Posted:
Dec 13, 2018 @ 6:51 am

The current avalanche danger for the West Central Montana backcountry is Considerable on wind loaded terrain. Large human triggered avalanches are likely in specific locations. On all non-wind loaded terrain the avalanche danger is moderate. Careful snowpack evaluation and cautious route finding are essential to safely travel in the backcountry today.

Good morning, this is Logan King with the West Central Montana Avalanche Center’s avalanche advisory for December 13th, 2018.  This danger rating does not apply to operating ski areas, expires at midnight tonight and is the sole responsibility of the U.S. Forest Service.

Weather and Snowpack

Over Tuesday night, winter made a return to the region and deposited between 8-14 inches of snow with some locations picking up an inch of SWE. The snow tapered off yesterday as the winds intensified. Since Tuesday afternoon the winds have calmed a bit and shifted from a westerly flow to a SSW-S and are currently 13 mph gusting to 20 mph at ridge tops. Over the past 24 hours area snotels have recorded an additional 1-3 inches of snow.

Yesterday we found the new snow had caused natural releases that were 8-10 inches deep over night, but conditions were already settling down by morning. Collapsing, whumphing and shooting cracks were localized to terrain where the wind was depositing snow. On a wind loaded slope we had and extended column test propagate at about 16″ deep on only 2 taps. This means that the primary avalanche concern today is wind drifted snow near ridglelines and on leeward slopes.

The secondary avalanche concern is storm slabs. The are some weaknesses within the new snow that will lead to reactive surface layers that will produce small loose snow sluffs. Loose snow avalanches are primarily problematic in steep exposed terrain where the consequences are higher. There is lots of loose snow that will easily be en-trained in small sluffs and can knock you off your feet or sled.

Finally, all of the new snow is sitting on a layer of surface hoar that formed over the last few weeks. The buried surface hoar is extremely weak but the new snow is lacking some of the slab properties needed to propagate and create large avalanches (except where wind affected). Below the buried surface hoar is a weak unconsolidated snowpack that is also suspect, but continues to be un-reactive in stability tests. Even though these weak layers are not overly reactive in tests, they will be hard to trust and will be a lingering problem for the foreseeable future.

Avalanche and Weather Outlook

A ridge of high pressure looks to set up for the next couple of days. Winds preceding the ridge will continue to load lee terrain but avalanche conditions will slowly settle as the snow adjusts and bonds over the next few days. The next few days should set up with above normal temperatures and gusty winds. The next loading event looks to impact the region sometime over the weekend (Forecast).

If you make it out into the backcountry feel free to share what you see on our public observations page. They are not only helpful to your community but extremely helpful in producing the forecasts.

Ski and ride safe.

READ FULL ADVISORY  

Problem 1 - Wind Drifted Snow

  • TYPE

    wind-slabs

    Wind Slabs

    Release of a cohesive layer of snow (a slab) formed by the wind.  Wind typically erodes snow from the upwind sides of terrain features and deposits snow on the downwind side.  Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

  • SIZE

    4-5

    2 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-5

    Likely

    The likelihood of an avalanche resulting from this problem.

  • IMAGE

Areas of wind deposited snow will be touchy today as the wind slabs and new snow are sitting on a very weak layer of buried surface hoar. Yesterday at Lolo pass we found wind loaded slopes to easily propagate in stability tests.

Problem 2 - Soft Storm Slabs

  • TYPE

    storm-slabs

    Storm Slabs

    Release of a soft cohesive layer (a slab) of new snow which breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

  • SIZE

    2-3

    1 (Small)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    Possible

    The likelihood of an avalanche resulting from this problem.

A significant load of new snow has been added to the snowpack and will take a few days to settle and bond.

Problem 3 - Persistent Slabs

  • TYPE

    persistent-slabs

    Persistent Slabs

    Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks.  Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Persistent, Deep-Slab.

  • SIZE

    4-5

    2 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    Possible

    The likelihood of an avalanche resulting from this problem.

  • IMAGE

Surface hoar and facets in the old snow will continue to be suspect until they are buried deep in the snowpack.

VIDEO

Observations 12/12/2018

Observation 12/12/2018

FORECAST & OUTLOOK

  • Danger Trend

    same

    Same Danger

  • Area Forecast

    Breezy

A ridge of high pressure should lead to dry and windy conditions for the next couple of days.

This information is the sole responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight but the information can help you make a more informed decision regarding travel in avalanche terrain for the next few days.

Our advisory area includes National Forest System lands in the Bitterroot Mountains from Lost Trail Pass north to Granite Pass, the Rattlesnake Mountains north of Missoula and the Southern Swan and Mission Mountains near Seeley Lake, MT. Avalanche information for the Lookout Pass/St. Regis Basin area is available from the Idaho Panhandle Avalanche Center.