GNFAC Avalanche Forecast for Tue Feb 21, 2023
<p>Very dangerous avalanche conditions exist in the Bridger Range and the mountains around Cooke City. Human-triggered and natural avalanches are likely. Cooke City is getting hammered by new snow with 30” + equal to 2.4” of <a href="https://www.mtavalanche.com/taxonomy/term/451"><span>snow water equivalent</span></a> (SWE) falling primarily in the last two days. The Bridger Range got a <em>slight</em> break in the last 24 hours, but the webcams at Bridger Bowl show heavy snowfall this morning with nearly 3” falling in the last hour. This adds up to a remarkable 36-40” of heavy snow since midday Saturday (a total of 3.8” of SWE). Winds gusting to 55 mph are adding further weight to slopes and are creating massive overhanging cornices.<span> </span></p>
<p>New snow and wind-slab avalanches will be large, and we could easily see slides break on buried weak layers deeper in the snowpack and run into flat terrain below. Other than how far they run, the specifics of any avalanche today are unimportant. Anything that slides could be unsurvivable and the strategy is pure avoidance.</p>
<p>Yesterday, Alex saw multiple slides in the Bridger Range on Saddle Peak and further south, including several on road cuts in the canyon (<strong><a href="https://www.mtavalanche.com/node/28224">observation and photos</a></strong>).</p>
<p>Play in the flats away from runout zones, go to the ski area, and enjoy the powder, but DO NOT go onto steep slopes in the backcountry (<a href="https://www.youtube.com/watch?v=ncycT237y64"><strong><span>video</span>…;
<p>The avalanche danger is HIGH on all slopes.</p>
<p>Dangerous avalanche conditions exist in the Madison and Gallatin Ranges and the Lionhead area. Yesterday, at Buck Ridge we saw a natural avalanche in Second Yellowmule that broke about 200 feet wide and 1-3’ deep on a steep, wind-loaded slope. Natural avalanches are free information about instability telling us that human-triggered avalanches are likely, and we should stay clear of steep slopes (<a href="https://www.youtube.com/watch?v=QjdtnwcWPVs"><strong><span>video</span>…;
<p>The snow totals in the last few days range from 12” (1.0” of SWE) in the Lionhead area to 18-20” near Big Sky and Hyalite (1.1-1.4” of SWE). While the snowfall totals vary, strong wind from last night into this morning is a constant. Wind-loaded slopes with recent drifts are the area of greatest concern for instability, but all steep terrain is suspect. Snowfall continues today and will only exacerbate instability.</p>
<p>New snow and wind-slab avalanches are likely and larger slides breaking on buried weak layers in the upper few feet of the snowpack are possible. Cautious route-finding, careful snowpack evaluation and conservative decision-making are essential.</p>
<p>The avalanche danger is CONSIDERABLE on all slopes.</p>
<p>Please share avalanche, snowpack or weather observations via our<a href="https://www.mtavalanche.com/node/add/snow_obs"><span> </span></a><a href="https://www.mtavalanche.com/node/add/snow_obs"><strong><span>website</s…;, email (<strong>mtavalanche@gmail.com</strong>), phone (406-587-6984), or Instagram (#gnfacobs). <span><span> </span></span><span> </span></p>
<p><span>Island Park received 8” of new snow in the last couple of days. Strong winds gusting to 55 mph are drifting this snow onto slopes where avalanches are possible. Watch for signs of instability related to wind-loading such as recent avalanche activity, large cornices, and cracks shooting from your sled or skis. Dig a quick snowpit to assess for instability in the upper few feet of the snowpack. Always follow safe travel protocols and carry a beacon, shovel, and probe in case something goes wrong. </span></p>
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Weather and Avalanche Log for Tue Feb 21, 2023
AVALANCHE WARNING: Bridger Range, Cooke City
Isolated wind loading on Mt Ellis
This afternoon we went for a tour up Mt Ellis. The winds ripped across the ridge from the west, heavily loading the east to northeast aspects and building up some impressive cornices. We avoided the wind-loaded slopes and dug a pit in a protected meadow on a northeast aspect below the ridge. There was little evidence of wind loading and we observed strong stability in our pit.
