February 22, 2010 Avalanche Advisory

Hello! This is Steve Karkanen at the West Central Montana Avalanche Center with backcountry avalanche information for Monday, February 22, 2010.

Current Avalanche Danger

Above 6000 feet on all terrain steeper than 35 degrees the avalanche danger is CONSIDERABLE, natural avalanches are possible and human triggered avalanches probable.

On other slopes the avalanche danger is MODERATE. Natural avalanches are unlikely, human triggered avalanches are possible. Below 6000 feet, the avalanche danger is LOW, natural and human triggered avalanches are unlikely.

Be especially vigilant during periods of rapid warming. Many wet snow point release avalanches occur during periods when the sun bakes a slope for a few minutes. While these smaller avalanches by themselves do not present much of a hazard, it is possible the rapid load to a weak snow structure can result in these slides stepping down to weaker layers within the snowpack.

Weather and Snowpack Analysis

Cool temperatures have allowed the buried surface hoar layers to persist and in some cases, grow larger. We continue to receive excellent reports from backcountry travelers who have either triggered avalanches or have seen recent avalanche activity. The most recent report was from a group who triggered an avalanche on a north facing slope near St. Mary’s Peak Saturday.

In the Rattlesnake Sunday our stability tests were consistent on all aspects with moderate range compression test scores but high quality fast clean energetic shear planes failing at the buried surface hoar about 25cm deep (CTM11, Q1). South and West aspects appear to be the most sensitive and are showing the ability to propagate fractures over distance (ECTP11 25cm deep). Wind loaded terrain may be especially troublesome especially in the higher terrain in the Bitterroot Mountains. Observations from the northern Bitterroots near Hoodoo Pass and further south near Lost Horse at elevations above 7000 feet show similar instability but a much deeper (40cm) slab.

Lower elevation sites such as Lolo and Lookout Passes have a much more stable snowpack and the surface hoar that is troublesome at most other sites was mostly destroyed by warmer temperatures prior to last week’s snow. The only way to know for sure is to dig down into the snow a couple of feet to see if it exists. The surface hoar crystals are well preserved and large so they won’t be hard to miss in a quick pit.

We watched many people ski 30-35 degree and some steeper terrain without incident so conditions are slowly improving over what we experienced a week ago.

Weather Forecast and Avalanche Outlook

The upper level pattern will transition from a northerly flow to a ridge Monday night and Tuesday. The air mass will be dry with mostly clear skies. Night time and early morning temperatures will be cold due to radiational cooling. Temperatures will begin to slowly warm in the higher elevations as an inversion sets up keeping valley temperatures cool. By Wednesday, most forecast models indicate a breakdown of the high pressure ridge with a moist westerly flow moving into western Montana.

The cooler temperatures will allow the weak layers to persist and the cold clear nights will allow a new layer of surface facets to start growing. Expect conditions to slowly improve as temperatures moderate. The next storm cycle will be potentially very dangerous if it comes in quick, warm and with a lot of precipitation.

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Avalanche Center Survey

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.