Avalanche forecast

Rattlesnake

West Central Montana Avalanche Center
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Avalanche danger

Wed
Alpine
4 · High
Treeline
4 · High
Below
3 · Considerable
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Highlights

An avalanche warning has been extended. 

At least another foot of snow in the past 12 hours will continue to raise the avalanche hazard. This brings the storm total in areas of Seeley Lake up to 5 feet and 3-4 in the Rattlesnake. Storm slab exists on all aspects at mid and upper elevations, especially in areas where the wind has built fresh wind slab. 

Travel in avalanche terrain is not recommended today. 

Avalanche problems

  • Storm Slabs

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

    Another foot of snow fell overnight in the Rattlesnake, bringing the storm total since Friday to 3-4 feet. There is a density inversion within the storm snow, with reports of high density snow falling yesterday on top of lower density snow from the start of the storm. Today, snow will return to low density as colder air arrives. There is also a thin crust midway through the storm snow that formed during a period of warmer temperatures and freezing rain on Sunday. 

    Yesterday, we received a report of a large natural avalanche in birthday chute in jenny bowl outside Snowbowl. This is the second time this path has run during this storm cycle indicating strong transport winds and re loading. 

    We do not have wind data for the past 12 hours from our primary weather station but models show moderate to strong west winds along the highest ridgelines overnight.  Mid day yesterday, we received a report of strong ridgetop winds in the Rattlesnake. The day before, observers noted strong winds stripping west aspects and transporting snow onto mid and upper elevations north through southeast slopes. Expect wind slabs on east aspects and complex terrain cross loaded by strong winds. 

    Travel in avalanche terrain is not recommended. Stay back from the edge of cornices, as they can break back further than expected. 

Avalanche Discussion

After a few weeks of relatively benign weather, a strong winter storm has been impacting the area since Saturday. Snow totals are impressive, especially around Seeley Lake with up to 5 feet of snow. South, in the Bitterroot, snow totals are significantly less but still 2-3 feet. 

Dangerous avalanche conditions exist in the mountains today. A density inversion in the storm snow brings the potential for storm slab avalanches. This forms when the storm starts cold, as it did Saturday with low density falling. Sunday, temperatures warmed, and snow densities increased. The dense snow forms a slab, with the low density snow below acting as the weak layer. These avalanches can be very sensitive and break slopes down to 30 degrees. The good news is that instability from storm slabs, especially from density inversions, is usually short lived and only lasts a few days. Give the storm snow time to settle and bond. Don’t get sucked into a powder panic today. 

We have been without our Point 6 weather station above Snowbowl for 24 hours now, likely due to a rime event. This complicates the understanding of overnight winds in the northern half of the forecast zone. Between field observations from yesterday and models of overnight winds, I have high confidence that winds continued to blow out of the west overnight. That being said, be suspect of all aspects for wind loading along ridgelines. 

During storms like this with degrees of spatial uncertainty, your observations help a lot! Even something as simple as letting us know how many inches of snow fell where you skied helps us fill in the gaps in the forecast area. A picture or video is even better! Please consider taking the time to submit an observation if you are in the mountains today. 

Ski and ride safe. 

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