March 1 Avalanche Advisory

Hello! This is Steve Karkanen at the West Central Montana Avalanche Center with backcountry avalanche information for Monday, March 1, 2010.

Current Avalanche Danger

Above 6000′ on terrain steeper than 35 degrees the avalanche danger is
MODERATE. Natural avalanches are unlikely, human triggered avalanches are possible.

Below 6000 feet, the avalanche danger is LOW, natural and human triggered avalanches are unlikely.

During periods of rapid warming or whenever it begins raining, the avalanche danger will quickly rise to CONSIDERABLE on any slope that has been exposed to the sun. Starting around 10-11am, these aspects have the most potential for wet snow avalanche activity; E to SE to S to SW to W. Under these conditions it is appropriate to retreat to either shaded or lower angle slopes and be mindful of the terrain above you. Any steep slope can quickly become dangerous shortly after it starts raining.

Weather and Snowpack Analysis

Mountain temperatures climbed into the 40’s at most SNOTEL sites in the afternoons then dropped back into the teens and lower 20’s overnight. Many sites picked up an inch or two of snow which really doesn’t matter much. We received reports from skiers in the Wisherd Ridge, Southern Bitterroots and the Rattlesnake areas all reporting low to moderate strength stability test scores associated with the buried surface hoar layer 20-30cm deep. Also the surface snow is undergoing radiation recrystallization (or temperature gradient snow) which is making for some very enjoyable riding but this snow once buried then becomes another layer to be concerned about. No problem today.

We were getting consistent stability test scores that made us a bit nervous. On a NE 35 degree aspect we scored consistent CTM12-14 Q1 failures 35cm deep, an ECTP14 Q1 failure and a Rutschblock 3 Q1. All failing on the buried surface hoar that formed the first part of February. What is noteworthy about this test site is that the snowpack is weaker today than it was Thursday. We opted not to ski this particular slope based on the above info. Other aspects were also showing the buried surface hoar failing with CTM12-14 Q1 30cm deep. South aspects have a nasty sun crust but also fail similarly.

A better illustration of our concern is a close call reported to us last night. The incident occurred on St. Mary’s Peak in the Bitterroot. The photos are posted on our photo gallery and the short report is in italics here.

Went out on Saturday to ski at St. Mary’s Peak in the Bitteroot. We had a good day out skiing and did not see any recent signs of instability or avalanche activity. However, from previous avalanche reports we knew that serious instability existed in the snowpack just a few days ago. Therefore we used caution and dug a snow pit on an 30 degree NE aspect near the ridge top. Our stability results did not suggest any serious concern, but did break easily on a small layer 10cm deep and provided a hint of what we would see lower down. We ski a NE-E aspect, 30 degree slope and had two good runs until we shifted to a N aspect almost at the bottom of the basin and triggers a small avalanche (size 2) on a slight convex slope (see attached photos). In hind sight it was shocking to see how heterogeneous the quality of the buried surface hoar layer was across the landscape. While we knew areas of instability existed the buried surface hoar layer we encountered was VERY reactive and we should of dug more pits on different aspects to really get a good idea of the stability of the entire cirque. I hope that you can pass this on to other folks. From our experience I would not say that the buried surface hoar layer on N aspects has gained anymore stability since the last avalanche cycle and would suggest people use extreme caution on N aspects.

So the hazard remains and it is certainly enough of a slab to be concerned about and worth looking for when you go out.

Weather Forecast and Avalanche Outlook

The Missoula Office of the National Weather Service is forecasting that a ridge of high pressure building over the Northern Rockies will bring drier air and clearing skies through Monday. On Monday the warmest temperatures of the winter are expected with near record highs possible. Moisture moving in ahead of a Pacific low pressure system Tuesday will usher in a cool down that will last into the latter part of the week.

That is a good and bad forecast as it relates to avalanche safety.

Here’s why: We have a persistent weak layer that continues to headline our concern. It is now deep enough (30cm) that it could easily injure, bury or kill someone. The cooler temperatures up to this week have not only preserved this layer, but actually made it weaker in many areas. Record breaking high temperatures might be just what it takes to eliminate this problem. But with the high temperatures the probability of wet snow avalanches will increase. Initially these avalanches will involve the snow currently sitting on this 30cm deep layer of buried surface hoar. As the snow melts and water percolates down the first thing it will encounter is the faceted snow and I would expect to see widespread avalanche activity as soon as this happens.

So we need the warm temperatures to stop the faceting process and get this snow to settle and bond but we may go through a period of very unstable conditions to get there.

We’ve been getting some of the best observations and photographs from skiers and riders we’ve seen since we started this web site several years ago. Your observations help us produce a more accurate advisory and may save a life. 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.