Posted:
Dec 29, 2016 @ 7:03 am

The current avalanche danger is HIGH in the Bitterroot Range of the west central Montana backcountry on steep(>30) wind-loaded slopes.  On all other slopes in the advisory area the avalanche danger is CONSIDERABLE.  Travel on wind-loaded slopes is not recommended. This means careful snowpack evaluation, cautious route-finding and conservative decision-making are essential to recreate today.

Good morning, this is Travis Craft with the West Central Montana Avalanche Center’s avalanche advisory for December 29, 2016. 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

Mountain temperatures range from 6 F to 14 F in the region. In the Bitterroot, winds are 13 mph with gusts of 18 mph out of the SE and Point Six, in the northern part of the advisory area, winds are reading at 15 mph with gusts of 20 mph out of the SW. The forecast area received 5 to 24 inches of new snow in the last 48 hours favoring the southern region. The new snow had SWE’s ranging from .4 to 1.5 inches of water.

Logan and I rode the sleds into Twin Lakes in the central Bitterroots yesterday.  We found over 2 feet of new snow and winds actively loading leeward terrain. We observed 4 natural wind slab avalanches that had released during the last 24 hours.  Downing Mountain Lodge sent us observations over the last 72 hours of wind slabs forming 1 to 2 feet deep and were touchy to triggers. Matt was at Lolo Pass and observed storm slabs that were easily triggered by skiers and leeward terrain being actively loaded by wind.

The primary avalanche problem today are wind slabs.  The region has received several inches of new snow and the winds have transported snow to leeward terrain.  We observed these slabs releasing naturally and John from Downing Mountain Lodge in the central Bitterroot, described them as touchy to human triggers.

The second avalanche concern is the storm slab.  These were observed by Matt at Lolo Pass and were easily triggered by a skier.  These range in depth throughout the area from 5 inches to over 2 feet.

The last concern are dry loose releases.  These sloughs can be very large and could potentially carry a rider into a terrain trap (rocks, cliffs,or trees).

Bullseye clues to instability from yesterday to aid in your assessment of instability in the mountains: natural avalanche activity, whumfing, shooting cracks from skis, new snow, and active wind transport of snow.  These signs help to identify the snowpack is unstable and should be given some time to adjust to the new load of snow.  Choose low angle terrain to get a feel for current conditions.

Avalanche and Weather Outlook 

The snowpack needs time to adjust to the new snow.  The advisory area will see gusty winds today which will continue to load leeward terrain.  The region is forecasted to have a new system move into the area tonight and Friday bringing with it low snow totals.  With the current winds expect the avalanche danger to remain the same.

I will issue the next advisory on December 31, 2016.

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

    4-5

    2 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-7

    Very Likely

    The likelihood of an avalanche resulting from this problem.

  • IMAGE

Wind slabs have developed on a number of different aspects.

Problem 2 - 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-6

    Likely/Very Likely

    The likelihood of an avalanche resulting from this problem.

  • IMAGE

New snow has created storm slabs which are sensitive to triggers.

Problem 3 - Loose Dry

  • TYPE

    loose-dry

    Loose Dry

    Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose-Dry Avalanches,they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose-wet avalanches can trigger slab avalanches that break into deeper snow layers.

  • SIZE

    3-4

    1-2 (Small-Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-7

    Very Likely

    The likelihood of an avalanche resulting from this problem.

  • IMAGE

In some locations the snow is unconsolidated and is easily triggered by riders.  These could be a problem if they carry you into a terrain trap(cliff, rocks, or gully).

VIDEO

Twin Lakes 12/28/2016

12/28/2016 Observations

FORECAST & OUTLOOK

  • Danger Trend

    same

    Same Danger

  • Area Forecast

    Wind

Gusty conditions

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.