Bollettino valanghe

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Northwest Avalanche Center
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Limite del bosco
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Evening Update: Dangerous avalanche conditions exist at all elevations on the heels of a warm and wet storm. Dangerously wide and deep wet avalanches remain possible. You may be able to trigger avalanches on older weak layers though the distribution is uncertain.

Problemi valanghivi

  • Storm Slabs

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

    Look for hard-over-soft layering in the new and recent snow. Watch for shooting cracks and test slopes that slide easily. If you see these clues, stick to slopes under 35 degrees. The recent warming trend with rain and heavy snow set the stage for storm slab avalanche conditions.

    Slab avalanches in recent snow will be bigger and more likely above treeline. Above 5,000ft the snow should be mostly dry. Winds may have been strong enough to form subtle drifts and thicker slabs on leeward slopes above treeline. 

    (1/8/2023)

    Example storm slab in terrain below treeline.

    Photo:

  • Wet Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2.5

    Wet avalanches could still occur where the snowpack remains moist and weak. In the low valleys and in areas of very shallow, weak snow, wet avalanches could involve the entire snowpack. Further into the mountains, there are weak layers in the upper snowpack that could become extra weak once wetted. Loose avalanches, while easier to predict, can be forceful enough to be dangerous. Use caution where you find wet, unsupportive snow or deeper weak layers below that have become wet. Stay well out from under large terrain features with wet snow. For older weak layers above the rain line, see the discussion below.

    (1/11/2023)

    Large wet loose and wet slab avalanche in the Methow Valley from earlier in the week. 

    Photo: Willie Webster

Avalanche Discussion

There's good evidence that a significant cycle of avalanches occurred on Friday in the lowland hills. Avalanche activity may have peaked from afternoon into evening. Friday morning, I saw a couple of small wet avalanches that ran to the ground on a steep bank north of Methow (see below). WSDOT forecaster reported a wide avalanche (~600ft) in the SR 153 corridor near Carlton. An observer reported a large avalanche put debris on Twisp River rd near the intersection of Poorman's Creek rd. Observations in the higher into the mountains are still limited. If you see avalanches, please submit an observation.

The weight of the recent snow may have stressed buried persistent weak layers. You're most likely to find these in wind-sheltered terrain and sparse trees. We suspect buried surface hoar from around January 5th and possibly weak snow above the Christmas crust could be problematic weak layers. Watch for long shooting cracks and audible collapses as you travel. Digging snow pits and looking for propagating test results can help you identify these weak layers in top 2-3 feet of the snowpack. 

Three recent avalanches may have involved older weak layers, though none have been confirmed- On Jan 8th, a skier was caught but uninjured in a slab avalanche near Varden Creek. On Jan 10th, a skier triggered a large slab avalanche on a north aspect at 4,800ft near Silverstar Creek.  Finally, snowboarders triggered a deep and wide avalanche on a southeast aspect at 6,180ft near Cutthroat Creek on the 12th (see photo below). While there's much uncertainty, all of these avalanches appear to have run on older layers of snow. We should take this as bullseye or red flag clues to use caution and assess the snowpack.

On Friday, observers near Cutthroat Creek reported upside-down, heavy snow. Below 5,000ft the snow was moist, though precipitation remain snow down to at least 4,000ft. On Thursday, I went to the Twisp River drainage to look for persistent weak layers. We didn't find anything that obviously pointed to the danger. We were able to trigger small loose dry avalanches in steep, unsupported terain at 4,900ft. At this same elevation on a northerly aspect we found tilt tests to fail down about 10" on an interface from around January 5th. This interface could harbor surface hoar in other locations. The Christmas crust was a series of thinner crusts down about 2ft. We found sudden planar tests and propagating results within the crust (CTM (SP), ECTP 31). Below 3,500ft, snow surfaces were moist in the afternoon.

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