February 21 Special Update
This is Steve Karkanen with a special update for the northern portions of the advisory area issued at 1230 February 21.
The Rattlesnake, southern Swan and southern Mission Mountains above 5000 feet now have HIGH avalanche danger. The northern mountains are receiving heavy snowfall and high winds. The ski patrol at Snowbowl is reporting that ski cuts are producing dangerous slab avalanches and that the overall snowpack is now very sensitive to disturbance. North Fork Jocko SNOTEL is reporting nearly an inch and a half of SWE which is a more than enough to stress the snowpack.
Temperatures are warm this morning so the immediate concern is the upside down nature of the snow with heavy wet snow on top of a layer of dryer, lighter snow as well as a more deeply buried layer of surface hoar crystals.
We will continue to monitor the situation and will update this information as conditions change.
In the Bitterroot mountains, the avalanche danger remains MODERATE but will change to HIGH when the area begins to receive heavy snow.
Weather and Snowpack Analysis
The mountains in West Central Montana have been steadily picking up snow since Friday morning. The amount of snow over the weekend varies from up to 10 inches in the Southern Missions and Southern Swans to a couple of inches in the southern part of the advisory area. Snotels indicate the new snow in the Southern Missions and Southern Swans is settling rapidly; but the snow will be more sensitive in the northern part of the advisory area due to the increased load.
I toured near Lolo Pass yesterday and found the buried surface hoar from early February underneath 30cm (1 foot) of snow. The snow was failing with moderate strength but very low energy on northeast and southeast slopes at 6300 feet in elevation. Steve got the same results with some test pits in the Rattlesnakes yesterday.
This is a bit if a tricky layer; you can plainly see the surface hoar in the snowpits but it is not reacting as a slab in many tests from Thursday and Sunday. On the other hand, we talked to three skiers who either got tumbled or pushed around from large sluffs running on this layer. We got a report from a fourth skier who had to manage large sluffs while skiing in the Southern Bitterroots. I don’t trust this layer; I am looking for it on any steep slope I might want to ski.
We got a great report from a snowmobiler who was partially buried by a 16 inch slab that ran on the buried surface hoar. This incident was just out of our advisory area (Northern Missions), but should serve as a warning as we get more of a load on the layer. These riders were reporting natural and triggered slabs on Saturday. I posted a couple of pictures of this slide; you can find them at the bottom of our 2012 season folder.
I wrote about conditions changing from our previous period of Low avalanche danger on Friday. All steep slopes are suspect in the advisory area.
Weather Forecast and Avalanche Outlook
It looks like things are about to change, big time. Snow showers should continue through tonight with little or no accumulation; then…La Nina returns with a vengeance. A winter storm is expected to move across the Northern Rockies beginning Tuesday and produce 1 to 2 feet of snow at the higher elevations. Snow levels should rise above 4000 feet; this may mean heavier, wetter snow contributing to severe loading of the snowpack. By Wednesday there may be winds gusting to 60mph; massively loading leeward slopes. I would expect the avalanche danger to bust through CONSIDERABLE and go right to HIGH during this period. You probably won’t recreate with 60mph winds but, it’s going to take some time for the snow to settle down if this storm comes to pass and deposits its load on our existing weak surface.
It’s time for my mid-winter rant about backcountry (I should say sidecountry) behavior. I was inspired Saturday when I watch three people ski an avalanche chute just outside the Snowbowl boundary together……at the same time. This particular slope had a large skier-triggered avalanche on it this year. I doubt these three riders read the avalanche advisory, so I’m aiming my comments at Snow Bowl parents who may have kids, teenagers or older, ducking the ropes or skiing out-of-bounds. Once you leave the ski area; you’re on your own. The areas just outside the ropes are backcountry. There is no slope management or patrol. You should be prepared to deal with a burial or trauma. It would be a major operation to extract someone from the Rankin Lake basin just outside Snowbowl. Folks are treating this terrain as part of the ski area. It’s not. Do you think people who would ski three at a time down an avalanche chute know when the snow is stable and when it isn’t?
We just had a terrible incident in the sidecountry near Stevens Pass, Washington. Bozeman experienced a major avalanche on Saddle Peak (where everyone hikes) just outside Bridger’s boundary a couple of seasons ago (fortunately no one was hurt). Everyone wants to ski the gnarly terrain; the sidecountry is popular. Have you talked to your kids about drugs, avalanches and skiing out-of-bounds?
Check out this video from the Gallatin Avalanche Center. http://www.mtavalanche.com/sidecountry