SW Montana Avalanche Bulletin

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This bulletin is from the Gallatin National Forest Avalanche Center - [Map]

Gallatin National Forest Avalanche Advisory

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Keywords: avalanche danger; cornice; cornices; crown; facets; rescue gear; slab; snowpits; stress; stresses; weak layers; wind-loaded;

This avalanche information bulletin is issued on April 11, 2011 and does not expire. The Gallatin National Forest Avalanche Center has stopped issuing avalanche advisories for the season. Traveling in the backcountry requires carefully snowpack evaluation on the slopes you intend to ride or ski.

Mountain Weather:

Snowpack and Avalanche Discussion:

Some avalanche concerns to keep in mind: 1. WIDESPREAD INSTABILITY The greatest avalanche concern is new snowfall and wind- loading. As of April 11 the mountains do not have widespread weak layers buried deep in the snowpack. The likelihood of triggering avalanches spikes during and immediately after significant snow storms. Additionally, wind-loaded slopes are dangerous too. The good news is that any instability associated with the new and/or windblown snow can easily be found in the upper few feet of the snowpack. Instabilities associated with new snowfall will typically be short lived.

2. LOCALIZED INSTABILITY Deep, slab avalanches are still a possibility on localized terrain. The big snowfalls in March and the beginning of April loaded slopes with many feet of snow. These big loads put enormous stress on slopes and revealed weak(er) snowpacks. Large avalanches broke on deep layers of facets that formed months ago. On most slopes these weakness did not survive, but on a few they did. A skier triggered a slide in the Lionhead area on April 10 that broke on buried facets ( photo1, photo 2)--a reminder that even during times of relative stability, there are still slopes that will avalanche.

3. CORNICES Cornices this year are some of the biggest I've seen. These tractor trailer sized overhangs of snow are dangerous for two reasons. First, they break away from ridge much further back than anticipated. Getting launched down a slope with tumbling cornice blocks will end in injury at the very least. Second, these backcountry bombs place huge, sudden stresses on the snowpack. They are one of Mother Nature's most effective triggers. Large, deep avalanches, even in relatively strong snowpacks, can be released from falling cornices. Give them a wide berth.

4. WET SNOW AVALANCHES Spring is synonymous with wet avalanches. As the sun climbs higher in the sky and daytime air temperatures stay above freezing, wet avalanche activity increases. More importantly, above freezing temperatures at night add to the danger and can create unstable conditions. South facing slopes get the brunt of incoming solar radiation, but the warming swings around to all aspects as spring progresses. Be aware that sunny aspects may have a wet snow danger while shadier slopes still have a dry snow avalanche danger. Pinwheels and large rollers of snow are a sign of increasing avalanche danger. Punching to the ground in wet, unsupportable snow is another bad sign. Wet avalanches, whether loose snow or slab, can be destructive and powerful.

5. DISCLAIMER Yes, there's always a disclaimer. If snowfall continues there's a very real possibility that new weak layers will be buried created lingering, and possibly widespread instabilities. Always assess the slope you're about to play on with diligence. Do not let your guard down. And always travel with a partner, carry rescue gear and only expose one person at a time to avalanche terrain. We live by these rules in the winter and they still apply in the spring.

Have a safe and enjoyable spring and summer! Doug Chabot Mark Staples Eric Knoff Audio: Audio for Apr 11, 2011 » * Printer-friendly version New Danger Scale The new North American Public Avalanche Danger Scale. Click on the image for a larger version.

[DangerScale.jpg] Photos and Video Lionhead Avalanche Crown Mon. Apr. 11, 2011 Lionhead Avalanche Crown This avalanche was triggered by a skier on a thin spot (24") of...

[view image, read more] Lionhead Avalanche Mon. Apr. 11, 2011 Lionhead Avalanche This avalanche was triggered from the ridge. It broke 3-5 feet...

16. http://www.wcc.nrcs.usda.gov/nwcc/site?sitenum=609&state=mt 17. http://www.wcc.nrcs.usda.gov/nwcc/site?sitenum=609&state=mt 18. http://www.wcc.nrcs.usda.gov/nwcc/site?sitenum=858&state=mt 19. http://www.wcc.nrcs.usda.gov/nwcc/site?sitenum=480&state=mt 20. http://www.wcc.nrcs.usda.gov/nwcc/site?sitenum=480&state=mt 21. http://www.youtube.com/user/AvalancheGuys .

50. http://www.facebook.com/pages/Bozeman-MT/Friends-of-the-Gallatin-National-Forest-Avalanche-Center/173768210029?ref=mf .

52. http://snowpilot.org/ .

59. http://www.avalanche.org/ .

63. http://twitter.com/AvalancheGuys 64. http://www.facebook.com/pages/Bozeman-MT/Friends-of-the-Gallatin-National-Forest-Avalanche-Center/173768210029?ref=ts .

66. http://www.mailermailer.com/x?oid=1004149j .

70. http://www.mailermailer.com/x?oid=1004149j 71. javascript:openheaderlink('gnfac-map-lorez', 'height = 750, width =770' ) 72. javascript:openheaderlink('Avalanche-Danger-Scale', 'height = 655, width =1010' ) 73. javascript:openglossary('Loading', 'height = 400, width =800' ) 74. javascript:openglossary('Weak Layer', 'height = 400, width =800' ) 75. javascript:openglossary('Trigger', 'height = 400, width =800' ) 76. javascript:openglossary('Wind Loading', 'height = 400, width =800' ) 77. javascript:openglossary('Slab', 'height = 400, width =800' ) 78. javascript:openglossary('Trigger', 'height = 400, width =800' ) 79. javascript:openglossary('Slide', 'height = 400, width =800' ) .

82. javascript:openglossary('Stability', 'height = 400, width =800' ) 83. javascript:openglossary('Cornice', 'height = 400, width =800' ) 84. javascript:openglossary('Cornice', 'height = 400, width =800' ) 85. javascript:openglossary('Trigger', 'height = 400, width =800' ) 86. javascript:openglossary('Cornice', 'height = 400, width =800' ) 87. javascript:openglossary('Aspect', 'height = 400, width =800' ) 88. javascript:openglossary('Aspect', 'height = 400, width =800' ) 89. javascript:openglossary('Dry Snow Avalanche', 'height = 400, width =800' ) 90. javascript:openglossary('Slab', 'height = 400, width =800' ) 91. javascript:openglossary('Weak Layer', 'height = 400, width =800' ) .

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