Posted:
Dec 10, 2015 @ 6:11 am

Good morning, this is Logan King at the West Central Montana Avalanche Center with an early season avalanche information update issued Thursday, December 10, 2015.

A series of storms moved through the region depositing a significant load earlier in the week. Many locations in west central Montana saw nearly 2 inches of snow water equivalent as snow levels climbed to around 7500 feet. High snow levels resulted in a combination of snow and rain that was seen throughout the region adding a significant load to the snowpack.

Wednesday morning more precipitation was added to the snowpack as snow levels remained fairly high. This event deposited much less snow water but significant winds were observed as the cold front pushed through in the afternoon. Ridge top winds were observed in the 40-50 mph range and gusting over 70 mph out of the west on Point Six.

Widespread storm slabs and more localized windslabs will be sitting on top of a weak snowpack that developed during the extended cold period that occurred around Thanksgiving. A good heavy slab sitting on a weak snowpack is going to warrant close attention.

Temperatures are expected to drop going into the weekend and more snow is in the forecast as well. If you choose to travel in the backcountry use safe travel protocols and be diligent with your assessments. Dependent on conditions, regular avalanche advisories will be posted starting later in December. As always let us know if you get out in the snow and share your observations on the public observations page. Think Snow.

READ FULL ADVISORY  

Problem 1 - New 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-3

    Possible

    The likelihood of an avalanche resulting from this problem.

  • ADDED DANGER

    Increased Slope Danger

    Increased/Added Danger

    There is an increased risk of avalanches on these slopes:

    N - North
    NE - Northeast
    E - East
    SE - Southeast

New Wind Slabs can be expected on lee terrain following the strong winds that were seen with the cold front passage on 12/9.

Problem 2 - Dense 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

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    Possible

    The likelihood of an avalanche resulting from this problem.

With dense new storm slabs and more snow in the forecast the snowpack will require time to adjust to the new load.

Problem 3 - Persistent Facets

  • 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-3

    Possible

    The likelihood of an avalanche resulting from this problem.

Both the wind and storm slabs will be sitting on the facets that developed during the cold snap around Thanksgiving. Although it was warmer for the last week, facets take much less time to develop than they do to round so you can expect to see them around for a while.

FORECAST & OUTLOOK

As the temperature drop so will the snow level to about 3000 feet and the majority of mountain ranges in the region are forecasted to get snow over the next few 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.