March 13 Avalanche Advisory
Good morning. This is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for March 13, 2009. This information is the responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight, but the advisory can provide valuable information for evaluating avalanche hazard for the next 48 hours. The advisory area includes the Bitterroot Mountains from Lost Trail Pass to Lookout Pass, the Rattlesnake Mountains and the Southern Swan and Mission Mountains near Seeley Lake. Detailed avalanche information about the St. Regis Basin can be found on the Idaho Panhandle National Forest Avalanche Center website.
Weather and Snowpack Analysis
Our region received modest amounts of snow since Monday morning. The SNOTEL site at Lolo Pass indicated 4.5 inches of snow Monday and Tuesday, other data showed very little snow elsewhere. We did have lots of wind from different directions and frigid temperatures. The cold light-density snow from Sunday through Tuesday was moved around by the wind and left wind slabs up above timberline. During field observations yesterday small skier-triggered slabs were noted as well as skier-triggered sluffing. The wind has also helped solidify the newer snow and, other than the wind slab formation and sluffing, reports from observers are indicating generally stable conditions.
On Northerly aspects we are still seeing a layer of surface hoar that formed the first week of February in some stability tests. This layer has about 30 inches of snow on it and continues to shear cleanly, but it takes a lot of force. Observers from Lost Trail Pass have been observing this layer for weeks and yesterday noted that it was not reacting to stability tests on Northwest slopes.
Right now, the main consideration is wind slabs that have formed above timberline on steep exposed slopes. These wind slabs can be found on the lee sides of ridge-tops and cross loaded in gullies or couloirs.
Current Avalanche Danger
On all slopes above timberline 35 degrees or steeper that are wind-loaded the avalanche danger is MODERATE. Natural avalanches are unlikely and human triggered avalanches are possible. Elsewhere in the region the avalanche danger is LOW. Natural and human triggered avalanches are unlikely.
Do not let our recent string of generally stable conditions lull you into complacency. Continue to practice good travel habits. Go one at a time, skiing or high-marking. Communicate with your travel companions and set safe routes and traverses when negotiating steep terrain.
Weather Forecast and Avalanche Outlook
It looks like a snow-producing system will move in late Saturday and persist through Sunday. The system may favor the Bitterroots with up to 5 inches of snow. Moderate winds are expected on Sunday.
Although we have already experienced some spring-like days this winter it’s time to start treating the backcountry snow in a late-season manner. Be aware of rapidly warming temperatures on South and West slopes in the afternoon as March progresses. The snow becomes more sensitive when it warms up quickly.
If you have snow observations or any snow information you’d like to share please contact us at [email protected].
The next avalanche advisory will be issued on Monday, March 16, 2009.
The avalanche danger rating scale is being revised. The opinion from folks who use the advisories is important. Please take time to fill out the avalanche danger rating survey.