18-19
The red circle shows our snowpit location at 10,100' on a SSE facing slope near Lulu Pass. There was 140cm of snow and we got a layer of facets to break 55cm from the surface with 22 hits on an ECT. We opted to not ski the slope. Photo: GNFAC
A layer of facets that formed in early December is breaking clean in stability tests (ECTP22) about 1.5 feet under the surface. Wind-loading or more snow will make this layer more reactive. Photo: GNFAC
GNFAC Avalanche Forecast for Wed Dec 26, 2018
Snowpack and Avalanche Discussion
<p>In the mountains from Big Sky to West Yellowstone, the snowpack is weak. The bottom half consists of sugary facets that do not have much structural integrity. These weak grains are easily scooped out of the snowpit wall and do not instill confidence in their ability to support a big snowfall. Videos from our field days to <a href="https://www.youtube.com/watch?v=3NMgoDNKUQg&index=6&t=0s&li… Rind</strong></a> and <a href="https://www.youtube.com/watch?v=rQ7fzq4X1-4&index=6&list=PLXu51…; </a>illustrate this well. On Saturday, a skier in Bacon Rind got three distinct “whumphs” when the snow collapsed under his weight (<u><strong><a href="https://www.mtavalanche.com/node/19372">details</a></strong></u>). This is a clear sign of instability. On Monday, a skier found a <em>very similar snowpack</em> on Emigrant Peak in Paradise Valley (outside our forecast area) and triggered a large avalanche (<a href="https://www.mtavalanche.com/node/19398"><strong>details</strong></a>). Yesterday’s 2-3” of snow (.2” <u><a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent</a></u> or SWE) will be drifted at the ridgelines from westerly winds, loading slopes even more. For today, stay off of wind-loaded slopes and avoid avalanche terrain if you experience collapsing. Triggering avalanches is possible and the danger is rated MODERATE on all slopes.</p>
<p>Snowfall in the last 24 hours were 2-5” around Bozeman, Big Sky and Cooke City. Winds are blowing westerly with gusts of 20 mph and this new snow will be easily blown into drifts. The snowpack in these mountains has a weak layer of facets and surface hoar buried about a foot under the surface. On some slopes this unstable layer is capped by wind-drifted snow which we chronicle in our videos from <strong><a href="https://www.youtube.com/watch?v=2lO7K1Y-gOs&t=0s&list=PLXu5151n… Peak</a></strong>, <strong><a href="https://www.youtube.com/watch?v=OTiERTNpw38&t=0s&list=PLXu5151n… Basin</a></strong>, and <strong><a href="https://www.youtube.com/watch?v=u520iMxlXOQ&t=0s&list=PLXu5151n… City</a></strong>.</p>
<p>Snowmobilers on Friday triggered a few small slides near Lulu Pass outside Cooke City (<strong><a href="https://www.mtavalanche.com/node/19381">details</a></strong>) and on Saturday a sledder triggered a small slide on Buck Ridge south of Big Sky (<strong><a href="https://www.mtavalanche.com/node/19386">details</a></strong>). In the last five days the snowpack has been incrementally loaded with new snow measuring 1.1” of SWE in Cooke City and .6-.8” of SWE elsewhere (<strong><a href="https://www.mtavalanche.com/weather/wx-avalanche-log">Weather Log</a></strong>). This snow is being blown into drifts and creates the possibility of triggering avalanches. For today, the avalanche danger is rated MODERATE on all wind-loaded slopes and LOW on all others.</p>
<p>If you get out and have any avalanche or snowpack observations to share, contact us via our <u><a href="https://www.mtavalanche.com/node/add/snow_observation">website</a></u>, email (<u><a href="mailto:mtavalanche@gmail.com">mtavalanche@gmail.com</a></u>), phone (406-587-6984), or Instagram (#gnfacobs).</p>
New webpage dedicated to the Centennial Range in Idaho and Montana
After last season's two snowmobiler avalanche fatalities on Reas Peak in the Centennial Range we created a webpage so riders could get good, relevant information regarding snowpack and avalanches.
