Posted:
Dec 31, 2017 @ 6:09 am

The Avalanche warning is terminated this morning.  The avalanche danger is high on wind loaded slopes in the West central Montana backcountry.  All other slopes are rated considerable.  Travel on or under wind loaded slopes is not recommended today.  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 31, 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

Mountain temperatures range from 13 F to 26 F in the region.  In the Bitterroot winds are 3 mph with gusts of 4 out of the SW.  In the northern part of the advisory area, at Point Six, winds are reading 21 mph with gusts of 26 mph out of the WNW.  The forecast area received 3 to 4 inches of new snow in the last 24 hours.

We are coming out of a large natural avalanche cycle.  Downing Mountain Lodge sent us observations of a natural wind slab avalanche that released on the Thanksgiving crust yesterday.  This slide ran 1800 feet, it released around 7800 ft in elevation and ran to 6000 ft in elevation.  Snowbowl ski patrol reported very sensitive wind slabs that were 1 to 2 feet in depth.

The primary avalanche problem today is wind slabs.  Leeward terrain will have large wind slabs.  Look for rounded pillows of snow near ridgelines and recognize signs of instability such as cracking in the surface snow.  These slabs will be very sensitive to human triggers.  Avoid traveling on or under wind loaded slopes.

The second avalanche problem is persistent slabs.  The layer of facets above the Thanksgiving crust has been stressed.  This layer has been shown to fail in the last two natural avalanche cycles. This means careful snowpack evaluation, cautious route-finding, and conservative decision-making are essential to recreate today.  Look for clues from the snowpack shooting cracks and localized collapsing.  Dig a pit on low angle terrain in a safe spot out of runout zones to see how the layers are adjusting to the new load.

The final avalanche problem is storm slabs.  These slabs are becoming less reactive as the snowpack adjusts to the weight of the new snow.  Look for shooting cracks and try small test slopes with low consequence to see how the new snow is bonding.

Avalanche and Weather Outlook

Our next chance for snow will be in the middle of the week.  With the forecast look for the avalanche danger to slowly decrease as the snowpack adjusts to the new snow.

If you are out in the backcountry, please send us your observation, these are very helpful in producing the advisory. I will issue a weather update tomorrow  January 01, 2018.

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

    5-6

    2-3 (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.

With Strong winds leeward terrain is loaded with large wind slabs sitting on the Thanksgiving crust.  We had reports of natural releases of wind slabs on the Thanksgiving crust in the  Central Bitterroot.  These slabs were very sensitive to human triggers in control work from the Snowbowl ski patrol.  The slides are running long distances.

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

    6-7

    3 (Large-Very Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-4

    Possible/Likely

    The likelihood of an avalanche resulting from this problem.

The layer of facets above the Thanksgiving crust has been stressed.  This layer has been shown to fail in the last two natural avalanche cycles. This means careful snowpack evaluation, cautious route-finding, and conservative decision-making are essential to recreate today.  Look for clues from the snowpack shooting cracks and localized collapsing.  Dig a pit on low angle terrain in a safe spot out of runout zones to see how the layers are adjusting to the new load.

 

Problem 3 - Strom 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

    5-6

    2-3 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    Possible

    The likelihood of an avalanche resulting from this problem.

The snowpack is adjusting to the new load and is slowly settling out.  We are still getting reports of whumphing.  Look for shooting cracks and try small test slopes with low consequence to see how the new snow is bonding.

FORECAST & OUTLOOK

Next chance for snow is mid week. With this forecast look for avalanche danger to slowly decrease. Here is a link to the discussion.

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.