Avalanche forecast

Rattlesnake

West Central Montana Avalanche Center
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Avalanche danger

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

Avalanche hazard will rise rapidly as the storm arrives this afternoon. Human triggered avalanches on persistent weak layers are likely this morning.

By this evening, triggering a wind slab will become very likely. Natural avalanches are possible as the new snow and wind overload the weak snowpack.

Stick to slopes under 30 degrees and stay out of all avalanche terrain today. 

Avalanche problems

  • Persistent Slabs

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

    Triggering an avalanche on persistent weak layers in the Rattlesnake is likely. These avalanches could break up to 3 feet deep and be triggered from above, to the side, or below. The two layers of concern are buried facets deep in the snowpack and a buried layer of surface hoar buried 2 feet down. 

    Observations from the area are showing the slab becoming stubborn. Collapses and whumpfing are being reported but less widespread and sensitive. Once a fracture starts, it propagates easily across the weak layers. Shallow areas of the slab or areas around rocks are the most likely trigger spots. 

    Avoiding slopes over 30 degrees is the only way to manage this problem right now. 

  • Wind Slabs

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

    Today's storm will add to an already sensitive wind slab problem at mid and upper elevations. Northeast winds will increase to 40mph by this evening, transporting both today's low density new snow and previous snow into easy to trigger slabs 2-3 feet deep. Previous winds have loaded east slopes complicating the avalanche problem and creating hazards on all aspects along ridgelines. 

    Any wind slab triggered today has the potential to step down and create a large dangerous avalanche on persistent weak layers

    Give potential cornices a wide berth and avoid any wind lips or pillows. Stay off slopes over 30 degrees along ridgelines. 

  • Storm Slabs

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

    The Rattlesnake zone is likely to get 4-5 inches of new snow today with .28” SWE (snow water equivalent). In sheltered areas, this may be enough to create small storm slab pockets failing on near surface facets. 

    Avoid terrain traps like cut banks and gullies, as even a small avalanche can bury in the wrong spot.

Avalanche Discussion

A significant winter storm with potentially historic low temperatures is at our doorstep, arriving mid afternoon today in Seeley Lake and in the Bitterroot by evening. With it will come high winds, snow, and bitterly cold temperatures. The highest precipitation rates will occur this evening and tonight, but periods of very high accumulation are likely this afternoon.

Weather for the past three days has been more benign, with moderate temperatures, winds, and only a few inches of new snow. This has not been enough to overload the existing snowpack or help it heal. Near surface facet formation has occurred in all zones creating a poor interface with the incoming snow. 

High spatial variability of weak layers and avalanche problems exist both between zones and inside each individual forecast zone. Careful route finding and attention to conditions are necessary to manage this many problems today. Do not let a lack of results in snowpit stability tests trick you into skiing steeper terrain. When dealing with poor structure, avoidance is the only way to manage high consequence but decreasing probability slab. Staying off of slopes over 30 degrees is still the only way to truly manage this problem. 

Temperatures for this storm have the potential to reach historic lows, especially in Seeley Lake. Lows for tonight will reach -20 Fahrenheit.  Think about the consequences of your travel decisions today. Tonight is not the night to be stuck out overnight because of a minor injury. Even a small avalanche without injury could prove deadly if it keeps you in the mountains tonight. 

Check the forecast for each zone you are thinking of skiing or riding today, as the information in them is specific to the zone. 

Please take the time to submit an observation of what you are seeing out there! We appreciate each one, and it truly does help create more accurate forecasts. 

Ski and ride safe… and stay warm!

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