Bollettino valanghe

Mt Hood

Northwest Avalanche Center
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Pericolo valanghe

Thu
Alta quota
4 · Forte
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Large slabs will be easy to trigger on Wednesday, particularly during the morning hours. Storm slabs 8" or thicker will be sensitive and wind drifts will be thick, widespread, difficult to identify, and even more dangerous. Avoid all terrain connected to upper elevation start zones where avalanches can start, stop, or run.

Problemi valanghivi

  • Wind Slabs

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

    Observers described touchy small slabs forming at middle elevations on Mt Hood on Tuesday. These slabs will be much larger, more connected across the terrain, with winds and continued snowfall maintaining reactivity on Wednesday. Don't expect these bruisers to lurk uniquely in the typical suspect spots in the lee of ridgelines. They may embed themselves on mid-slopes convexities, down-canyon moraines, or connect widely across bowls and below corniced ridgelines. Winds may switch directions during the day on Wednesday, so don't rely too heavily on the direction of active snow transport to highlight slab accumulation. Recently formed sharp cornices may help a bit more, but in general, it's not a good idea to try to out-fox the problem as you extend into heavily wind-exposed terrain.  Shooting cracks serve as a reminder that a steeper wind-loaded slope would avalanche. But you may not get that feedback if slabs lie beneath lower-density snowfall.

    How to manage a tricky day like Wednesday? Stick to wind-sheltered terrain, scoured faces, or ridgelines; avoid large wind-loaded open slopes, particularly those connected to the upper mountain where a D3 avalanche that can span multiple elevation bands isn't out of the question.

    Example: Hard slab debris. This is the nature of the beast on Wednesday.

    Photo: Joe Dellaporta

  • Storm Slabs

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

    12-18" of fresh snow should arrive right-side-up, but high snowfall rates may still produce storm instabilities in wind sheltered or weakly wind-loaded terrain due to high snowfall rates Tuesday night and at times on Wednesday. These layers could fail within the storm snow on density or grain size differences. It could also fail or step down to the suncrust buried 1/6 on SW-S-SE aspects, or a weakness about an inch above the 1/3 rain crust. That rain crust exists on all aspects and could provide an efficient bedsurfaces for avalanches to run on. The distribution of this avalanche problem remains limited to the distribution of slopes with rideable snow. So, if there's enough snow to ride, there's enough snow to slide. Use your favorite column test to evaluate the snow above the last crust and recognize that the underlying interfaces may shift as you change aspects, so don't rely on any single test location. Reactive storm slabs can be found anywhere 8" or more of snow exists above the last crust.

    Example: A widely propagating storm slab like we might see on Wednesday.

     Photo: AJ

Avalanche Discussion

4-5" of snow arrived on a warming trend on Tuesday, although temperatures stayed cold enough for snow above 4000 ft. Strong to extreme winds redistributed that snow into touchy 3-6" slabs that were easy to trigger but generally small at middle elevations, and likely growing large at upper elevations. In the Salmon River Canyon, these slabs were forming on a variaty of aspects due to terrain channeling. Strong sunshine on 1/5 formed a sun crust buried by the onset of the snowfall 1/6. On other aspects, Tuesday's winds (strongest in the early morning hours before snow started to fall) tapped into plenty of dry snow remaining available for transport from 6-10" of snow that accumulated from January 3-6th. Mt Hood Meadows patrol reported triggering soft slabs in wind-loaded terrain on January 5th. Crown depths ranged from 6 to 8 inches, and most failed on an interface just above the prominent 1/3 rain crust. That crust likely extends up to 6500 or 7000 ft. 

In the gallery you will find images of the touchy 3-6" slabs forming Tuesday that will be much thicker and more dangerous on Wednesday, potentially stepping down to any of the crusts and interfaces described in the previous paragraph. You will also find photos of the soft slab activity and state of the snowpack prior to the current storm.

Although we're expecting up to 2 ft of storm total snow by the end of the day Wednesday, at lower elevations and on wind-scoured aspects, early-season conditions exists. A transition from shallow snow hazard to very real avalanche hazard may occur over short distances at lower elevations. Many avalanche paths are broken up with rocks, creeks, and early-season terrain depressions. Look for slopes with a clean run-out free bone-breaking trees, creeks, and rocks. 

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