January 18 Avalanche Advisory

Weekend Avalanche Advisory
Posted January 18th at 0700

Hello! This is Steve Karkanen at the West Central Montana Avalanche Center with the weekend avalanche advisory for January 18th -20th, 2008. The avalanche danger rating expires at midnight January 18th. 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.

Weather and Snowpack Analysis

We continued to receive short bursts of snow the past few days. All mountain SNOTEL sites report 1.2” of Snow Water Equivalent (SWE) or about 12” of new snow. Twin Lakes and North Fork Jocko each received 2.4” so there are a few places that picked up even more snow. High elevation winds pounded the area last Friday and temperatures began dropping into the single digits. Yesterday temperatures moderated but are expected to begin dropping into the sub-zero zone sometime this weekend. The mountains North of Missoula picked up an additional 6” overnight. The main weather concern has been the wind and the ability to form tricky slabs on leeward terrain.

Observers report mostly stable conditions and very good skiing and riding conditions at all locations this week. Everyone reported significant snow transport and wind slab formation at the higher elevation sites or any location exposed to wind. Overall stability continues to improve with a couple of notable exceptions. These wind slabs formed over a layer of cold, low density snow and were showing moderately easy failures when force was applied. This condition can be found in nearly all locations above 6000′. A good video clip of this slab can be seen here:

http://www.youtube.com/watch?v=c0obgp1oChQ

We received a report of a skier triggering this wind slab on Sheep Mountain last Sunday and going for a 400′ ride in Bowl 3. He was not injured and very lucky as this small slide strained him through small trees.

The other concern is the now deeply buried faceted snow that formed in early December. Although this feature is taking a lot of force to fail, it remains a concern because when it fails, it does so cleanly with energy and will be destructive. This is the layer that failed in the tragic accident in Canyon Creek near Whitefish where 2 people died last Sunday. During periods of heavy loading, this weak layer may become reactive.

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. Above 6000′ there may be pockets where the avalanche danger is CONSIDERABLE on recently wind loaded terrain steeper than 35 degrees. Natural avalanches are possible and human triggered avalanches probable. The most likely place to find these pockets of unstable snow is on East to North aspects or wherever the wind has recently deposited new snow.

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

A strong short wave will move into the Northern Rockies tonight bringing an increased chance for moisture and accumulating snowfall. 6000′-8000′ winds will again be a factor as this system passes. Temperatures are expected to drop into the single digits and lower as cold arctic air moves in from the Northeast and imbeds itself in the Western valleys.

Expect snowpack stability conditions to worsen during periods of heavy snowfall and high wind. Pay close attention to areas recently wind loaded. Avoid rocky areas or pockets where the snowpack is thin. These are likely places where it would be possible to initiate a fracture propagation involving the deepest weak layer. This is not the year to completely trust what you see out there. The avalanche near Whitefish earlier this week is an unfortunate reminder of that.

We’ve been getting awesome reports from many different areas recently. These reports really do help us piece together an advisory that covers a large area. If you want to send us an observation or report an avalanche, contact us by sending an email to [email protected]

The next avalanche advisory will be posted on January 25th, 2008.