West Central Montana Avalanche Advisory

Good morning! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for December 22nd, 2008. 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.

Weather and Snowpack Analysis

Most locations received just a few more inches of snow since Thursday and the temperatures started to moderate back into the single digits and teens at most locations. The wind Sunday turned more East-Southeasterly at 15-20 and was gusting into the 30-40mph range. This morning temperatures are close to 20 degrees at most SNOTEL sites with 2-3” of new snow overnight. More snow is in the forecast and stability conditions are getting touchier as the snow piles up and the wind blows it around.

We received a great report from the Northern Bitterroot Mountains near Hoodoo Pass where an observer’s stability testing showed failures occurring at the level of the rain crust that formed on 12/6. While it took moderate force on compression tests to produce failure (CT14-20), the isolated columns slid cleanly and easily (Q1). Further North in the St. Regis Basin area, the same party noted fracture propagations, and collapse indicators on steep wind loaded slopes on mostly Southern aspects. They wisely choose to ski on less steep terrain in both locations and found decent (but thin) skiing.

In the Rattlesnake Sunday, we found highly variable (and thin) snow conditions on all aspects. A strong ESE wind was loading WNW aspects through the day and scoured the more East facing terrain. We triggered an 18-20” wind slab while crossing beneath a steep embankment on a road. While this was a small slide what was notable was the distance the fracture propagated. The fracture ran parallel to the road for approximately 450′. It failed on faceted snow that formed on a hard snow surface on what is usually a wind scoured area. Compression test scores were CTV to CTE2 with a Q1 shear plane. On slopes adjacent to this slide that turned just a few degrees to the North, our compression test scores were much higher with dirtier shear quality CTM12-24 with Q2 shear planes. This YouTube video shows a compression test on this particular aspect.

Current Avalanche Danger

On steep wind loaded terrain at the highest elevations (7000′ – 9000′) the avalanche danger is CONSIDERABLE. Natural avalanches are possible and human triggered avalanches are probable. Unstable slabs can be found in isolated areas that have been recently wind loaded. These are typically at the higher elevations on the leeward side of ridges especially above the tree line.

At other on all aspects and elevations from 7000′ to 9000′ the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. There are some very weak features that need close attention during the next couple of storms.

Below 5000′ the avalanche danger is LOW. Natural avalanches are very unlikely and human triggered avalanches are unlikely. There is very little snow at most locations below 5000′.

Weather Forecast and Avalanche Outlook

Expect snowfall to continue through the day today as a low pressure system digs into the Northwest. This system has us under a Southwest flow which is pumping in ample moisture. The best location for snow in our area is South of I-90. A weak ridge of high pressure builds Tuesday ahead of the next storm system. The next good chance for snow appears to be coming Wednesday just in time to fill our stockings for Christmas.

Expect avalanche danger to increase as the mountains receive more snow and wind. Pay particular attention to areas that have been recently wind loaded and any steep open slope where the anchors are covered. As always, never expose more than 1 person at a time to a slope steep enough to slide, have a transceiver, probe pole and shovel on you and know how to use them.

If you have any avalanche or snowpack information you’d like to share or have questions about the upcoming season, please contact us at [email protected]

The next avalanche advisory will be posted December 26th, 2008.