Posted:
Mar 17, 2018 @ 6:04 am

The avalanche danger for the West Central Montana backcountry is MODERATE. Snow has returned to the region and brought instabilities along. Small avalanches will be possible in some terrain, and large avalanches are possible in isolated locations. Carefully assess snow bonding to identify areas of increased concern.

Good morning, this is Logan King with the West Central Montana Avalanche Center’s avalanche advisory for Saturday, March 17th, 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

Mountain temperatures this morning range from 18-26 degrees. Light snow showers have brought a trace to two inches of snow to isolated locations across the advisory area. Winds are shifting to the E this morning and are currently 21 mph gusting to 26 mph at Point 6. In the southern Bitterroot they are 9 mph gusting to 12 mph from the E.

Travis and I toured near Lost Trail Pass yesterday. We found about 6 inches of new snow that had fallen on a crust that was suspect and sensitive to triggers in specific areas. We also found that wind loaded terrain produced small shooting cracks at some locations and a few test slopes produced small soft wind slabs that released with high energy.

The primary concern today will be wind slabs. Wind slabs are growing and sensitive to human triggers. With more snow and  moderate wind in the forecast today the wind slab problem will be increasing. Look for signs like cracking in the snow surface or rounded pillows of snow to determine where wind loading has occurred. Small wind loaded test slopes yesterday were easily triggered up to about 8 inches deep and released with high energy. Avoid traveling on wind loaded terrain because human triggered avalanches will be possible and increasing through the storm cycle.

Storm slabs are the other significant concern today. They are not as widespread as usual but are suspect in areas where a stiffer slab developed or in locations where they sit on top of a crust. The new snow has not bonded well with the old snow surface. Dig a quick pit or use test slopes to identify if the storm snow will be susceptible to failure before committing to avalanche terrain.

Avalanche and Weather Outlook

The forecast suggests light snow for the northern half of the advisory area, while the southern portion of the region looks to have more substantial snowfall today through tonight. The avalanche danger will be increasing today specifically for the Bitterroot range while the rest of the advisory area should have a more consistent avalanche danger.

If you are out in the backcountry, please send us your observation, these are very helpful in producing the advisory.

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-3

    Possible

    The likelihood of an avalanche resulting from this problem.

  • IMAGE

Soft wind slabs have formed on lee terrain that produced cracking and propagated on test slopes.

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

    2-3

    1 (Small)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-3

    Possible

    The likelihood of an avalanche resulting from this problem.

Small storm slabs have formed in isolated areas, they are soft but can easily run on crust surfaces or in ares where wind has affected the storm snow creating a slab.

VIDEO

Audio is poor due to the wind, but great look at a wind slab on a test slope.

FORECAST & OUTLOOK

Light snow is possible in the northern half of the advisory area while more accumulation is expected for the southern half of the region.

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.