Weather and Avalanche Log for Wed Dec 26, 2018
measured at 0600
A skier triggered this avalanche near Emigrant Peak in Paradise Valley. He was not caught. From his email, " North Facing aspect at approx. 9800' on a west to east ridge. Slope angles are 35-40. Crown varied from 1-3'. Hard slab on the the cornice propagated to two other small starting zones and pulled the cornices and all the faceted snow near the ridgeline. Avalanche ran about 800'." Photo: A. Whitmore
Avalanche Details: Skier triggered on Emigrant Peak
Forecast link: GNFAC Avalanche Forecast for Wed Dec 26, 2018
Out of Advisory Area, 2018-12-25
Forecast link: GNFAC Avalanche Forecast for Wed Dec 26, 2018
This map shows where a skier triggered a wind slab from the ridgetop near Emigrant Peak in Paradise Valley. Photo: A. Whitmore
Avalanche Details: Skier triggered on Emigrant Peak
Out of Advisory Area, 2018-12-25
Skier triggered on Emigrant Peak
Emigrant Peak
Out of Advisory Area
Code
HS-ASu-R3-D3-O
Elevation
9800
Aspect
N
Latitude
45.26290
Longitude
-110.70700
Notes
From an email:
This was triggered Christmas Eve day while skinning up a ridge in the mountains east of Emigrant Peak. It broke about 8' back from the cornice/drift right near my ski tips. I instinctively did a quick drop and roll away even though it was unnecessary. Pretty pulse quickening. I didn't get great pictures as I was heading up the ridge and the light was bad. You can pick out the debris flow by its lighter shade of white and see chunks of cornice/slab that got hung up in the starting zone.
North Facing aspect at approx. 9800' on a west to east ridge.Slope angles are 35-40. Crown varied from 1-3'. Hard slab on the the cornice propagated to two other small starting zones and pulled the cornices and all the faceted snow near the ridgeline. Avalanche ran about 800'.
The last few week's weather has really weakened what was starting off to be a decent base. Now the winds have capped a a lot of zones with a hard slab pockets.
Number caught
0
Number buried
0
Avalanche Type
Hard slab avalanche
Trigger
Skier
Trigger Modifier
u-An unintentional release
R size
3
D size
3
Bed Surface
O - Old snow
Problem Type
Persistent Weak Layer
Slab Thickness
100.0 centimeters
Vertical Fall
800ft
Slab Width
100.00ft
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Single Avalanche
Advisory Year
GNFAC Avalanche Forecast for Tue Dec 25, 2018
Snowpack and Avalanche Discussion
<p>Since yesterday morning the Bridger Range got two rounds of snowfall totaling 6”. Remarkably, winds were light. About 12” under the snow surface is a persistent weak layer of facets or surface hoar. Alex and I dug on Saddle Peak on Sunday and found this to be the main instability in a snowpack that is generally strong (<strong><a href="https://www.youtube.com/watch?v=2lO7K1Y-gOs&list=PLXu5151nmAvRNl9ku…;). The new snow (.4 inches of <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent</a>) creates the possibility of triggering avalanches on this faceted layer, especially in areas where it is buried under hard wind drifts. For today, the avalanche danger is rated MODERATE on all slopes.</p>
<p>In the mountains from Big Sky to West Yellowstone the snowpack structure is weak. The lower half consists of sugary facets that are breaking in stability tests, and more concerning, collapsing with “whumphs” from the weight of a skier. This occurred at Bacon Rind (<u><strong><a href="https://www.mtavalanche.com/node/19372">details</a></strong></u>) where Eric’s snowpit revealed a terrible snow structure (<u><strong><a href="https://www.youtube.com/watch?v=3NMgoDNKUQg&t=0s&list=PLXu5151n…;) that mirrored my findings in Lionhead near West Yellowstone (<u><strong><a href="https://www.youtube.com/watch?v=rQ7fzq4X1-4&index=2&t=0s&li…;). Given the overall snowpack weakness and instability identified by collapses, triggering avalanches is possible today and the danger is rated MODERATE. Avoid steep slopes that have a thick slab of snow overlying sugary facets.</p>
<p>Yesterday, 2-3” fell outside Cooke City and south of Bozeman in Hyalite, while the Big Sky area only got a dusting. The snowpack in the northern Gallatin and northern Madison Ranges, and also around Cooke City, is close to 3 feet deep and generally strong and stable. A weak layer of facets and surface hoar is buried 8-12” deep and can be found on many slopes. In some areas this layer is buried under a hard wind slab where it would be possible to trigger a slide. This instability is not widespread, but requires careful assessment by digging and testing. Eric’s <strong><a href="https://www.youtube.com/watch?v=OTiERTNpw38&index=1&list=PLXu51…; from Beehive Basin and Alex’s <strong><a href="https://www.youtube.com/watch?v=u520iMxlXOQ&list=PLXu5151nmAvRNl9ku…; </strong>from Cooke City show the weak layer breaking under the wind-drifted snow. For today, it is possible to trigger small wind slabs on these facets and the avalanche danger is MODERATE on wind-loaded slopes and LOW elsewhere.</p>
<p>If you get out and have any avalanche or snowpack observations to share, contact us via our <u><a href="https://www.mtavalanche.com/node/add/snow_observation">website</a></u>, email (<u><a href="mailto:mtavalanche@gmail.com">mtavalanche@gmail.com</a></u>), phone (406-587-6984), or Instagram (#gnfacobs).</p>
New webpage dedicated to the Centennial Range in Idaho and Montana
After last season's two snowmobiler avalanche fatalities on Reas Peak in the Centennial Range we created a webpage so riders could get good, relevant information regarding snowpack and avalanches.