GNFAC Avalanche Forecast for Tue Dec 3, 2019
<p>The Lionhead, Southern Madison, and Southern Gallatin ranges received one to two feet of snow in the last week on top of a weak base. Two days ago, Doug and I went into Lionhead to assess how the snowpack is dealing with this stress. We had unstable test results and heard loud “whumping” as we walked (<a href="https://www.youtube.com/watch?v=mAbxNKrJb2U&list=PLXu5151nmAvSbcbVf…;). Today, obvious signs of instability will be harder to find as the snowpack adjusts and stability gradually improves. However, because we only have two field days at Lionhead, the snowpack structure is weak, and our weather data is limited, I am playing a conservative card and recommending more time before going into avalanche terrain. </p>
<p>Human triggered avalanches remain likely on steep slopes and where wind deposited snow is adding weight. These avalanches could be large, and the danger is rated CONSIDERABLE. Careful route finding to avoid avalanche paths and runout zones is essential.</p>
<p>In the Bridger, Northern Gallatin, and Northern Madison Ranges, and the Beartooth Mountains around Cooke City the snowpack is generally stable. Those hunting for deeper instability can find it on isolated slopes and slopes with fresh drifts. Yesterday, while shoveling cornice, the Yellowstone Club Ski Patrol triggered a small avalanche that gouged down to the ground. And, on a tour to the Thone, Alex and I found unstable snow on an isolated slope with an extended column test and choose to minimize our exposure to avalanche terrain (<a href="https://www.youtube.com/watch?v=5U2vv30wHmw&list=PLXu5151nmAvSbcbVf…;). These two events differ from other reports of avalanche activity this season because they failed deeper in the snowpack rather than as fresh drifts or at the new snow interface (<strong><a href="https://www.mtavalanche.com/avalanche-activity">recent avalanche activity</a></strong><strong> page</strong>, <a href="https://www.youtube.com/watch?v=jocNqCVRsZI&list=PLXu5151nmAvSbcbVf…;). This illustrates the importance of weighing signs of instability more than signs of stability.</p>
<p>Moderate winds continue today from the southwest building cornices and fresh drifts that will avalanche in specific areas. Avalanches are unlikely on slopes not affected by the wind. The danger is rated <strong>MODERATE</strong> on wind loaded slopes and <strong>LOW</strong> everywhere else. Evaluate the effect of the wind carefully and hunt for isolated instabilities with a stability test before committing to steep terrain.</p>
<p>Please send us your observations (no matter how brief) of avalanches, snow structure and stability, new snow amounts or wind effects. You can fill out an <a href="https://www.mtavalanche.com/node/add/snow_observation"><strong>observat… form</strong></a>, email us (<a href="mailto:mtavalanche@gmail.com"><strong>mtavalanche@gmail.com</strong></a…;), leave a VM at 406-587-6984, or Instagram (#gnfacobs).</p>
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Forecast link: GNFAC Avalanche Forecast for Thu Dec 5, 2019
We skied in the northern Bridgers (12/2/19), and an unstable test result (ECTP14) prompted us to adjust our travel plans. In the mountains around Bozeman and Big Sky the snowpack is generally stable, and will get more stable without snow the next few days, but it is just as important as ever to carefully assess stability of each slope, each day you plan to ride in avalanche terrain. East aspect. 8,100' elev. Photo: GNFAC
Forecast link: GNFAC Avalanche Forecast for Tue Dec 3, 2019
GNFAC Avalanche Forecast for Mon Dec 2, 2019
<p>Last night an additional 2-4” of snow fell on top of the 12-16” from the previous five days. The weight of the 1.7” of <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent</a> (SWE) is keeping this weak snowpack on edge. Doug and I rode and skied into the Lionhead Range yesterday to see how the sugary foundation is supporting the new load. We heard the snowpack collapse with a “whumph” sound and had unstable results in our snowpit (<a href="https://www.youtube.com/watch?v=mAbxNKrJb2U&list=PLXu5151nmAvSbcbVf…;). This is consistent with what other skiers in the area reported on Saturday (<a href="https://www.mtavalanche.com/images/19/unstable-snowpack-lionhead"><stro…;). Several inches of snow last night along and wind will keep the avalanche hazard elevated today. Over the next few days, as the storm ends and the snowpack adjusts to the new weight, it will become harder to trigger avalanches, but the weak layer near the ground will be with us going forward and cause problems with each snowstorm.</p>
