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GNFAC Avalanche Advisory for Thu Apr 8, 2010

Not Current Advisory

Good Morning. This is Mark Staples with the Gallatin National Forest Avalanche Advisory issued on Thursday, April 8, at 7:30 a.m.  ProLite Gear, in cooperation with the Friends of the Avalanche Center, sponsors today's advisory.  This advisory does not apply to operating ski areas.  Bridger Bowl is now closed for the season and backcountry conditions exist.

Mountain Weather: 

This morning temperatures were in the low 20s F with SW winds blowing 10-25 mph.  The ridge responsible for yesterday's sunny weather is flattening and an approaching cold front will bring increased winds later this morning and isolated snow showers.  Winds will increase to 20-40 mph, and high temperatures shouldn't climb above 32 degrees.  This cold front is moving fast and doesn't contain much moisture.  In some areas snowfall may be intense but short lived and only 1-3 inches of snow will fall by tomorrow morning.

Snowpack and Avalanche Discussion: 

The Bridger, Gallatin and Madison Ranges, the Lionhead area near West Yellowstone, the mountains around Cooke City and the Washburn Range:

The snowpack contains a layer of weak faceted snow buried 1-3 ft deep under the snow from several recent storms.  Over the weekend this layer produced many avalanches (summary in Tuesday's advisory) that demonstrated its ability to propagate fractures over long distances and produce large avalanches.  In some cases avalanches initiated in wind slabs but quickly stepped down to this layer.  Wind-blown snow from continued strong winds will keep this layer sensitive.  It can be found on most slopes but is weakest on northerly facing ones where the best skiing and riding conditions exist.

Evaluating the strength of this layer will require multiple stability tests.  A single test or quick hand pit may yield false stable results.  This means the test indicates stable conditions when unstable conditions actually exist.  For more information on false stable results, read this paper by Doug and Karl or watch the presentation.  Basically dig more, well placed pits and keep in mind snowpack history, recent weather, and avalanche activity.  In southwest Montana the snowpack has been unreliable and generally unstable this season.  Recent weather has provided a significant load, and notable avalanche activity occurred just a few days ago.  With this information in mind, numerous pits with good stability test results will be required to safely play in avalanche terrain.   For these reasons today, a CONSIDERABLE avalanche danger exists on wind loaded slopes steeper than 35 degrees.  A MODERATE avalanche danger exists on all other slopes.

Eric will issue the next advisory tomorrow morning at 7:30 a.m. If you get out in the backcountry let us know what you find.  You can reach us at 587-6984 or email us at mtavalanche@gmail.com.

Last Advisory: This Sunday, April 11, we will issue the final advisory for the 2009/2010 season.

Audio: 
Audio for Apr 8, 2010