Avalanche forecast

Silver Valley & Bitterroot Mountains

Idaho Panhandle Avalanche Center
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Avalanche danger

Sat
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Avalanche danger will increase by this afternoon with the potential for development of storm slab and loading of persistent weak layers in the upper snowpack. Keep an eye on changing surface conditions and if you see signs of slab development or instability be prepared to move to safe terrain.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    The weather models are predicting the possibility of 8-13” of new snow to accumulate from last night through tomorrow morning, with winds shifting from the northeast to the southwest. The new snow will fall on a variety of old snow surfaces, including a thin breakable crust we found on solar aspects yesterday. The temperature during the storm is forecast to increase this afternoon, possibly creating conditions for an “upside down” storm slab, heavy wet snow resting on cold light snow. Taken together we have several variables that create the possibility of a storm slab problem increasing throughout the day. If you start seeing cracking or other signs of instability, it is time to move to lower angle slopes.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    1.5–2.5

    A well-preserved layer of graupel sitting just above the February 19th ice lens is buried 1-2’ feet from surface on solar aspects, while a layer of buried surface hoar that developed on March 1st is buried anywhere from 10-16” around the compass rose. Although these layers have not shown a propensity to propagate in our recent stability tests, the buried surface hoar was the weak layer in last week’s slide on West Willow. The load from today’s storm will stress the weak layers in the upper snowpack, which may trigger a round of natural or human triggered avalanches.

    Buried surface hoar from March 1st.

Avalanche Discussion

Yesterday, Kyla, Katrina and I enjoyed the mild temps and glimpses of sun poking through the clouds as we toured around Burke Summit. After a week of fairly innocuous weather, we found good stability and decided to descend via the Pipeline Couloir.

However, don’t count on yesterday’s snowpack evaluation to apply today. Today’s snowstorm is changing the equation and the potential for avalanches will increase by this afternoon. Weather models show 0.5-1.0” SWE and 15” of new snow possible through tonight. It looks like temperatures will rise mid-storm this afternoon before falling again this evening, which may create an “upside down” storm slab (warm dense snow resting on cold light snow). Adding to the mix are winds shifting from northeast to southwest through the storm.

We won’t know for sure how this will all play out until the storm subsides and we get a chance to investigate the snowpack, but we do know that avalanche hazards increase with rapid loading. The big questions that influence the avalanche danger today and tomorrow are: How is the new snow bonding to the old snow surface? What effect does the new snow load have on the weak layers in the upper snowpack? And, how hard and from what direction are the winds blowing?

Add a margin of safety into your plan if you head out today. Be leery of steep and open terrain this afternoon. Tree skiing/riding and lower angle slopes will be the safe bet as avalanche danger increases.

Thank you to the folks who have submitted observations in the past week. Our forecasts are better when we have data points from across the zone that help paint a more complete picture of the snowpack. Information that is helpful in an observation include: recent avalanches, signs of instability, snowpack test results, weather observations, height of snow, depth of persistent layers, elevation and aspect, and overall impression of stability. Feel free to submit anonymously and don’t feel obliged to pin-point your honey hole on the map. Thanks for supporting the backcountry snow going community with your observations.

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