Dec 27, 2013 @ 7:16 am

December 27, 2013 Avalanche Advisory

The avalanche danger in the west central Montana backcountry above 7000 feet is MODERATE .  Natural avalanches are unlikely but human triggered avalanches are possible. It is possible to trigger an avalanche on wind loaded terrain steeper than 35 degrees and on shaded north to east facing terrain steeper than 35 degrees.

On other terrain and in areas below 7000 feet, the avalanche danger is LOW. Although conditions are generally safe in these areas, you need to be aware of the potential for unstable snow on isolated terrain features.

Good morning!  This is Steve Karkanen with the West Central Montana Avalanche Center’s Avalanche Advisory for Friday, December 27th, 2013.  The avalanche danger rating expires at midnight tonight and does not apply to operating ski areas.

Current Weather and Snowpack

This morning skies are clear with mountain temperatures in the upper 20’s.  Above 8000 feet the wind is gusting to nearly 30 mph. We’ve received no new snow since early Christmas Eve.

Our primary avalanche problem is recent wind loading on steep leeward terrain due to high winds that have been present the past few days.  Ridge top winds have been mostly from the west with gusts into the 40 mph range.  Much of the snow that was on the ground earlier this week has been scoured away and redeposited as a stiff slab on the lee sides of ridges and outcrops.

These slabs are stubborn. It takes a lot of force to get them to react, but when they do, they fail as a hard slab. A low probability-high consequence scenario. You don’t want to get caught in a hard slab avalanche especially with the thin snowpack we currently have.

A secondary avalanche problem is a persistent weak layer present on many shaded aspects and in areas where the snowpack is shallow or less than 3 feet deep.

At all the locations the avalanche specialists and observers visited this week, we’re finding a mostly stable snowpack in areas where the snow is deeper than 3 feet, but a weaker structure in areas that are shaded or less than 3 feet deep.

The faceted snow that formed during the early December Arctic blast is the most prominent feature in many of our profiles but it is showing signs of strengthening. Isolated columns will not propagate fractures during extended column tests and the weak layers fail only after a lot of force is applied in other stability tests.

This is all good and not much of an avalanche problem right now, but this structure may not support a big load of new snow if and when we get it.


Weather and Avalanche Forecast

A high pressure ridge continues to be the dominant weather feature. A weak disturbance will move through the northern Rockies tonight and Saturday causing widespread light snow with the mountains and passes receiving a couple of inches of snow.

With the moderate temperatures and continued wind, I expect the avalanche danger to remain similar until the next significant snowfall.

Dudley will issue the next avalanche advisory on New Years Eve.



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.