Posted:
Feb 20, 2016 @ 6:17 am

The current avalanche danger for the west central Montana backcountry is MODERATE. Human triggered avalanches are possible in isolated areas and on specific terrain. 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.

Good morning, this is Logan King with the West Central Montana Avalanche Center’s avalanche advisory for Saturday, February 20th, 2016.

Weather and Snowpack

Mountain temperatures for the advisory area are currently averaging in the mid twenties and the region has seen an additional 2-5 inches of snow overnight. Strong wind gusts continue with Deer Mountain recording winds of 16mph gusting to 49mph from the west early this morning. 

The primary avalanche concern today will be storm slabs. We have seen about 10 inches of cold snow over the last few days and this new snow has not bonded to the older and warmer snow. Travis and I were in the Rattlesnake yesterday and found this interface to fail and propagate in extended column tests on north aspects and a skier trigger slide likely slid on this interface on an eastern aspect in Jenny Bowl as well (pic1). We were able to perform stability tests on a NE aspect adjacent to a slope that had a small skier triggered slide (pic2) and found this interface to be sensitive on the NE aspect as well. The cold/warm interface was reactive on variable aspects so take a look at the snow before recreating and see if the new snow is bonding with the old surface, and though the slab is not very large, carefully evaluate the consequences of getting caught on this storm slab.

Winds have primarily been out of the south for the later portion of the week and wind slabs were seen in areas that are usually free of wind affected snow. Some of these wind slabs have proven to be reactive over the week and with more low density snow for transport they continue to grow. Evaluate all slopes carefully and look for signs of wind deposit and ultimately wind slabs. With unusual weather you get unusual avalanche problems and pockets harboring small wind slabs are a real problem as of late, specifically on crossed loaded terrain features. These signs are obvious but easily ignored, be suspicious of any terrain feature that appears to have been wind loaded.

Lastly facets continue to persist in the snowpack, and have been most reactive in areas where they are found under a melt freeze crust. Yesterday the facets at mid elevations on an east aspect were found to be wet and failed and propagated readily in stability tests. Unfortunately facets can be found on nearly all aspects. Particularly problematic are wet and saturated facets on aspects with high solar radiation but facets have also been found on cooler shaded aspects. The more northern aspects are not as reactive but the facets are further developed and still warrant a watchful eye.

Avalanche and Weather Outlook

The current pattern of light intermittent snow will likely continue into tonight bring modest accumulations to the area. Winds are also expected to maintain current intensities through today. Sunday will likely bring more settled conditions before another pulse of snow sunday night followed by warming temperatures again setting up early in the week. With no major changes expected for the next 48 hours avalanche conditions will remain relatively constant through the weekend.

The next avalanche advisory will be issued on Tuesday.

READ FULL ADVISORY  

Problem 1 - Storm Slab

  • 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.

  • IMAGE

The interface between the new colder snow has not bonded to the old and warmer snow surface yet. This layer was reactive yesterday with shallow skier triggered slides and propagated in stability test.

Problem 2 - 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-2

    Unlikely/Possible

    The likelihood of an avalanche resulting from this problem.

Windslabs are widespread and the winds have been uncharacteristic yielding windslabs in diverse locations.

Problem 3 - 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

    3-4

    1-2 (Small-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 layer of facets below the melt freeze crust continues to propagate in stability test.

FORECAST & OUTLOOK

Intermittent snow and wind will continue into tonight bringing modest accumulations.

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.