Jan 29, 2013 @ 6:46 am

January 29, 2013 Avalanche Advisory

There is CONSIDERABLE avalanche danger in the west central Montana backcountry on terrain steeper than 35 degrees. Natural avalanches are possible and human-triggered avalanches are likely. On all other terrain, the avalanche danger is MODERATE.

Good Morning. This is Tim Laroche with the West Central Montana Avalanche Center’s backcountry avalanche advisory for Tuesday, January 29th, 2013.


Weather and Snowpack Analysis

Scattered snowfall has continued overnight and the most recent storm has deposited 8-10 inches in the Bitterroots, a foot in the Rattlesnakes, and 16 inches in the southern Missions and Swans. Winds were moderate out of the south and west and temperatures hovered in the mid twenties. Currently,  snow is falling, temperatures are in the upper teens or low twenties, and winds are out of the west 4-6mph.

The primary avalanche concern is storm snow avalanches. Yesterday in the northern Bitterroots, Steve and I were able to easily trigger 12 inch slabs in steep terrain (photo). The new snow is sliding on a thin sun crust on those aspects that were exposed to the sun last week or a layer of facets that formed on shaded aspects. The storm snow has not yet formed a slab in all areas. Where the slab exists, expect to trigger soft slabs from one to two feet deep on steep terrain. Where the snow has not formed a slab, expect to trigger loose snow avalanches that could catch and carry you down slope.

The secondary avalanche concern is wind slabs that have developed on leeward slopes and terrain features. This will become more of a concern today as the winds are still predicted to blow in the 20mph range. The newest snow will be transported easily onto leeward slopes and be sensitive to human triggers. This problem will be found at mid and upper elevations on ridgetops and cross-loaded gullies. Avoid steep terrain where fresh wind drifts have formed.

The new snow has continued to add weight to a buried weak layer now down 2-3 feet. This weak layer of buried surface hoar has been found in some localized areas in the Bitterroots. It has been gaining strength, but observations indicate that it will still fail cleanly when a lot of force is applied. I found it last week near Lolo Pass (video), and it was also found by a party in the southern Bitterroots this past weekend (public observation). This persistent weak layer is worth looking for if you intend to play on steep, open terrain.


Weather Forecast and Avalanche Outlook

Today expect snow to continue with the chance of another 3-6 inches. Winds will be out of the north and west in the 15-20mph range and temperatures will be in the mid 20’s. The northwest flow should last a couple of more days with snow showers remaining in the forecast until later in the week.

I expect the avalanche danger to stay the same as we continue to receive moderate amounts of mountain snow.

I will issue the next advisory on Friday, February 1st.

If you get out and have the time to send us some information about what you are seeing, please use our “public observations” link on our website or send us a quick note at [email protected]. Thank you for your continued support!





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.