January 15, 2010 Avalanche Advisory

Good Morning! This is Steve Karkanen at the West Central Montana Avalanche Center with backcountry avalanche information for Friday, January 15, 2010.

Current Avalanche Danger

On slopes steeper than 35 degrees and on all aspects at elevations above 5000 feet, the avalanche danger is MODERATE. Natural avalanches are unlikely, human triggered avalanches are possible.

Below 5000 feet, the avalanche danger is LOW.

Weather and Snowpack Analysis

For the past five days afternoon temperatures at all SNOTEL sites reached into the upper 30’s and many locations even saw a bit of sun for a few hours.

Our concern all winter has been the depth hoar or facets at the base of our snowpack that formed during the December deep freeze and later buried and sort of preserved/insulated from later moderate temperatures. The more recent concern is a surface hoar layer that formed over the holidays. Many avalanches ran during the last heavy precipitation event January 4-5 and most of these involved the upper layer of buried surface hoar.

The lack of new snow and moderate temperatures this past week have not done much to improve the skiing and riding conditions but it has sped up the settlement process which strengthens these weaknesses we’ve been describing.

All observers are talking about failures at the level of the buried surface hoar, about 10 to 15 inches deep, but it is taking a lot of force to get this layer to fail. Compression test scores at this level were in the moderate to hard failure range. Some test sites display the potential for this layer to propagate across a slope but much stronger than reported earlier. Other sites are showing a very strong condition.

A good example of this variability is the results observers at Lookout Pass and in the Rattlesnake were getting with the Rutschblock test.

Rattlesnake: RB score 6, Q1 failure at 35cm on the buried facets (depth hoar) above an old melt-freeze crust.

Lookout Pass: RB score 3, Q1 failure at 70cm on the buried surface hoar.

This surface hoar layer is widespread throughout the region but the Lookout Pass test area at 5800 feet has remained cooler than the higher elevation site in the Rattlesnake at 7200 feet. Cooler temperatures help prolong persistence of weak layers. In the case of the Rattlesnake test site, the warmer temperatures have settled the snowpack quicker than normal for this time of year so the surface hoar layer had a good opportunity to gain strength during the warm spell.

We believe the super weak facets at the ground (50cm at the end of December) have collapsed in many areas and are not as much of a stability factor as in our earlier observations. We see about 20cm of facets near the ground where we were seeing 50cm 3 weeks ago. A very good sign of strengthening but when a test area failed, it failed with a lot of energy to the ground and propagated across the slope. South aspects are more suspect than other aspects. We see easier failures at the level of both features on south aspects or any slope where the snow is shallow or rocky.

Take home message: Never trust a layer of faceted snow.

Weather Forecast and Avalanche Outlook

A weak ridge of high pressure will build over the area Friday. Moisture streaming into northwest Montana may cause showers Friday night into Saturday. A mid-level jet stream will develop over the area late Friday causing extremely strong westerly wind at higher elevations near the continental divide. Expect little precipitation for the next 48 hours with snow levels near 5000 feet.

Continued milder conditions will strengthen the overall snow structure but we are not completely free from seeing larger slabs bust out under the right amount of force.

We rely heavily on the many excellent observations people are sending us this winter. They help us produce a more accurate avalanche forecast since our resources are limited. Special thanks to everyone who sent us quick emails, left messages or talked to us in the backcountry during our travels to let us know what they were seeing during the period when conditions were sketchy over the holidays. Your information may save a life some day.

If you get out and see avalanche activity or want to send us quick snow observations, send us a note at [email protected] or call us at 406-530-9766. 530-9SNO.

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