January 22, 2010 Avalanche Advisory

Good Morning! This is Steve Karkanen at the West Central Montana Avalanche Center with backcountry avalanche information for Friday, January 22, 2010.

Current Avalanche Danger

On slopes steeper than 35 degrees and on all aspects at elevations above 5000 feet, the avalanche danger is MODERATE. Natural avalanches are unlikely, human triggered avalanches are possible.

Below 5000 feet, the avalanche danger is LOW.

Weather and Snowpack Analysis

Temperatures have remained moderate since the MLK holiday and most sites received just a couple of inches of new snow. Enough to “freshen up” the snow on shaded slopes. On the more south facing aspects, a sun crust formed during the sunny afternoon Wednesday.

These moderate temperatures continue to help strengthen the overall snow stability condition and most areas are now mostly stable but we still have a couple of persistent features that are keeping us in the moderate rating.

Near Lost Trail Pass observers are reporting a layer of buried surface hoar has actually weakened as stability tests are producing failures 45cm deep with moderate force but with high quality shears with energy and propagation across the column in extended column testing. Temperatures have been cooler and the snowpack shallower in this area than in our northern locations.

All other locations including an area near Camas Peak in the central Bitterroots at 8600 feet, Tom Beal Park near Powell ID at 7900 feet, Lookout Pass and the Rattlesnake at 7600 feet are all showing similar snow structure but strengthening of the weaker layers with the moderate temperatures.

South aspects are shallow and are going to be touchy if we receive significant snowfall in the next few days. Sun crusts that formed 2 and 4 days ago are showing near-surface facet growth between two sun crusts, which easily fail during skiing, and testing. You can see an example of this on our photo page. Steep open south slopes will be weak once loaded. Moreover, the depth hoar we have been talking about all winter is still present on all aspects. Shallow pockets will be of more concern for deep failures at this layer.

Weather Forecast and Avalanche Outlook

Mild and dry conditions will transition to cool and snowy over high terrain during the next few days. A broad Pacific storm system will split in two over the Pacific Northwest early Saturday with snow developing and snow ratios increasing with time. Expect the main snow event to end during Sunday with high pressure bringing more dry and mild weather.

We currently have the most stable conditions of the winter this week. Check out this YouTube video of a Rutschblock in the Rattlesnake for an example of what we are seeing. The two weak layers described in earlier reports may become reactive if the area receives a heavy load of snow. The places to watch will be any open wind loaded slope steeper than 35 degrees. It is possible that higher elevations could see 12-16 inches of snow this weekend. That will certainly be enough to bring our snowpack back to life if it comes in all at once. If it spreads out over time, we should be in good shape stability wise.

If you get out and see avalanche activity or want to send us quick snow observations, send us a note at [email protected] or call us at 406-530-9766. 530-9SNO.