Posted:
Feb 21, 2017 @ 6:26 am

The current avalanche danger for the West Central Montana backcountry is Considerable on wind loaded terrain above 7,000ft. Human triggered avalanches are likely on steep wind loaded terrain at upper elevations. Below 7,000ft and on non-wind loaded terrain the avalanche danger is moderate, small human triggered avalanches are possible in specific areas.

Good morning, this is Logan King with the West Central Montana Avalanche Center’s avalanche advisory for Tuesday, February 21, 2017. 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

The northern part of the advisory area received 5-8 inches of snow in the past 24 hours, and has accumulated a total of 6-10 inches of snow since the storm system moved in late Sunday night. Strong mountain winds have been observed for the last 24 hours at Point 6 peaking yesterday evening at 33mph gusting to 53mph. Current winds this morning have shifted towards the west and have settled in the last few hours to calm. SWE’s for the past 24 hours are .6-.8 inches for the northern parts of the advisory area. The southern part of the region has accumulated 1-4 inches of snow and the winds were not as strong yesterday. At Deer Mountain the winds this morning are 10mph gusting to 16mph from the SE.

The primary avalanche concern today is wind slabs. Josh and I toured in the Rattlesnake yesterday and found small soft wind slabs forming early in the day that were already starting to have some shooting cracks and signs of instability in tests (Video). With significantly more snow becoming available for transport through the day and winds reaching gale force, wind slabs will have further developed and will be touchy today. Winds have shifted directions from easterly to westerly so wind slabs and cross-loading can potentially be found on any aspect. Wind slabs will be suspect today and traveling on wind loaded terrain is not advised.

Persistent weak layers are the secondary concern today. Buried surface hoar and near surface facets continue to fail with clean shears but are not propagating as readily. The persistent weak layers still warrant attention so take the time to dig to see if these layers are present and propagating. The facets and buried surface hoar continue to break down and have become very small and are hard to identify until you perform stability tests.

The third concern today will depend on where you are in the advisory area. In the northern half of the advisory area small soft storm slabs will be possible. A good amount of new snow paired with wind effect will take some time for the new snow to bond to the old snow surface. In the southern portion of the advisory area where less snow has fallen loose avalanches will be more of a concern in steeper terrain.

Avalanche and Weather Outlook

Snow continues to fall in the mountains and an additional few inches are expected today. With more snow, the avalanche danger will slowly increasing throughout the day. Cooler temperatures and more snow are in the forecast for the next couple of days as winter looks to return.

If you are out in the backcountry, please send us your observations, these are very helpful in producing the advisory. The next regular advisory will be issued on Thursday, February 23, 2017.

Ski and ride safe.

READ FULL ADVISORY  

Problem 1 - Wind Slabs

  • 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

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-5

    Likely

    The likelihood of an avalanche resulting from this problem.

  • IMAGE

Strong winds and new snow have created wind slabs above 7,000 feet.

Problem 2 - 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

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-2

    Unlikely/Possible

    The likelihood of an avalanche resulting from this problem.

Buried surface hoar and near surface facets continue to give clean shears and intermittent propagations.

Problem 3 - Storm Slab

  • 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-2

    Unlikely/Possible

    The likelihood of an avalanche resulting from this problem.

Small soft storm slabs are possible in the norther part of the advisory area.

VIDEO

FORECAST & OUTLOOK

  • Danger Trend

    increasing

    Increasing Danger

  • Area Forecast

    Snow

Cooler temperatures today bringing more accumulating snow.

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.