March 7, 2008 Avalanche Advisory

Weekend Avalanche Advisory
Posted March 7th at 0600

Good morning! This is Steve Karkanen at the West Central Montana Avalanche Center with the weekend avalanche advisory for March 7th – 9th, 2008. The avalanche danger rating expires at midnight March 7th. This report is based on field observations and data collected on Thursday and describes conditions seen at that time. This advisory is the sole responsibility of the Forest Service and does not apply to operating ski areas.

Weather and Snowpack Analysis

West Central Montana mountain locations received an average of 2.25” of snow water equivalent since last Friday (2/29). North Fork Jocko, Stuart Peak and Twin Lakes SNOTEL stations picked up about 25” of new snow most of which came in hard on Tuesday and early Wednesday. Other Bitterroot SNOTEL sites received 12” of snow at Lost Trail and Lolo passes to closer to 18”-20” at Hoodoo and Lookout passes. This new snow is very dense or heavier and wetter than earlier storms we’ve seen this winter and was deposited onto a firm snow surface that formed during warm sunny weather 2 weeks ago. There are also many areas where surface hoar crystals formed on this snow surface which was subsequently buried by a few inches of snow. We have also seen graupel deposited onto this old snow surface during last weekend’s snow.

On Thursday, temperatures moderated, the sky cleared and the recent storm snow began to settle out. We received several excellent reports this week from skiers in many locations in the Bitterroot who noted weakness associated with either buried surface hoar or faceted snow on or just under the late February crusts. Many of these sites were getting very low stability scores prior to the snow that came in on the 4th and many of these backcountry skiers and riders wisely backed away from tempting powder shots. These reports are significant in that many of these locations now have a much deeper slab. A good example of what we’re seeing in the Rattlesnake and hearing about in other locations can be seen on YouTube here:

This particular video shows how easily and quickly a small slab of dense snow can pull someone off balance and send them tumbling down the hill. RB2 Q1.

The Rattlesnake snowfall was a big surprise for everyone Wednesday. As in most other locations, this new snow has not yet had the time needed to adjust and bond to the old snow surface. Another day or two should be enough time needed to stabilize the condition we saw Thursday but clear sky conditions and warm air temperatures will increase the danger of wet snow avalanches on any steep, sun-exposed terrain.

Today’s Avalanche Danger

In the Rattlesnake Mountains North of Missoula, the Southern Mission Mountains and the Southern Swan Range near Seeley Lake, MT, the avalanche danger is CONSIDERABLE above 6000′ on all North to Southeast aspects steeper than 35 degrees or on any wind loaded or cross loaded slope steeper than 35 degrees. On other aspects above 6000′ the avalanche danger is MODERATE. Below 6000′ the avalanche danger is LOW.

In the Bitterroot Mountains from Lost Trail Pass North to Lookout Pass the avalanche danger is CONSIDERABLE above 5000′ on all North to Southeast aspects steeper than 35 degrees or on any wind loaded or cross loaded slope steeper than 35 degrees. On other aspects above 5000′ the avalanche danger is MODERATE. Below 5000′ the avalanche danger is LOW.

CONSIDERABLE avalanche danger means natural avalanches are possible and human triggered avalanches are probable. Unstable slabs are probable on steep terrain so be increasingly cautious on steeper terrain.

MODERATE avalanche danger means natural avalanches are unlikely and human triggered avalanches are possible. Unstable slabs are possible on steep terrain.

Weather Forecast and Avalanche Outlook

A weak upper level shortwave weather system Friday will be followed by a stronger Pacific system Friday night and Saturday. This second system is expected to bring only light snow accumulation through Saturday evening. A ridge of high pressure will then build for the remainder of the weekend bringing warming and dry conditions through Monday when another Pacific system enters the area with warm moist air increasing the chance for rain in the valleys and snow showers in the mountains.

As mentioned above, the current instability will need a bit more time to settle out and glue itself to the hard surfaces it came in on. Moderate temperatures will certainly help speed this process but if we see temperatures quickly rise above freezing, wet snow avalanche danger will also quickly rise.

The next avalanche advisory will be posted on March 14th, 2008. If you are seeking further information please contact the local Forest Service Ranger District Office in the area where you wish to recreate or call us at 406-329-3752.