Posted:
Feb 10, 2018 @ 6:33 am

The avalanche danger for the West Central Montana backcountry is considerable on wind-loaded terrain.  moderate avalanche danger exists on all other terrain. Large avalanches are possible in specific terrain. Carefully identify areas of increased avalanche risk and utilize cautious route-finding techniques to travel in the backcountry today.

Good morning, this is Logan King with the West Central Montana Avalanche Center’s avalanche advisory for Saturday, February 10th, 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

Winter has finally returned to West Central Montana. Temperatures across the advisory area this morning range from 3-19 degrees. Winds are light from the NW at 12 mph gusting to 14 mph. Light snow continued through the day yesterday and last night adding another couple of inches. The snow is much lighter and colder than we have seen for the last month and only .2 inches of water were added to the snowpack since yesterday morning.

Travis and I rode up at Wisherd ridge yesterday. We found about 6-8 inches of low density snow. At lower elevations the new snow was sitting on saturated snow and runnels were observed to 6,400 feet. Above that the snow was much colder and had preserved graupel throughout the upper layers of the snowpack. Overall we found the snow that is continuing to trend towards stability with wind slabs as the glaring concern.

The greatest concern continues to be wind slabs. Strong winds did a number above about 6,000 feet. Large wind slabs have formed on lee terrain and will be susceptible to triggers from skiers and riders. Look for rounded pillows of snow, cornices, and shooting cracks that will key you into locations where wind loading has occurred. Avoid traveling on or under wind loaded terrain as these wind slabs are primed and waiting for a trigger.

Non-wind loaded terrain continues to gain strength. Loose snow avalanches are the secondary concern today. The new light snow lacks cohesion to form a slab but will easily sluff in steeper terrain. The biggest concern for these sluff avalanches are terrain traps that increase the consequences of being swept off your feet or sled. Carefully evaluate the consequences of even a small slide before committing to steep terrain.

Finally, the buried surface hoar and associated crusts are now found deep in the snowpack. This means that deep persistent slabs are the third concern today. The buried surface hoar is now deep enough in the snowpack that it has become very hard to trigger, but it is a still a possibility, albeit an unlikely one. Be cautious around shallow or rocky areas as they will be the prime location to trigger slides on these deep layers.

Avalanche and Weather Outlook

A weak high pressure system looks to set up for the next 36 hours or so. Cold and settled conditions will persist through the day today before the next system moves in late Sunday into early Monday. Avalanche danger will continue to trend towards stability.

The next advisory will be issued on Tuesday, February 13th. 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.

Large wind slabs have formed on leeward terrain and small wind slabs can be found on cross-loaded features.

Problem 2 - Loose Snow

  • 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

    1-2

    < 1 (Small)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-5

    Likely

    The likelihood of an avalanche resulting from this problem.

Small loose snow avalanches are likely in steep terrain.

Problem 3 - Deep Persistent

  • TYPE

    deep-persistent-slabs

    Deep Persistent Slabs

    Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground.  The most common persistent weak layers involved in deep, persistent slabs are depth hoar, deeply-buried surface hoar, or facets surrounding a deeply-buried crust. Persistent, Deep-Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply-buried over time.

  • SIZE

    7-8

    3-4 (Very Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-2

    Unlikely/Possible

    The likelihood of an avalanche resulting from this problem.

The buried surface hoar is now deeply buried but should be considered before committing to avalanche terrain.

GALLERY

preserved graupel 2/9/18
preserved graupel 2/9/18
Wind Loading in Bowl 3
Wind Loading in Bowl 3
Wind loaded ridge
Wind loaded ridge
Wind loaded rollover
Wind loaded rollover
Sheep Mountain 2/9/18
Sheep Mountain 2/9/18
Overcast with rolling fog
Overcast with rolling fog
Runnels at Lower elevations
Runnels at Lower elevations
Bowl 1
Bowl 1
02_10_18Wisherd-09-Feb
02_10_18Wisherd-09-Feb

FORECAST & OUTLOOK

A weak high pressure system looks to set up for the next 36 hours or so. Cold and settled conditions will persist through the day today before the next system moves in late Sunday into early Monday.

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.