Avalanche forecast

Seeley Lake

West Central Montana Avalanche Center
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Avalanche danger

Wed
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Seventeen hours of sustained high winds have loaded easterly aspects with wind slab. Warming temperatures will create wet snow problems at all elevations on solar aspects and on all aspects at lower elevations. Buried surface hoar in the Lolo Pass area could result in larger human triggered avalanches. Avoid wet snow, and stay alert for signs of instability, such as rollerballs. Avoid wind loaded start zones over 35º. In the Lolo Pass area, stay off slopes over 35º and avoid convexities. As slopes warm, dial back travel to 30º and less.

Avalanche problems

  • Persistent Slabs

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

    Primarily in the Lolo Pass area, buried surface hoar became more reactive over the weekend with warm temperatures that consolidated the new snow from last week. In tests, the surface hoar was failing and propagating. Whumpfing was also experienced. This layer was somewhat stubborn to human triggering. However, given the extent of the surface hoar, large avalanches are possible if you find the trigger spot. This layer could become touchier with further warming and wet, heavy snow on it. Slides could have characteristics of wet slab avalanches.

    Isolated pockets of buried surface hoar exist throughout the forecast area. North and east aspects, in areas sheltered from the wind, are the most likely places to harbor it. 

    As you travel today, especially in the Lolo Pass area, check in the snow for a thin strip of lower-density snow. Avoid slopes over 35º, and stay out from under steeper slopes as you may be able to trigger avalanches on the surface hoar from below. 

    If there is significant warming and roller balls are starting, dial travel back to slopes 30º and lower as you make your way to shadier aspects or home. 

  • Wind Slabs

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

    Strong sustained winds were transporting snow for the past 17 hours. While transport was limited in many places due to the presence of a crust and moist snow surfaces, the possibility exists to find reactive pockets of windslab. Upper elevations below ridgelines are the most likely places to trigger windslab, but cross loaded gullies may also hold pockets. Look for wind lips and smooth pillows to indicate wind transport. Shooting cracks indicate instability. Avoid wind loaded start zones over 35º. 

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–1.5

    With warming today, wet loose avalanches will become a problem. They are most likely on solar aspects. Terrain traps will increase the consequences of being caught in a wet loose slide. There is a slight possibility that wet slab avalanches could occur in the Lolo Pass area, where buried surface hoar is more widespread.

    As snow surfaces warm-up and roller balls start, move to shadier aspects or higher elevations. Plan a safe exit from the mountains that does not cross or pass below steep slopes with wet snow.

Avalanche Discussion

The past 72 hours have seen a transition back to high pressure with warm days and cold nights. A sunny weekend had temperatures reach 30º at 8000 feet and sun affecting southerly slopes. Yesterday temperatures rose as high as 47º at 6400’ in the central Bitterroot. Overcast skies limited solar warming in the afternoon, and strong to extreme winds out of the west kept temperatures feeling cooler. 

Today, temperatures are forecast to reach 45º below 6000 feet and 39º above this. The freezing level could climb to 7000’, and the Bitterroot will be warmer than the Rattlesnake and Seeley lake area. Partial cloud cover may limit the solar impact on the snow surface, but expect rapid sun warming of the snow when the skies are clear. Winds are forecast to be light with strong gusts, although Point 6 is still recording winds at 31mph out of the west. 

Wet snow problems can be expected at all elevations on solar aspects today and on all aspects at elevations lower than 6000’. Windslab is a problem at upper elevations and on multiple aspects due to strong winds and local terrain effects. The Lolo Pass area has a buried surface hoar that was reactive and propagating in tests over the weekend. Buried surface hoar exists in isolated pockets in other parts of the forecast area.

Yesterday in the Rattlesnake, we found a widespread sun crust topped with surface hoar on all aspects but north. Strong westerly winds blew exposed surface hoar off throughout the afternoon and evening, but further snow transport was limited due to the crust. Stable conditions were found with no signs of instability. However, pockets of reactive windslab are likely after 17 hours of sustained strong wind.

Lolo Pass saw lots of warming, with roller balls to 7200’. Winds were extreme, with gusts to 75mph recorded at the visitors center. Upper elevations saw much stronger winds than this. Lower elevations had saturated snow surfaces. There were reports of wet loose snow sliding on a layer up to 20” deep, likely the buried surface hoar that propagated in tests over the weekend. There is some possibility of wet slab avalanches sliding on this layer with continued warming. 

The central Bitterroot had extreme winds out of the west, transporting significant snow. The winds kept the snow surface cool despite sunnier skies than the Rattlesnake. There were no signs of instability while touring in the Glen Lake area, but a large natural avalanche was spotted coming off the Sweathouse Crags in north-facing unsupported terrain. Isolated large avalanches are a possibility with continued warming.

The southern Bitterroot saw an increase in reactivity over the weekend with the consolidation of the surface snow. Yesterday, stability had increased, but wind slab was building throughout the day. Expect reactive windslab in the region today and a possible decreasing stability as surface layers warm and lose cohesion. 

We received several high-quality public observations over that past week. Thanks to all of you who submitted them. You can submit further observations here. 

Ski and ride safe

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