January 8 Avalanche Advisory

Posted 0630 on January 8, 2010.

Good Morning! This is Steve Karkanen at the West Central Montana Avalanche Center with backcountry avalanche information for Friday, January 8, 2010.

Current Avalanche Danger

We have downgraded the Avalanche Danger issued Wednesday from HIGH to MODERATE. There are isolated areas where the avalanche danger is CONSIDERABLE. This rating is for all elevations and aspects above 5000 feet on slopes steeper than 35 degrees. This higher avalanche danger exists on steep open terrain where it is possible to trigger an avalanche from rock outcrops or shallow places where the weight of a sled or skier can cause failure that propagates across a larger area.

Below 5000 feet, the avalanche danger is LOW.

Weather and Snowpack Analysis

We have two major concerns with the current situation.

Surface hoar developed during a period of cold clear weather over a week ago, that is now buried across widespread mountain locations. Many small avalanches were reported during and after the storm cycle that passed through western Montana Monday and Tuesday when high moisture content snow overloaded this weak layer. This layer is showing signs of gaining strength but was still failing with a moderate amount of force and with fast clean fracture propagations during stability tests Thursday.

The second concern is the layer of facets at the ground that are continuing to fail in stability tests as we have shown in our YouTube videos. All locations are reporting that it is getting harder to initiate failure at this level but again, when it fails, it fails with energy, cleanly and wants to propagate across the slope. These two layers are widespread throughout Western Montana on all aspects above 6000 feet in areas North of Missoula and above 5000 feet in the Lolo and Lookout Pass areas. The buried surface hoar is spotty but all our observers found it Thursday. Slopes that avalanche on the surface hoar layer may easily entrain enough snow and weight to initiate failure of the faceted snow near the ground. These may be large destructive avalanches similar to what we experienced last year at this time.

Please review our recent YouTube videos here:

Missoula Avalanche YouTube Videos

Weather Forecast and Avalanche Outlook

The Missoula Office of the National Weather Service is forecasting a ridge of high pressure to remain anchored over the northern Rockies until a Pacific weather system pushes into the area late Friday. The upper airflow will come from the southwest bringing mild temperatures and light snowfall. Weather models depict the ridge of high pressure to rebuild with dry mild conditions.

The anticipated mild weather conditions will allow our snowpack to gain strength. It is going to take several weeks for the weakness at the base of the upper elevation snowpack to gain strength. The big problem with making go, no-go decisions in avalanche terrain these days is the incredible variation in results we see in our stability tests. The bottom line is that we have a significant layer of rotten, faceted snow at the ground that cannot be trusted and now another weak layer in the upper snowpack that caused several avalanches earlier this week.

We noted surface hoar was developing again with the cold clear weather so keep track of where this feature is located and what happens to it as the weather changes.

We rely heavily on the many excellent observations people are sending us this winter. They help us produce a more accurate avalanche forecast since our resources are limited.

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.