Maid of the Mist
On Saturday (2/18) we toured into Maid of the Mist and dug a pit on a northern aspect at the base of the skinny maid. Our results indicated that where there was wind loading, it was very likely to fail and propagate. In addition to the ECTP 1 on the wind slab, we noticed shooting cracks on isolated pockets of wind loading.
Beehive, Middle and Bear basin
Our group toured around beehive, middle and bear basin. The wind has made the cornices huge at ridge lines and created obvious pillows and drifts and added texture to the snow surface and made the new snow noticeably denser than yesterday. The newest 15-20cm fell upside down. In our pit we noticed the top 20cm of new snow was F+ consisting of primarily large graupel, below that, the 20cm of new snow from yesterday was F- with snow trending from 4F to 1F over the next meter below. We had ECTN4 where this denser new snow from today overlayed yesterdays lower density snow. The NSF layer was about a meter from the surface and unreactive in our tests. Today was almost unskiably deep, and the nature of the new snow structure made burying a tip and pearling an expectation.
We saw a natural avalanche that occurred overnight or the morning of February 20, in 2nd Yellowmule. It appeared to be ~200' wide and broke 1-3' deep. Photo: GNFAC
Forecast link: GNFAC Avalanche Forecast for Tue Feb 21, 2023GNFAC Avalanche Forecast for Wed Feb 22, 2023
Natural avalanches Bridgers
Drove up to Bridger at noon and saw a large avalanche on along the road north of the fire station. It was 75’ wide, 1-1.5’ deep and 25’ vertical. HS-N-R4-D1.5/2. It was on an east facing slope, south of the long slope that has cornices. The slope with cornices hadn’t slid at this point. On the way home at 3:30 the larger slope with cornices had slid. It was very big, possibly R5. 1.5-2’ deep, 200’ wide, huge chunks of hard slab and cornice. Looks like new wind-loaded snow with some gouges into older snow.
From Olson creek I had a cloud free flat light view of the ridge from Saddle to Bridger Peak and looked with binoculars. There was a wind slab just north of quarter saddle that did not go over the cliffs. Probably 1-2’ deep, 30’ wide of new snow. There was a large wind slab on the north half of Between the Peaks (250’ wide) and one similar depth wind slab in the Pinnacles (100’ wide). Both of these broke 1-3’ deep immediately below the cornice and did not entrain much snow or propagate very wide or downslope given how much new snow there was. I could see the debris from the slide between the peaks which ran over 1000’ vertical to the top of the runout zone but relatively low volume.
Big cornices in beehive
Biggest of the cronies not pictured but around 3-5 m in height at the biggest on the west side of the ridge
Natural Avalanche and Thick Drifts at Buck Ridge
We rode through first and second Yellowmule at Buck Ridge. There was a natural avalanche below the cornice line on the headwall above second Yellowmule that broke within the last 12 hours as it was not covered by any new snow. From a distance, it looked to be ~200' wide and broke 1-3' deep. I believe it failed just under the storm snow, not of deeper weak layers, but a very large cornice, hang fire (snow that didn't avalanche above the crown), and ongoing loading from snow and wind made it unsafe to investigate more closely.
The storm total farther back in the area was 20" of thick snow. Strong winds were creating heavy drifts in many areas. Human-triggered avalanches are likely on steep slopes, especially those loaded by fresh drifts. We dug one pit near the entrance of 2nd Yellowmule, we saw the buried weak layers, but they did not react in our stability test... We tested the snowpack for scientific interest. We planned to, and did, stay off steep slopes and the runout zones of slide paths and recommend that folks do the same until the storm is over and the snow has a chance to stabilize. The lack of propagation is a good sign for longer term stability, and I expect slopes to stabilize relatively quickly once the storm is over and wind-loading lets up.