Avalanche forecast

Bitterroot

West Central Montana Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Increasing winds, rising temperatures, and continued snowfall are mixing the fresh and low-density storm snow we received in the past few days into slabs capable of producing large avalanches. Be wary of recently drifted or wind-hardened snow surfaces. Dangerous avalanche conditions exist on wind-loaded slopes at upper elevations. You may still be able to trigger slabs that break on buried surface hoar one to two feet deep on protected shaded slopes in the Lolo area. Avoiding loaded slopes steeper than 35 degrees and in areas that you find buried surface hoar is the best way to reduce your risk of triggering slab avalanches today. 

Avalanche problems

  • Wind Slabs

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

    While the new snow yesterday was of low density and lacked the cohesion nessary for destructive slab avalanches in most areas. The storm is finishing off warm with increasing winds. This will change the recipe, leading to more touchy slabs, especially where winds are in the mix. There is plentiful low-density snow for wind transport. Last night winds blew moderate to strong out of the west and northwest. More winds out of the west are expected to be in the light to moderate range with strong gusts today. Wind slabs will form over various melt-freeze crusts, providing a good surface for them to slide on. 

    Stay alert for changing conditions, use small safe representative slopes for tests, and watch for features that indicate the presence of wind slabs such as cornices, wind lips, and stiff hollow sounding snow. Avoid them by sticking to protected or wind-scoured areas. If in doubt, stick to slopes less than 35º.

    (2/16/2022) Example of wind slab from the Rattlesnake. These will grow in size and consequence today.   Photo: Ryan Sorenson

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    1–2.5

    In the Lolo Pass area, buried surface hoar layers exist. These layers are somewhat stubborn to human triggering and generally unlikely. However, avalanches are still possible. Be suspicious of northerly aspects with large trees or small wind-blocking terrain features and harbor steep convex rollovers. Triggering will be more likely on slopes that don't have hard melt-freeze crusts below the new snow. The surface hoar failed and propagated in tests, and whumpfing was experienced last weekend (2/12-2/13) on Mount Fuji.

    Look for a grey stripe in snow pits down about 10 to 30 inches (25 to 75 cm) deep. A column test can be a helpful tool to identify and begin to gather clues about their stability; however, it's best to err on the side of caution and use terrain to your advantage. Avoid them by steering clear of slopes with angles over 35 degrees where you find buried surface hoar.

    (2/16/2022) Example of buried surface hoar from the Lolo area.  Photo: Ryan Sorenson

Avalanche Discussion

Recently, a low-pressure system moved south out of Canada, causing snowfall in west-central Montana since late Valentines' night (2/14). The fresh snow fell with moderate northwest winds Tuesday (2/15) night and early yesterday morning (2/16). Calmer winds were observed after approximately 10 a.m. yesterday. Recent observations had storm totals at 4 to 13 inches across the forecast area, with the northern regions favored. We had 8 hours of productive moderate winds last night. The Rattlesnake and Central Bitterroots could pick up another 1 to 6 inches before noon today.  The Seeley area will pick up another 1 to 2 inches. 

Yesterday's storm snow was of low density and lacked the cohesion for slab formation. However, the snow will likely behave drastically differently today with increasing winds and rising temperatures. 

Last week, a dry period saw shrinking snow depths as warming continued to melt and consolidate the snowpack. The fresh snow sits over various melt-freeze crusts on all aspects below approximately 7,000 feet. Above that, various crusts and windboards existed.

The buried surface hoar and facets near January freeze/thaw crusts remaining 10 to 35 inches (25 to 90 cm) deep appear to have healed some over the last week of warm temperatures and sold nightly refreezes. The surface hoar is knocked over and rounding in most areas we've found. However, reactive persistent layers are still a problem and could still exist elsewhere in the forecast area. 

Many thanks to the folks who have been submitting observations. They are beneficial for assessing conditions throughout the forecast area. Please continue to submit them here:

https://missoulaavalanche.org/submit-an-observation/

Ski and ride safe.

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