January 30, 2009 Avalanche Advisory

Posted Friday January 30th, 2009 at 0600.

Good morning! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for January 30th, 2009. This information is the sole responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight but you can expect avalanche conditions to remain similar unless weather conditions change significantly. Our advisory area includes the Bitterroot Mountains from Lost Trail Pass North to near Lookout Pass, the Rattlesnake Mountains and the Southern Swan and Mission Mountains near Seeley Lake. Avalanche information about the St. Regis Basin can be found on the Idaho Panhandle National Forest Avalanche Center website.

Weather and Snowpack Analysis

Most mountain locations received a few inches of new snow since Sunday with a few Bitterroot mountain locations picking up close to 15”. Temperatures started to moderate from the negative and single digits back up into the twenties on Thursday. Winds have been strong from the SW and have been moving snow as soon as it falls. Surface conditions range from several inches of blower powder in the sheltered pockets to hard scrabble where wind has scoured open terrain.

All observers are reporting the new snow is failing on the sun crust but it is not yet a significant problem. We’re seeing easy failures on this crust but they do not snap out of the pit with energy and the shear plane is rough meaning it is bonding fairly well to this hard surface. There are near-surface-facets just beneath the crust and many areas had ample opportunity for surface hoar development during the cold clear weather that ended last week. Locations near the transition from fog to clear (elevations up to 5500′) have a surface hoar layer that is now buried and needs to be watched. These are tricky and can be a big surprise after skiing all day at higher elevations with no sign of instability. Watch for this on lower terrain in drainages where the fog persisted last week or wherever there is open running water.

The factors to watch for now are how well the new snow bonds to the hard surface; how much weight will the sun crust support; and can we trust the weak faceted snow at the ground? So our worries are features close to the surface and the mature depth hoar now deeply buried at the ground in some areas. The former is more of an immediate concern than the latter but it all depends on how much weight and how quickly it is added to our current snowpack. Areas where the snow is thin are not to be trusted regardless of how much snow we get.

Current Avalanche Danger

At all advisory area locations above 5000′ the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. Unstable slabs may be found on steep terrain. Below 5000′ the avalanche danger is LOW. Natural avalanches are very unlikely, human triggered avalanches are unlikely. Dangerous avalanche conditions exist in isolated areas where the snow is thin. These are typically found on West, South and East aspects, in rocky areas or where wind has scoured the snowpack. It takes a lot of force to produce failure of the snowpack in these areas, but it fails to the ground on the faceted snow that formed in December.

Weather Forecast and Avalanche Outlook

The Missoula Office of the National Weather Service is forecasting significant snowfall in Western Montana from early Saturday through early Sunday. Wind will be a factor as the pressure gradient is expected to increase at the surface.

Expect avalanche conditions to worsen in areas that receive significant new snow and wind. Conditions Thursday were mostly safe but it won’t take much snow with high winds moving it around to bump the avalanche danger into the considerable range.

If you have any information you’d like to share or have questions about anything related to snow safety, please contact us at [email protected]

The next avalanche advisory will be posted on Groundhog Day, 2009.