Bollettino valanghe

Mt Hood

Northwest Avalanche Center
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Pericolo valanghe

Fri
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Slabs will increase in size and reactivity on Thursday and it could be easy to trigger one in wind-loaded terrain. Navigate away from freshly loading slopes at middle and upper elevations, particularly beneath potentially sensitive cornices. In sheltered terrain, test storm snow for reactivity before engaging with steeper slopes.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1.5–2.5

    Slabs will grow in size and reactivity as the snow falls and loads onto lee slopes. Where slab development remains soft, these slabs will become easy to trigger, but if the slabs are hard, they could be larger, more dangerous, and widely propagating. Note the difference as you travel, using small test slopes to check for firm over weak layering. If the slabs become punchy and more supportive, you're into hard slabs, a sign that the winds have pulverized the snow grains.  In most cases, slabs will form over a new/old snow interface, but the old snow frequently overlies crusts. If you engage with wind loaded aspects, choose slopes less than 35 degrees and expect that slabs could form well down below ridgelines and would be difficult to navigate around given the strength and duration of the winds.

    Example: Hard slab debris. You don't want to get caught in something like this.

    Photo: Joe Dellaporta

  • Storm Slabs

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

    Storm slabs will build throughout the day. They may not be reactive until you have around 6-8" of new storm snow or if the snow comes in higher density than the snowfall earlier in the week. They will grow in size and reactivity with new snow loading and will peak in reactivity after periods of heavy snow. While most will remain small, they could entrain more snow than you expect. They could also push you into trees, obstacles, or terrain traps such as gullies, amplifying their impact. We can't predict exactly how reactive this problem will be because the factors are very subtle, so you'll need to assess representative slopes before engaging with terrain steeper than 35 degrees. If hand shears, skin track tests, or shooting cracks occur, dial back your terrain selection and stay off lower elevation open slopes steeper than 35 degrees on all aspects. Keep in mind that if the snow remains loose and unconsolidated, you can still trigger dry loose avalanches on slopes approaching 40 degrees.

    Example: A small skier triggered a storm slab on Pea Gravel Ridge. 

    E aspect, 5400 ft. 

    Photo: Joe Dellaporta

Avalanche Discussion

After a break in the train in storms Tuesday night and Wednesday, the train is rolling again starting Thursday evening. With moderate snowfall and strong winds, reactivity will be significantly more widespread and building on Thursday. Dial back your terrain selection significantly. 

An observer headed up the Palmer Snowfield on Wednesday, finding variable crusts and wind deposits. On wind loaded slopes, he was able to trigger stubborn slabs 8-12" thick. Another crust 2-3 ft down had a clean shear in snowpack tests. There is an outside chance that a larger loading event or a cornice fall could trigger this slab, but in most cases, it is bridged by firmer snow layers and should gradually gain strength with time. A rider triggered a size D1.5 cornice fall in White River, which entrained some dry snow, but the rider rode out unscathed. Such cornice falls should remain on your mind both along ridges and near the edges of escarpments, particularly late in the season when cornices are at peak size and actively building during a storm cycle.

Mt Hood Meadows Patrol triggered two large hard slabs (snowcat and explosive triggers) with crowns up to 4 ft deep on S aspects at 7200 ft. These were likely loaded during periods of NW wind earlier in the week. It would probably take more than the weight of a human to trigger these. At middle elevations, they triggered soft slabs running several hundred feet with 1 ft crowns.

On March 13th, riders triggered a large slab that killed one person on Mt Bailey in the Central Oregon Cascades. To our north, folks have been caught and carried in the Washington Cascades nearly every day this week and we don't think there's any reason to believe Mt Hood is immune. Dangers are present, and instabilities are slower to heal during periods of cooler temperatures. Mt Hood has picked up around 60" of snow in the past week. The snowpack is layered, wind-affected, and it's difficult to keep up with the interfaces, crusts, and dynamic slab properties at middle and upper elevations during relatively brief weather windows. All that is to say, it's worth dialing it back and taking the travel advice seriously in times like this. 

You can help us gather more data by submitting an observation HERE or via the Avy App.

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