<p>Today human triggered avalanches are likely on steep and wind loaded slopes. These avalanches could be large and it will still be possible to trigger these avalanches from the bottom of slopes. The avalanche danger is rated <strong>CONSIDERABLE</strong>. Avoid travel on and underneath avalanche terrain and pay special attention to wind loaded areas.</p>
<p>The foundation of the snowpack around Bozeman, Big Sky, and Cooke City is generally stable and was able to handle the 6-12” of snow during the last week. The avalanche concern today is fresh drifts of snow. Skiers and climbers at Bridger Bowl, Beehive, and around Hyalite Canyon all reported wind slabs avalanching. One skier triggered a small drift from one hundred feet away (<a href="https://www.mtavalanche.com/images/19/strong-winds-loading-snow-blackmo…;, <a href="https://www.mtavalanche.com/images/19/skier-triggered-wind-slab-beehive…;, <a href="https://www.mtavalanche.com/images/19/natural-slide-mineral-mountain"><…;, <a href="https://www.mtavalanche.com/images/19/wind-slab-triggered-100-away-hyal…;). On a tour to Beehive Saturday, Alex found this strong foundation with instability isolated to the new snow and wind drifts (<a href="https://www.youtube.com/watch?v=jocNqCVRsZI&list=PLXu5151nmAvSbcbVf…;, <a href="https://www.mtavalanche.com/sites/default/files/styles/very_large_1200w…;). Moderate winds from the west to southwest will continue to form fresh wind slabs, but the new snow elsewhere stabilized.</p>
![https://www.mtavalanche.com/sites/default/files/styles/very_large_1200w…](https://www.mtavalanche.com/sites/default/files/styles/very_large_1200w_/public/2019-12/IMG_5064.JPG?itok=tH-igZxj"><strong>photo</strong></a>){kind=link}
<p>Today, it will be possible to trigger fresh drifts of snow. Avalanches will be unlikely on slopes unaffected by the wind. The danger is rated <strong>MODERATE</strong> on wind loaded slopes and <strong>LOW</strong> everywhere else. Be cautious around ridgelines and where you see the wind affecting the snow and consider the consequences of small avalanches that could push you into unforgiving terrain. </p>
<p>Please send us your observations (no matter how brief) of avalanches, snow structure and stability, new snow amounts or wind effects. You can fill out an <a href="https://www.mtavalanche.com/node/add/snow_observation"><strong>observat… form</strong></a>, email us (<a href="mailto:mtavalanche@gmail.com"><strong>mtavalanche@gmail.com</strong></a…;), leave a VM at 406-587-6984, or Instagram (#gnfacobs).</p>
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BOZEMAN
Natural and human triggered wind slab avalanches in Hyalite
Observations from Hyalite today:
1. When approaching the base of Mt. Blackmore this morning at roughly 8:45am we observed strong swirling winds at higher elevations (summit of Blackmore and the Elephant/Blackmore Saddle). The winds we observed were primarily loading snow onto E and SE facing aspects. Furthermore, we witnessed four naturally triggered avalanches over a twenty minute span. All slides appeared to be D1/D2 on E and SE aspects and, seemingly, restricted to the newly loaded snow.
2. My partner and I climbed Zach Attack today. We intentionally triggered numerous size 1 wind slabs in the gully approaching the climb, which has slopes up to 35 degrees. These slabs were between 5 and 15 cm thick, up to 5m wide, 4F in hardness, touchy, and failed on lower density new snow. Although none of them ran more than 20m in this terrain, it would be a different story on steeper slopes, and if you were in an exposed position they had enough mass to push you around. Strong down and cross-slope winds formed these slabs at and below treeline, while the more alpine terrain of the climb itself was scoured.
3. Skied up in the main drainage of Hyalite today. Triggered some small wind slabs and propagating cracks. We backed off our main objective (a north facing couloir) because of these red flags and multiple crown lines on the same aspect and elevation.