Posted:
Dec 27, 2018 @ 6:19 am

The current avalanche danger is considerable for the West Central Montana backcountry. Human triggered avalanches are likely in specific areas across the region. Safe travel in the backcountry will be achieved with keen hazard identification, cautious route-finding and careful snowpack evaluation.

Good morning, this is Logan King with the West Central Montana Avalanche Center’s avalanche advisory for Thursday, December 27th, 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

Light snowfall developed overnight and across the region 1-4 inches of snow has accumulated. Mountain temperatures were cold overnight keeping the snow drier with SWE values of 0.1-0.2 inches being recorded. As of this morning mountain temperatures are 14-22 degrees. Winds are light from the WSW at 2 mph gusting to 3 mph, however strong winds were reported over the weekend and holiday for parts of the advisory area.

The snowpack varies greatly across the region. As some areas gain strength others continue to have a weak structure that can not be trusted. The biggest problem that we continue to see are persistent weak layers. The buried surface hoar is still reactive when present and facets near the ground continue to cause problems in certain areas. At the southern end of our forecast area the snowpack is unconsolidated, and consists of sugary weak snow. While stronger snow can be found there is a mixed bag of snow structure for the norther half of the forecast area.

Persistent slabs are the main concern today. In areas where there is any sort of slab overlying a layer of buried surface hoar or facets continues to be reactive in stability tests and will be easily triggered if traveled on or near. Careful snowpack evaluation is needed to identify areas of heightened concern. The only way to know if facets or buried surface hoar is present on a slope is to dig a pit and perform stability tests.

Wind slabs continue to gain strength and are becoming less reactive but are ideally suited to fail in locations where facets or buried surface hoar are also present. The combination of very weak layers and firm wind slabs will create dangerous avalanche conditions. Verify that any location that has a slab doesn’t also have facets. The strong winds over the holiday have created some big wind slabs and will make for some touchy conditions across specific terrain.

The take away today is that conditions are complex and the consequences are high if you make a mistake. Don’t be lulled into a false sense of security. Take the time to do your due diligence; dig a pit and test the snowpack before travelling in avalanche terrain.

 

Avalanche and Weather Outlook

Light snow showers are expected to continue through the morning before a break in the weather. Avalanche conditions will not change much with light accumulations and dissipating snowfall. The next round of snow is setting up to arrive Friday night but there is uncertainty around snow levels and the snow has the potential to be warm and wet (forecast).

If you make it out into the mountains feel free to share what you see on our public observations page. They are not only helpful to your community but extremely helpful to us.

Ski and ride safe.

READ FULL ADVISORY  

Problem 1 - 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

    5-6

    2-3 (Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-5

    Likely

    The likelihood of an avalanche resulting from this problem.

Buried surface hoar and facets are weak layers that present a serious hazard.

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

    6-7

    3 (Large-Very Large)

    The potential size of avalanche resulting from this problem.

  • LIKELIHOOD

    Likelihood-5

    Likely

    The likelihood of an avalanche resulting from this problem.

Wind slabs are touchy and very suspect in areas with facets and hoar.

FORECAST & OUTLOOK

Scattered snow showers this morning should subside by the afternoon.

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.