Bollettino valanghe

West Slopes North

Northwest Avalanche Center
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In sintesi

Be ready for a big change in the weather on Sunday. Heavy snowfall and strong winds should create unstable slabs on all aspects. Shift to simpler, less exposed terrain as the snow piles up and conditions deteriorate.

Problemi valanghivi

  • Storm Slabs

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

    The first significant storm in weeks is heading our way with periods of heavy snowfall and strong winds in the forecast. Avalanche danger should increase throughout the day. You are more like to trigger a fresh slab in wind-loaded terrain. Still, up to 1’ of new snow falling on a weak snow surface is reason enough to be suspect of all steep slopes, wind-loaded or not. 

    Don't ignore obvious signs of instability like whumphing, shooting cracks, and recent avalanches. Hand shears and test slopes can help you gauge the bond of new snow layers, but with weak layers in the upper snowpack, heavy snowfall, and stormy conditions, the best policy might be to simply avoid big, steep slopes.  

    (3/25/2023)

    Example of the kind of slab avalanche you could trigger on Sunday. 

    Photo: Jason Speer

Avalanche Discussion

Low elevations should see rain for a good chunk of this storm until snow levels lower a bit Sunday night. Rain on dry snow usually makes for some amount of avalanche activity. On the other hand, most low-elevation locations just don’t have the consistent snow cover needed for legitimate avalanche activity. 

Look for rollerballs and snow shedding from cliffs if traveling down low. Use extra caution around low-elevation terrain that is steep, smooth, and white. If snow levels are higher than forecast you could start seeing rain and wet avalanche activity at mid-elevations as well.

Observers in the Baker BC reported lower wind speeds and less wind-transported snow than anticipated on Saturday. Lenticular clouds up on Shuksan indicated winds in the alpine were much stronger than at mid-elevations. Intermittent clouds kept most wet avalanche activity in check. Temperatures at 5000’ stayed below freezing most of the day.

In light of the coming storm, the most relevant observations have to do with layering in the upper snowpack above the Early February Crust  - or the EFC for short. Consider the following factors:

  • Multiple rounds of light to moderate snowfall at cool temperatures in the last week
  • Light winds for most of the week, even at upper elevations
  • Some clear nights enabled faceting and surface hoar formation
  • Light-density snow facets more easily than dense wind slab
  • Snowall at low wind speeds does a better job of capping and protecting a weak snow surface

The result is that most areas have multiple layers above the EFC consisting of facets, crust, wind slab, and powder. While it’s probably not the setup for a long-lasting persistent weak layer problem, it’s likely we could see higher-than-average reactivity at the new/old interface or at others not far from the old snow surface. Be prepared for touchier conditions than you might normally expect from a foot of new snow and some wind.

If you make it out, please share what you found with an observation.

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