December 20, 2013 Avalanche Advisory
The avalanche danger in the west central Montana backcountry is MODERATE. It’s possible to trigger an avalanche on slopes above 7000 feet that are wind-loaded and 35 degrees and steeper. It’s also possible to trigger an avalanche on slopes above 7000 feet that have shaded, northerly aspects and are 35 degrees and steeper. On other terrain in the advisory area the avalanche danger is LOW.
Good Morning! This is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for Friday December 20, 2013. This danger rating expires at midnight tonight. (Dec. 20, 2013).
Weather and Snowpack Analysis
This morning, mountain temperatures are ranging from single digits to the mid-teens. West and northwest above 5000 feet are blowing 5 to 15mph, with gusts in the 20’s. The area received 2 to 3 inches of snow Wednesday night and Thursday morning. The small amount of snow freshened up the surface and made for some softer riding and skiing conditions. High pressure after the snow allowed surface hoar to develop on some aspects.
You should still be on the lookout for possible wind slabs on leeward slopes at exposed higher elevations. Observers near Lookout Pass found mostly stable conditions, but were wary of remnant wind slabs (PIT PROFILE).
Shaded, northerly slopes are harboring facets that formed during the cold weather. These weaknesses have been found near Lolo Pass, the central Bitterroots and in the Rattlesnakes (VIDEO) . I would be suspect of these colder slopes above 7000 feet. I would pay particular attention to slopes with shallow snow depths (2 ½ feet or less). Near Lolo Pass, Matt Young had energetic snow pit test failures higher in the snowpack on buried surface hoar; again on a northerly, cold slope (PIT PROFILE).
Weather Forecast and Avalanche Outlook
A moist air mass, of tropical origins, will interact with a cold northwest flow over central Idaho; this disturbance may produce snow in western Montana. It looks like the Idaho border (Lookout, Lolo and Lost Trail) will be the main recipient. If we get the snow predicted the avalanche danger will rise. The surface hoar formation from the last couple of nights could form a weak layer underneath this new snow.
Avalanche advisories are a great place to start assessing your risk. Advisories are a general area forecast and not a specific slope forecast. When a ski area or a highway department bombs a slope and opens the terrain or travel corridor to the public – that is a specific slope forecast.
Advisories have limitations:
-the fluctuations and randomness of the weather
-the spatial variability of the snow pack (isolated pockets of instability)
-extreme terrain (i.e. a steep couloir above 9000)
-the individual interpretation of the avalanche hazard
Bull’s eye field data trumps avalanche advisories; i.e. recent activity, collapsing of the snowpack, high winds. Use an advisory a part of your risk assessment.
Ski and ride safe!
Steve will issue the next advisory Christmas Eve, 2013.