Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Natural and skier/rider triggered wet loose avalanches are likely today. Look for signs of increasing wet snow instability, like rollerballs and point releases. If you find yourself trenching through wet and unsupportable snow, stay off slopes over 30 degrees and out from under avalanche terrain. 

It remains possible for skiers and riders to trigger slab avalanches on buried persistent weak layers on southern aspects and areas with previously buried sun crusts.  Digging into the snow and looking will be the best way to identify and avoid areas with this problem layer.

This will be our last regular forecast for the 22/23 season. We will issue a general avalanche information next week and avalanche warnings if conditions warrant it. 

Problemi valanghivi

  • Loose Wet

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Warm temperatures, light wind speeds, and cloud cover will heat up the snowpack significantly today, and natural wet loose avalanches are likely.  Everywhere except the highest peaks has now gone 24 hours with above freezing temperatures. 

    Today, wet snow avalanches will most likely be on southeast through west aspects during periods of sun. If we get warming due to clouds, the danger will move to all aspects and may happen rapidly on northern aspects that have not seen a warming event yet this season. Dense cloudcover and warm temperatures can heat the snowpack much more rapidly than direct sun and can prevent the snow from re freezing overnight.  

    The timing of wet snow avalanches can be difficult to predict. Most areas have a superficial melt freeze crust at the surface that formed overnight last night. Mid elevations are likely still wet from overnight rain.  This crust will need to break down today before the snow gets wet. How long this takes depends on the crust's thickness and strength. 

    As you are traveling around today, be alert for signs of increasing instability, like rollerballs or point releases. These can be early indicators that the snowpack is trending toward wet snow avalanches. If you find wet and unsupportable snow where you are traveling wet and unsupportable, move off slopes over 30 degrees. Know what terrain you are traveling below, especially as you exit this afternoon. Avoid traveling under terrain capable of producing large wet snow avalanches if possible.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Across all zones, we are still concerned about a persistent weak layer of small facets just above a crust buried 1-3 feet down. This layer is primarily present in areas that had a sun crust just prior to the snow event last weekend. In the past week, we have seen one large avalanche on this layer in the Gash and multiple D1 slabs failing above this crust. 

    This is a tricky problem to manage. Although the poor structure is present in many areas, it is not consistently reactive or propagating. Further complicating this issue, as the snow above it gets wet, percolating melt water may pool above the crust and decrease the strength of the facets leading to wet slabs in the future. 

    This layer is not producing the widespread whumfing and collapsing it was just a few days ago. The best way to identify this problem will be to dig in the snow. If you find a crust buried 1-3 feet down with some weak snow directly above it, stick to slope angles below 30 degrees. 

     

Avalanche Discussion

This season has been unusual in that our snowpack is only just starting to resemble a springtime snowpack. Overnight a few inches of dense snow fell at upper elevations. Rain lines were likely around 6000'.

The forecast often talks about the sun and wet snow, but how do clouds heat the snow? Clouds can absorb some of the heat radiating from the snowpack itself and re emit heat back into the snowpack. This is known as the greenhouse effect. It can form a positive feedback loop and rapidly cause the snow to lose strength as it gets wet. The greenhouse effect is also a way for wet snow avalanches to occur on sheltered north facing slopes. We will likely see a greenhouse effect around Seeley Lake and in other areas with full cloudcover and warm temperatures today. 

We still have concerns about the buried persistent weak layer on southern aspects. It is getting stronger and less reactive, but the travel advice remains the same to stay off slopes over 30 degrees if you find the buried crust/facets. Why 30 degrees and not 33 or 35? On the human factors side, most groups vary in their measurements by 2-3 degrees, especially newer skiers and riders. Being off by a few degrees when trying to stick to a 30 degree slope will be safer than being off by a few degrees when pushing your terrain selection to 33 or 35 degrees. Risk tolerance and exposure acceptance vary from person to person to what slope angle they will expose themself to with a given persistent weak layer. From a non human factors avalanche perspective, a slab avalanche is very unlikely to occur on a slope below 30 degrees in slope angle. 

Don’t grab the mountain bike or head to the desert quite yet. There is excellent skiing and riding still in the mountains.  If you are continuing to recreate or travel in the mountains this spring, please take the time to submit an observation. Even something as simple as uploading a few pictures, taking a snow depth measurement, or writing a few sentences about conditions helps. Observations are not just used by us as forecasters or fellow backcountry skiers and riders to monitor avalanche danger. Hydrologists, fire and fuels managers, fisheries managers, and other land management people all want to know how much snow is still out there and what is happening in the mountains.

Have a great spring and ski and ride safe! 

 

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