January 11 Avalanche Advisory

Weekend Avalanche Advisory
Posted January 11th at 0700

Hello! This is Steve Karkanen at the West Central Montana Avalanche Center with the weekend avalanche advisory for January 11th -13th, 2008. The avalanche danger rating expires at midnight January 11th. This report is based on field observations and data collected on Thursday and describes conditions seen at that time. This advisory is the sole responsibility of the Forest Service and does not apply to operating ski areas.

Weather and Snowpack Analysis

The mountains of West Central Montana received another shot of moisture this past week with most Bitterroot Mountain Range SNOTEL sites picking up 2.2” to 2.6” of snow water equivalent (SWE) since last Friday. This translates to 20” to 25” of new snow. Saddle Mountain SNOTEL and Stuart SNOTEL picked up 1.8” and 1.6” or about 15” of new snow. All this snow was spread out over the course of the past 7 days so there has been plenty of time for the snowpack to adjust to this new weight. 8000′ wind speeds have been in the 30 to 35mph range for most of the week. Last Friday high winds from the East then South scoured many sites and left a very hard surface near ridgelines.

Observers report mostly stable conditions and very good skiing and riding conditions at all locations except the very Southern Bitterroot near Lost Trail Pass. Skiing conditions were fine however stability testing produced easy failure at the level of the deeply buried faceted layer associated with the early December crust. While this may be an isolated weak spot, it is worth noting as a potential problem. Observers here felt the avalanche danger is CONSIDERABLE on steeper wind loaded slopes. In the Rattlesnake, observers noted fracture propagations and easy failures on recently formed wind slabs. These were not very big slabs but a lot of snow has been moved onto leeward terrain in the past few hours. Expect to find sensitive and deeper wind slabs near the ridgelines. The wind this morning is out of the W-SW averaging 30mph at 8000′.

Today’s Avalanche Danger

Above 5000′ in all advisory area locations, the avalanche danger is MODERATE. Natural avalanches are unlikely. Human triggered avalanches are possible. Above 6000′ there may be pockets where the avalanche danger is CONSIDERABLE on recently wind loaded terrain steeper than 35 degrees. Natural avalanches are possible and human triggered avalanches probable. The most likely place to find these pockets of unstable snow is on East to North aspects or wherever the wind has recently deposited snow on the hard surface created during the big wind last week.

Below 5000′ the avalanche danger is LOW. Natural avalanches are very unlikely and human triggered avalanches are unlikely. Low avalanche danger does not mean there is no avalanche danger. Snow is as variable as the mountains we like to play on and you can almost always find a pocket of snow that will avalanche given enough weight or shock applied to it.

Weather Forecast and Avalanche Outlook

A Heavy Snow Warning is in effect for the Bitterroot-Sapphire Mountains, the Clearwater Mountains and the Lower Clark Fork region until 3PM Friday. Expect several inches of new snow by tonight with another system expected to move into the area Saturday.

Expect snowpack stability conditions to worsen during periods of heavy snowfall and high wind. Overall snowpack conditions continue to strengthen and the faceted weak layer we’ve been describing for a while now is deeply buried and able to take a lot of weight and force before it fails. The main avalanche concern this weekend will be how well new snow bonds to the snow surface and how heavily the wind loads leeward terrain.

The next avalanche advisory will be posted on January 18th, 2008.