February 15 Avalanche Advisory

Weekend Avalanche Advisory
Posted February 15th at 0600

This is Steve Karkanen at the West Central Montana Avalanche Center with the weekend avalanche advisory for the Presidents Day Weekend, 2008. The avalanche danger rating expires at midnight February 15th. 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. Thanks to John Lehrman of Backcountryfocus.com for the help in the Southern Bitterroot this week.

Weather and Snowpack Analysis

Warm temperatures, heavy snowfall and rain were the big weather events last weekend. Most areas received rain up to about 5000′ Sunday then temperatures dropped with a fast moving system that also dropped several inches of snow. This newest snow is bonding well in most locations but as noted in the Rattlesnake, some wind loaded pockets were failing on density changes. This will continue to gain strength over time. You can see an example of this on YouTube:

http://www.youtube.com/watch?v=aomoWcaLwqM

There is evidence of avalanche activity that occurred last Sunday in many areas of the Bitterroot Mountains. Many of these avalanches ran on the MLK layer (a thin layer of faceted snow and surface hoar now 4′ deep) that continues to rear its ugly head whenever we get a big dump or major change in temperatures. This layer can still be found in most locations above 5000′ but has strengthened to the point where it is not much of a concern under the current weather and snow conditions. However, when and where we get it to fail, it fails cleanly with a lot of energy. It’s something to think about when we start to warm up or get rain above 5000′.

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.

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.

Weather Forecast and Avalanche Outlook

The Missoula Office of the National Weather Service is forecasting high pressure to build today then a disturbance will brush through Friday night and Saturday with only minor amounts of precipitation. Another stronger ridge of high pressure begins building Sunday into early next week. Mountain temperatures are expected to warm as an inversion sets up and intensifies in the western valleys.

Expect conditions to continue to improve with the current forecast until temperatures rise above the freezing point or whenever the higher elevations receive rain or heavy snowfall. The weakness that formed after the MLK holiday is now deeply buried and takes a lot of force to produce failures. But we still find that it is unable to support a rapid load or quick change in conditions in many areas. Avalanches that ran to the river on Highway 12 earlier this week remind us that the right combination of new snow load, warm temperatures or rain can produce large destructive avalanches in areas that seem stable.

Dudley Improta will post the next avalanche advisory on February 22, 2008.