On slopes steeper than 30 degrees and on wind-loaded slopes there is MODERATE Avalanche Danger in the west central Montana backcountry. The Avalanche Danger will rise to CONSIDERABLE during the warmest times of the day and if it rains at 5000 feet and above.
Good morning, this is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for March 23, 2012.
Weather and Snowpack Analysis
Spring officially began Tuesday, and it can be cold and snowy or warm and sunny; yesterday it was both. Since Monday the advisory area received 12 to 18 inches of snow, packing up to 4 inches of snow water equivalent. The storms mainly hit the central to northern Bitterroots along the Idaho border. Variable winds accompanied the storms and loaded 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. slopes. Yesterday afternoon winds were predominately out of the southwest. The area should pick up another inch or two of snow this morning.
Slopes throughout the advisory area had surface releases on Wednesday. At Lolo Pass, where it rained up to 5000 feet on Wednesday, some slides on the road cuts went to the ground. This activity shows the immediate, destabilizing effect rain can have on the snowpack.
Yesterday, in the Rattlesnakes, we saw surface debris on all aspects from loose snow avalanches that ran on Wednesday; and one small soft slabA relatively cohesive snowpack layer. A layer of snow stronger than underlying layers.A relatively cohesive snowpack layer. A layer of snow stronger than underlying layers. avalancheA slabA relatively cohesive snowpack layer. A layer of snow stronger than underlying layers.A relatively cohesive snowpack layer. A layer of snow stronger than underlying layers. avalanche of soft or low densityThe mass of snow per unit volume, but often expressed as a percent water content. New fallen powder has a low densityThe mass of snow per unit volume, but often expressed as a percent water content. New fallen powder has a low density (3-10%), while heavy or wet snow is more dense (10-20%). (3-10%), while heavy or wet snow is more dense (10-20%). snow. that appeared to be skier or snowboard triggered. The Snow Bowl Patrol had a soft slabA relatively cohesive snowpack layer. A layer of snow stronger than underlying layers.A relatively cohesive snowpack layer. A layer of snow stronger than underlying layers. release on a southwest exposure with their explosives yesterday morning. We posted this video that shows some of the layers from this week’s storms.
Our main concern is the storm snow from the past week and the wind-loadingThe addition of weight on top of a snowpack, usually from precipitation, wind drifting, or a person. that occurred. If you are recreating on steep slopes this weekend you should look for instabilities in the top 2 to 3 feet of the snowpack.
This week’s storm bypassed the southern part of the advisory area and things are a bit different. Near Lost Trail Pass, observers felt that wet slabsA relatively cohesive snowpack layer. A layer of snow stronger than underlying layers. or large wet sluffs during the afternoon are currently the main avalanche concerns.
Any rapid warming or rain will immediately weaken the snowpack and the large cornices that have formed this year.
Weather Forecast and Avalanche Outlook
Light snow this morning should give way to a weak high pressure lasting through Saturday and Sunday. Sunday may see the warmest temperatures of the year (possibly in the high 40’s at 7000 feet) with a return to cooler conditions Monday.
The avalanche danger will rise with the very warm temperatures predicted. Any rain on the snowpack will also elevate avalanche danger. During warm temperatures and/or rain beware of the possibility of corniceA mass of snow deposited by the wind, often overhanging, and usually near a sharp terrain break such as a ridge. Cornices can break off unexpectedly and should be approached with caution. failure.
If you head into the backcountry this weekend, as always, ski and ride safe!
Steve will issue the next scheduled advisory Monday, March 26.
As we slideA mass of snow sliding, tumbling, or flowing down an inclined surface. Same as avalanche. toward the end of another great skiing & riding season; I would like to extend thanks to all the folks who posted public observations or sent us reports and information. We have a large forecast area and it helps to hear if you missed something or get a confirmation of the forecast. I believe everyone who uses the advisory appreciates those contributions.