18-19

Avalanches breaking 2-4’ deep on sugary facets are becoming more difficult to trigger, but have big consequences. Unfortunately, evaluating slope stability is tricky. Obvious signs the snowpack is unstable are less frequent, such as cracking or collapsing, and multiple people can put tracks on a slope before a large avalanche is triggered. The simplest strategy to avoid large avalanches is to avoid avalanche terrain. Otherwise, you have to confidently rule out the presence of weak, sugary snow buried 2-4’ deep. Today, dangerous avalanches are possible to trigger and avalanche danger is MODERATE.

In the mountains near Bozeman, Big Sky and Cooke City no avalanches have been reported in over a week with only isolated signs of unstable buried weak layers. Today it is unlikely you will trigger an avalanche on weak layers in the middle or bottom of the snowpack. However, before riding steep terrain it is worth digging 3-4 feet to evaluate snowpack structure and stability. The weakest and potentially unstable slopes are where the snowpack is relatively shallow, less than 4 feet (<a href="https://www.youtube.com/watch?v=6nk39vqLWlQ&list=PLXu5151nmAvRNl9ku…;, <a href="https://www.youtube.com/watch?v=Z0dOv0z4GDI&t=0s&index=9&li…;), or where snow depth changes greatly across a slope due to variation in wind loading.

A more likely, though less consequential type of avalanche is small drifts formed from moderate southerly and east wind (<a href="https://www.mtavalanche.com/images/19/cracking-small-wind-slabs">photo<…;). These small slabs are possible near ridgelines and should be avoided where a small slide could carry you into trees, over cliffs, or through other hazardous terrain. Cracking of the snow surface is an obvious sign these drifts are unstable. Be on the lookout for these slabs in unusual locations due to less common east winds. Today, small fresh drifts of snow are likely and wind loaded slopes have a MODERATE avalanche danger. Avalanche danger is LOW on non-wind loaded slopes.

If you get out and have any avalanche or snowpack observations to share, contact us via our <a href="https://www.mtavalanche.com/node/add/snow_observation">website</a>, email (<a href="mailto:mtavalanche@gmail.com">mtavalanche@gmail.com</a>), phone (406-587-6984), or Instagram (#gnfacobs).

Read more about GNFAC Avalanche Forecast for Sat Feb 9, 2019

Story Mill Beacon Park

Stop by the public beacon park at the Bozeman Parks North Recreation Center at Story Mill on Rouse St. It is operational from 8:30 a.m to 5:00 p.m. 7 days a week. Here’s a fun video outlining how to use the park: https://www.facebook.com/friendsgnfac/videos/279522799401278/

Weather and Avalanche Log for Sat Feb 9, 2019

Read more about Weather and Avalanche Log for Sat Feb 9, 2019

Read more about Small, but deep natural avalanche

This avalanche broke on weak snow in the lower 1/3 of the snowpack after a storm dropped 2-3' of heavy snow between 2/3 and 2/5/19. Photo: GNFAC

Avalanche Details: Natural avalanches at Lionhead

Lionhead Range, 2019-02-08

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GNFAC Avalanche Forecast for Fri Feb 8, 2019

Snowpack and Avalanche Discussion

As we drove down the highway toward West Yellowstone yesterday, Alex and I spotted four impressive avalanches on Lionhead ridge (<a href="https://www.youtube.com/watch?v=xlsoevxJbKc&index=2&list=PLXu51…;, <a href="https://www.mtavalanche.com/photos">5 photos</a>). The largest was 4+ feet deep, a few hundred feet wide and triggered by a sledder on Wednesday. It was big enough to kill. This avalanche, as did two others, broke on the 1-2 foot thick layer of sugary facets that underlay our season’s snowpack. The fourth slide involved new snow. These avalanches ripped out older snowmobile tracks shattering the myth that tracks mean a slope is safe.

Two feet of snow fell over the weekend and prompted an avalanche warning on Monday. Since then, triggering avalanches are more difficult, which is good news. However, evaluating which slopes are dangerous and which are safe is challenging. Alex and I were careful as we rode under bigger slopes, only going one at a time. The snowpack structure is worrisome in the mountains south of Big Sky to West Yellowstone (<a href="https://www.youtube.com/watch?v=NIwKogFUSDY&list=PLXu5151nmAvRNl9ku…;, <a href="https://www.mtavalanche.com/images/19/bacon-rind-snowpit-profile-0">sno…;). A skier or rider could trigger a large slide by either breaking off a cornice or hitting the weakest part of a slope, typically the shallowest.

The avalanche danger is rated MODERATE today, but it’s a serious Moderate.

In the mountains around Bozeman, Big Sky and Cooke City there has been no avalanche activity in over a week and only isolated signs of instability. Yesterday in Beehive Basin a skier got cracking on wind slabs (<a href="https://www.mtavalanche.com/images/19/cracking-small-wind-slabs">photo<…;) and another party stuck to low angled trees when their tests broke on sugary facets on the bottom 2 feet of the snowpack, similar to Eric’s finding on Monday (<a href="https://www.youtube.com/watch?v=kbvE5YGGJOQ&index=2&list=PLXu51…;). A couple days ago skiers outside of Cooke City felt a large collapse while they dug a pit, fracturing a faceted layer in the middle of a 190 cm deep snowpack (<a href="https://www.mtavalanche.com/node/20005">details</a>). And last Friday, Alex got a rumbling collapse to the north of Cooke City on a shallow, weak slope (<a href="https://www.youtube.com/watch?v=6nk39vqLWlQ&list=PLXu5151nmAvRNl9ku…;). These disparate signs of instability are not unusual and luckily we have protocol for these types of situations: we dig and assess the snow under our feet, and we retreat to low angled terrain when we get collapses or cracks. Winds yesterday and last night likely created wind drifts at the ridgelines and in gullies that will crack and potentially avalanche. For today, avalanches on wind drifts and deeper in the snowpack are possible and the danger is rated MODERATE.

Read more about GNFAC Avalanche Forecast for Fri Feb 8, 2019

Story Mill Beacon Park

Weather and Avalanche Log for Fri Feb 8, 2019

Read more about Weather and Avalanche Log for Fri Feb 8, 2019

Natural avalanches at Lionhead

Lionhead Ridge

Lionhead Range

2/5/2019

Code

SS-N-R2-D2

Elevation

9000

Aspect Range

E-SE

Notes

GNFAC forecasters' observation: On 2/8/19 we observed 3 recent avalanches with unknown triggers (assumed natural) and one snowmobile triggered avalanche (listed separately as AMu on 2/6/19) along Lionhead Ridge. Three of them, including the snowmobile triggered slide, broke 3-4+ feet deep on sugary weak snow in the lower 1/3 of the snowpack. One avalanche broke 2' deep in the recent storm snow.

Multiple Avalanches

Number of slides

Number caught

Number buried

Avalanche Type

Soft slab avalanche

Trigger

Natural trigger

R size

D size