March 27 Avalanche Advisory
Posted Friday March 27th, 2009 at 0600.
Hello! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for March 27th, 2009. 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 but you can use the information we provide below to help you make more informed decisions regarding travel in avalanche terrain for the next few days. Our advisory area includes the Bitterroot Mountains from Lost Trail Pass North to near Lookout Pass, the Rattlesnake Mountains and the Southern Swan and Mission Mountains near Seeley Lake. Avalanche information about the St. Regis Basin can be found on the Idaho Panhandle NF Avalanche Center website.
Weather and Snowpack Analysis
Last week we talked about how the warm sunny spring weather was affecting the snowpack, this week we’ll backtrack and talk about what a winter storm has done. All SNOTEL sites picked up several inches of moisture starting out as rain last Sunday then turning to snow in abundant quantity by mid-week. Yesterday conditions were more like the end of January with many sites receiving 10-20 inches of new snow the previous 48 hours. Northern locations appeared to be the biggest winners with the Rattlesnake picking up 2.5 inches of SWE (snow water equivalent) in the past week and the Bitterroot Mountains about half as much. This added a lot of weight to what has been a mostly stable snowpack. In some areas, not enough time has passed to allow the snow to adjust to this new load. stability tests were showing mostly stable conditions in the Bitterroot Mountains and somewhat disconcerting conditions in the Rattlesnake.
In the Rattlesnake, we saw several natural slab and sluff avalanches that released during the storm Wednesday night on nearly all aspects. The recent storm winds came from many different directions and all slopes seemed to be treated equally this time around. While touring and skiing we triggered other small slab and loose snow avalanches on all aspects. Anything steeper than 40 degrees was touchy. If you don’t have strong skiing and slope assessment skills, it’s not a good time to blindly jump into steep terrain. I would expect this condition to persist for the next 24-48 hours or until weather conditions change. The primary concern now is the bond between the new snow and the hard crust that formed earlier in the week. Wind loaded pockets are particularly sensitive. The secondary concern is the weak layer we talked about last week that is now about 85cm or 35 inches deep. Though it takes a lot of force to get it to fail, it fails cleanly with energy and propagates over distance. The culprit is small grained faceted snow that formed on or below a sun crust that formed in late February. I simply wouldn’t trust steep shallow areas, rock outcrops, convex slopes or any potential thin spot where it’s possible to initiate an avalanche. This type of instability is hard to assess and can lead you into thinking a slope is safe based on high stability scores. And if a slope does fail at that level, it’s going to be nasty.
Current Avalanche Danger
Rattlesnake, Southern Swan and Southern Mission Mountains near Seeley Lake: Above 6000 feet, on wind loaded terrain steeper than 35 degrees, the avalanche danger is CONSIDERABLE. Human triggered avalanches are probable, natural avalanches are possible. On all other slopes above 6000 feet the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. Below 6000 feet the avalanche danger is LOW, natural avalanches are very unlikely and human triggered avalanches are unlikely.
Bitterroot Range from Lost Trail Pass to near Lookout Pass:
Above 6000 feet the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. Below 6000 feet the avalanche danger is LOW, natural avalanches are very unlikely and human triggered avalanches are unlikely. Please consider that there is a great deal of local variation in snowfall amounts. Even under low to moderate avalanche conditions, you can find isolated pockets of unstable snow on some terrain features.
Weather Forecast and Avalanche Outlook
A high pressure ridge brought clear skies and cold temperatures to the area Thursday and Friday. Expect warmer temperatures and higher snow levels Friday and Saturday. The next winter weather disturbance is expected to impact mountain locations Saturday night and Sunday with snow reaching valley floors by Sunday morning. Weather models predict significant moisture associated with this fast moving system but the timing is uncertain. The upcoming week will remain moist with below average temperatures under a Northwesterly flow aloft.
Expect avalanche danger to improve during the next few days as the snow slowly adjusts to the recent heavy load. During periods of clear weather, pay close attention to how the sun is affecting the snow surface. The sun angle is now high enough that just a few minutes of direct sunlight can warm a slope rapidly enough to bring it down.
We have received many excellent reports from backcountry snowmobilers and skiers this winter. These informal observations are very valuable to us in that they help us produce a more accurate avalanche advisory. The information you provide may save a life. Many thanks to everyone who sent observations or a quick email to us this winter!
If you have any information you’d like to share or have questions about anything related to snow safety, please contact us at [email protected].
We will no longer issue regular avalanche advisories on Mondays and Fridays. We will post information statements as needed after April 1st.
