notable observations from Mt Hood Meadows. The problem didn't go away, and then return. These are old hard slabs whose longevity has been prolongued due to a thin layer of faceting snow sitting just above the 12/24 crust. Sunday and Monday, the structure produced column failures on isolation and propagation on extended column tests. One on a SE slope at 6600 ft in Heather Canyon as well, the other on previously unmitigated slopes at lower elevations with slab depths in the 8" to 20" range. Warmth and time reduced the slab reactivity on Tuesday and allowed the weak snow to start gaining some strength. We're less confident that this healing process is occurring as rapidly on colder, shaded aspects, hence your focus on specific (mostly shaded) upper elevation aspects for the wind slab problem.

The late-December high pressure brought the warmest temperatures to middle and upper elevations on Mt Hood on Tuesday. Temperatures at Mt Hood Meadows Upper station (6500 ft) were 51F, while the base station recorded 36F, during the peak heating of the day. Despite the warm temperatures, observers reported plenty of soft snow to be found, moist snow was limited to steeper slopes receiving direct sunshine, and limited wet loose activity.

Moving forward, Wednesday will be similar to Tuesday, but more of that warmth is likely to reach down to lower elevations and shift the wet loose activity along with it. High clouds are also likely to move in during the afternoon, potentially reducing some of that direct solar heating, and could redirect some of the sun's energy onto shaded slopes. 

Despite recent accumulations, the snowpack remains thin by late December standards and snow depths increase markedly above 6,000 ft. Keep an eye out for shallow coverage down low, scoured west-facing aspects, or thinly bridged crevasses in alpine terrain. 

 

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