The avalanche danger in the west central Montana backcountry is MODERATE. New snow and strong southwest winds have created areas of unstable snow in our advisory area.

Good Morning! This is Tim Laroche with the West Central Montana Avalanche Center’s backcountry avalanche advisory for Tuesday, January 8th, 2013.

Weather and Snowpack Analysis

The storm that rolled in yesterday left 3-9 inches(.4-1.3” SWE) of new snow and produced winds in the 15-20mph range out of the southwest. The southern Mission and Swan ranges received the higher amounts of snowfall with the southern Bitterroots receiving the lesser amounts. Currently mountain temperatures are in the high 20’s, winds are blowing steadily from 10-20mph out of the southwest, and bands of snow are tracking through our forecast area.

We have been enjoying some fantastic snow days in the mountains this past week on a mostly stable snowpack(profile). Now the weather is changing and we are headed into another welcomed storm cycle. Last week’s weather left behind a myriad of weak snow surfaces including 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., near surface facetsAngular snow with poor bonding created from large temperature gradients within the snowpack., sun crustsA snow layer melted by radiation from the sun and subsequently refrozen., wind crusts, and a thin freezing rain crustA crust is a hard layer of snow where liquid water has refrozen into grain fabric. Crusts usually result from sun, rain or wind.A Rain CrustA crust is a hard layer of snow where liquid water has refrozen into grain fabric. Crusts usually result from sun, rain or wind. is formed by the freezing of infiltrated rain water. It can be thin (from a glaze to a few millimeters thick) or if there is hard rain there may be more of an ice surface than a distinct crustA crust is a hard layer of snow where liquid water has refrozen into grain fabric. Crusts usually result from sun, rain or wind..A clear layer of ice formed when rain falls on the snow surface then freezes. in some locations. It is important to pay particular attention to how the new snow comes in and how it will react sitting on the old snow surfaces.

The primary avalanche concern is fresh wind slabsA relatively cohesive snowpack layer. A layer of snow stronger than underlying layers.A cohesive layer of snow formed when wind deposits snow onto 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. terrain. Wind slabsA relatively cohesive snowpack layer. A layer of snow stronger than underlying layers. are often smooth and rounded and sometimes sound hollow.A cohesive layer of snow formed when wind deposits snow onto 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. terrain. Wind slabsA relatively cohesive snowpack layer. A layer of snow stronger than underlying layers.A cohesive layer of snow formed when wind deposits snow onto 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. terrain. Wind slabsA relatively cohesive snowpack layer. A layer of snow stronger than underlying layers. are often smooth and rounded and sometimes sound hollow. are often smooth and rounded and sometimes sound hollow. that have 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 cross-loaded gullies. The past week’s weather helped continue to settle and strengthen our snowpack, but it created a weak snow surface in many locations. Strong southwest winds have created fresh drifts 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. terrain and will be sensitive today. Yesterday, we observed weak faceted snowAngular snow with poor bonding created from large temperature gradients within the snowpack. getting buried on an east aspectThe compass direction a slope faces i.e. North, South, East, West.. This was not a major concern at the time, but will be if it is buried by a fresh drift or storm 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.. Pay attention to how the snow is reacting. Watch for shooting cracks and whumpfingWhumpf has actually been adopted as a technical avalanche term to describe the sound of a collapsingWhumpf has actually been adopted as a technical avalanche term to describe the sound of a collapsing snowpack when you cross the snow. A sign of instability. snowpack when you cross the snow. A sign of instability. for sure signs of instability.

The secondary avalanche concern will be storm 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. avalanches on all aspects as we continue to receive more snow. If you are recreating in the southern Mission and Swan ranges this will be something to pay attention to today. As other areas receive more snow this will become a concern. Dig down below the storm snow to see how it is bonding to the old snow surface before you commit to a steep slope. Pay attention to obvious clues of instability like recent avalanche activity, periods of intense precipitation, strong winds transporting snow, or the feel of a “slabby” snowpack. Move off and out from under steep terrain until the storm snow has had time to settle.

Weather Forecast and Avalanche Outlook

Today a moist and warm weather system is set to impact our advisory area. Forecasts are calling for 1-3 inches of new snow today and another 2-5 inches tonight with continued precipitation through Thursday. Temperatures will be in the low to mid thirties, winds will be out of the southwest between 15 and 20mph and snow levels around 5000’.

Continued precipitation, warm temperatures, and strong winds will cause the avalanche danger to increase over the next 24 hours.

I will issue the next advisory on Friday, January 11^th.

If you get out and have the time to send us some information about what you are seeing, please use our “public observations” link on our website or send us a quick note at info@missoulaavalanche.org. Thank you for your continued support!