Archived Advisory for Fri Apr 4 2008

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Friday, April 4

Good morning. This is Mark Staples with the Gallatin National Forest Avalanche Advisory issued on Friday, April 4, at 7:30 a.m. Bridger Bowl, in partnership with the Friends of the Avalanche Center, sponsor today’s advisory. This advisory does not apply to operating ski areas.

MOUNTAIN WEATHER

Only a trace of snow has fallen in southern parts of the advisory area. At 4 a.m. mountain temperatures were in the upper teens F, and ridgetop winds were 15-30 mph from the west and southwest. This morning will see a mix of sunshine and cloudy skies, and temperatures will approach 30 degrees F. This afternoon more clouds will move into the area, and southwesterly winds will increase from 20 to 30 mph as a storm approaches with snow falling late this evening. 2-4 inches will accumulate by tomorrow morning, but isolated areas at higher elevations could see snowfall up to 7 inches.

SNOWPACK AND AVALANCHE DISCUSSION

The Bridger Range

Stability in the Bridger Range has been changing as fast as the weather. On Wednesday, my partners and I confidently skied steep slopes just south of Bridger Bowl under clear, cold skies. By afternoon an ice crust had formed within the new snow, and stellar snowflakes on top of this crust had become small grained facets. Yesterday, winds increased and deposited snow on top of these weak facets. While driving to the northern Bridgers, my partner and I observed an avalanche just south of Bridger Bowl. A skier cut a cornice which triggered this avalanche. While skiing and riding near Sacajawea Peak, we observed another cornice triggered avalanche. Using a ski cut, we then triggered an avalanche on a steep wind-loaded slope.

See a photo of a cornice triggered avalanche at:

http://www.mtavalanche.com/photos/photos.php

Watch a video clip of our avalanche at:

http://www.youtube.com/watch?v=p0VEWis-FDg

The avalanche danger in the Bridger Range has quickly risen because wind slabs have formed on this layer of weak faceted snow. For today, wind-loaded slopes have a CONSIDERABLE danger if they are steeper than 35 degrees and a MODERATE danger if they are less steep. Slopes without a wind load will have a LOW danger.

The northern Gallatin and northern Madison Ranges:

A similar layer of faceted snow likely did not form in the northern Gallatin and northern Madison Ranges. These areas should not have wind slabs which will fracture as easily as in the Bridger Range. Cornices have become very big, and yesterday skiers on Fan Mountain dropped a huge cornice which they estimated to weigh 3000 lbs. Their stability tests indicated stable conditions, but the snowpack was no match for such a large cornice which triggered an avalanche. These skiers were surprised where this cornice broke and almost fell with it. They said “We were almost on rock, but we needed to be ON rock.” In addition to exercising caution near cornices, watch out for slopes with wind deposited snow where human triggered avalanches are possible. For today, wind loaded slopes steeper than 35 degrees have a MODERATE avalanche danger. All other slopes have a LOW danger.

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

On Tuesday, Doug and his partner found a layer of weak faceted snow and surface hoar buried 3 feet deep. Their stability tests indicated that this layer requires significant force to fracture, but when it does, the overlying snow will pop off with a very clean shear. This layer exists in the mountains around West Yellowstone and Cooke City where human triggered avalanches are possible. For today with this layer present, a MODERATE avalanche danger exists on slopes steeper than 35 degrees, and a LOW danger exists on less steep slopes.

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

LAST ADVISORY

Our 126th and last avalanche advisory of the season will be issued on Sunday, April 6th.