The avalanche danger is rated MODERATE on slopes steeper than 35 degrees in the southern Mission and southern Swan mountains of our advisory area. Natural avalanches unlikely; human-triggered avalanches possible.
The avalanche danger is generally LOW elsewhere in the mountains of west central Montana. However, there are pockets of MODERATE avalanche danger for triggering a small wind drift on steep slopes. These wind drifts can mostly be found at higher elevations on exposed ridges.
Good Morning! This is Tim Laroche at the West Central Montana Avalanche Center with your backcountry avalanche advisory for Friday, December 16th, 2011.
Weather and Snowpack Analysis
Yesterday’s storm rolled in and out before we got a chance to really enjoy it, but it did drop 6-8 inches of new snow in the southern Missions and southern Swan mountains. Most other advisory area locations only received 1-2 inches of new snow. The winds were blowing out of the west and north at 7-10 gusting to 12. Currently, skies are mostly cloudy, mountain temperatures are in the mid and upper 20’s, and the winds are blowing out of the southwest 6-8 gusting to 12.
The storm snow that fell yesterday, fell on a mixed bag of surface conditions. There was surface hoarFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dewFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dew. Forms a persistent weak layer once buried.. Forms a persistent weak layer once buried. (photo), sun crustsA snow layer melted by radiation from the sun and subsequently refrozen., wind crusts, and recrystallizedSnow metamorphism that occurs near the snow surface when radiational heat loss forms a strong temperature gradient that allows faceted crystal growth. snow found on the snow surface throughout our advisory area due to the cold and dry weather we have been experiencing the past 3 weeks. Those kinds of weak surface conditions will not support a new load of snow and you should approach the areas that received the most snow cautiously. The new storm snow is the main avalanche concern right now. Wind drifts have likely formed on leewardWind erodes snow from the windward (upwind) side of an obstacle and deposits snow on the leeward (downwind) side. Deposited snow looks smooth and rounded. You should always beware of recent deposits of wind drifted snow on steep slopes. aspects and in gullies near ridgelines. These areas will be sensitive to a human triggerA disturbance that initiates fracture within the weak layer causing an avalanche . In 90 percent of avalanche accidents , the victim or someone in the victims party triggers the avalanche .A disturbance that initiates fracture within the weak layer causing an avalanche . In 90 percent of avalanche accidents , the victim or someone in the victims party triggers the avalanche . and should be approached with due caution. In the areas that received only 1-2 inches of new snow you may find pockets of wind drifts where you will be able to triggerA disturbance that initiates fracture within the weak layer causing an avalanche . In 90 percent of avalanche accidents , the victim or someone in the victims party triggers the avalanche .A disturbance that initiates fracture within the weak layer causing an avalanche . In 90 percent of avalanche accidents , the victim or someone in the victims party triggers the avalanche . a small slideA mass of snow sliding, tumbling, or flowing down an inclined surface. Same as avalanche.. Not a huge concern unless you are knocked off your feet and dragged through the rocks. Otherwise, the conditions are mostly stable where you can find enough snow to recreate on.
The good news is we have a mostly stable and well structured base to our snowpack (video). Although it’s shallow for this time of year, it has held its integrity thus far. This profile sent to us by one of our observers is a good example of the supportable base we have been finding on our tours (photo). So, give the new snow some time to settle out and gain strength before you commit to a steep wind-loaded slope.
We have had a couple of reports of glideWhen the entire snowpack slowly moves as a unit on the ground, similar to a glacier. crackThe spreading of a fractureThe physical separation of the slab from the bed surface and surrounding snow during the initiation of a slab avalanche. or crack within the snowpack. failures to the ground in the Bitterroot Mountains recently. Although not common this time of year, these features are hard to predict when failure may occur. Move quickly when you need to cross a slope above or below one of these features.
Weather Forecast and Avalanche Outlook
Skies should remain mostly cloudy today with a slight chance of a trace of snow. Then, back into high pressure for the weekend with temperatures climbing steadily into the mid thirties. Winds will be out of the southwest 8-10 in the afternoons. The next good shot at moisture is not until mid next week.
I expect the avalanche danger to stay the same in most advisory locations, but will increase significantly if we were to get a juicy storm.
Dudley will issue the next advisory on Friday, December 23rd. We will start issuing Monday advisories when we get more snow.
If you get out and have the time to send us some information about what you are seeing, please use our “submit observation” link on our website or send us a quick note at firstname.lastname@example.org. This information is invaluable to us and in turn comes back to you in the form of a better forecast.