February 13, 2009 Avalanche Advisory
Posted Friday February 13th, 2009 at 0600.
Good morning! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for February 13th, 2009. 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. Expect avalanche conditions to change as weather conditions change. Our advisory area includes the Bitterroot Mountains from Lost Trail Pass North to near Lookout Pass, the Rattlesnake Mountains and the Southern Swan and Mission Mountains near Seeley Lake. Avalanche information about the St. Regis Basin can be found on the Idaho Panhandle National Forest Avalanche Center website.
Weather and Snowpack Analysis
During the past 4 days all mountain locations received very light snowfall and experienced diminished winds. Observers report good skiing and riding conditions and continued strengthening of the deeply buried facets we have been talking about since December. We received several excellent reports from skiers and riders in the Bitterroot and on Wisherd Ridge (near Sheep Mountain NE of Missoula) all of whom reported loose snow avalanching on steep open slopes. This activity is occurring on slopes steeper than 40° and is involving the newest storm snow, which in most areas is less than about 8”. It fails on the interface of the new snow and faceted crystals or surface hoar on a prominent sun crust that formed several days ago. Some isolated locations, such as Wisherd Ridge, appear to have received a bit more snowfall (up to 14”) and have more potential for a soft slab already set up on this weak layer.
This will be a serious avalanche problem once a slab forms. The ease of the new loose snow failing on these facets is a huge indicator of what’s to come when we receive several inches of heavy snow or high wind. At this point, all we really need is a high wind to lay down a stiff wind slab on the weaker (and now protected) surface hoar and facets associated with the upper level crust.
Current Avalanche Danger
At all advisory area locations above 6000′ on wind loaded terrain steeper than 35°, the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. All other slopes have a LOW avalanche danger where natural avalanches are very unlikely and human triggered avalanches are unlikely. You can almost always find isolated pockets where it is possible to trigger an avalanche with the right amount of force, but why would you want to do that?
If temperatures warm up that alone will turn this light snow into a slab that is just thick enough to be dangerous. Right now, it’s really not much of a problem unless you’re in terrain where you could get knocked over a cliff or pushed through trees on a steep slope. Just be aware of this and don’t take any chances if you’re not an accomplished rider or skier.
Weather Forecast and Avalanche Outlook
The Missoula Office of the National Weather Service is forecasting a weak upper level flow through the weekend with a large upper level trough dominating the weather pattern for the Western US. Any significant moisture and instability will pass well south of our area through the weekend.
Expect avalanche conditions to remain similar and to slowly improve for the next few days. A significant new load on steep slopes from wind or storm snow will quickly raise the avalanche danger to considerable or high in areas where the above described weakness exists. Also keep in mind that many of the sheltered lower elevations that were in the fog during the inversion 2-3 weeks ago have an obvious buried surface hoar layer that will become reactive with a new load. This is most pronounced in the Lolo Creek drainage at elevations below 5000′.
If you have any information you’d like to share or have questions about anything related to snow safety, please contact us at [email protected].
The next avalanche advisory will be posted on Monday, February 16th, 2009.