December 24, 2010 Avalanche Advisory

Good Morning! This is Tim Laroche at the West Central Montana Avalanche Center with the backcountry avalanche advisory for Christmas Eve 2010.

Current Avalanche Danger

Above 6000 feet on all terrain 35 degrees and steeper the avalanche danger is Moderate, natural avalanches are unlikely and human-triggered avalanches are possible. On all other terrain above and below 6000 feet, the avalanche danger is Low, natural and human-triggered avalanches are unlikely.

Weather and Snowpack Analysis

The relatively benign and stable weather pattern that has been with us the past few days has allowed the snow pack to strengthen. At the same time, the cool clear nights have enabled a new crop of surface hoar to develop on shady slopes and melt-freeze crusts to form on sunny aspects. This will be something to investigate when we get our next good dump of snow.

There are currently two main concerns within our snow pack.  First, is the rain crust that has formed up to an elevation of about 6600 feet. Second, is the persistent buried surface hoar layer that formed on December 6 and has strengthened over the past week, but remains a concern in high elevation steep terrain. This weak layer is now buried from 40 to 60cm deep and is still moderately reactive in places.

We toured the Southern Missions on Wednesday and found the rain crust that developed just after the December 14th rain event (photo). The 35cm slab that sits on top of this crust was reactive to stability tests in the moderate range on a North aspect at 6600 feet(CT13 and ECTP15) and came off as a clean shear (Q1). This rain crust is evident on all aspects up to the 6600 foot elevation. Below 6000 feet, we have not received much snow in the past week and therefore, the avalanche danger is significantly reduced. The snow that we did get in some areas below 6000 feet has bonded well to the old snow.

During our tour of the Rattlesnakes on Thursday, we dug 3 separate pits on a North, East, and West aspect above 7000 feet. The buried surface hoar that we have been talking about the past week is present on all of those aspects and is showing signs of strengthening (video). Although this weak layer has gained strength during the past week, the hoar crystals, which are easily visible are in the size range of 5-7mm and so should remain suspect on steep slopes above 6000 feet. Observers in the Southern and Northern Bitterroot, as well as the Swan mountains were also able to see the buried surface hoar layer and found it to react to stability tests in the difficult range. They all reported mostly stable skiing and riding conditions.

Weather Forecast and Avalanche Outlook

High pressure will grow stronger across the region today effectively causing valley temperature inversions to become more pronounced. Most locations have experienced good radiational cooling overnight which will factor into keeping this afternoon’s temperatures quite cool. However conditions will warm quite nicely in the mountains under the building high pressure ridge. The models continue to indicate a significant feature moving into the Northern Rockies on Sunday that could bring widespread precipitation and effectively put us into a more active and wet weather pattern.

Given this weather forecast, avalanche conditions should continue to improve until the low pressure system starts to affect our area on Sunday.

The next advisory will be issued Monday December 27th.