GNFAC Avalanche Forecast for Thu Dec 23, 2021
<p>The spigot is turned on for Cooke City. Seven inches of snow (.7” <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/null/?cid=nrcseprd1314… water equivalent</u></a>) and more forecasted with strong west wind will raise the avalanche danger. Today will be stormy and avalanches are likely to be triggered on slopes that are wind-loaded. <em> </em>A natural avalanche on Sheep Mountain on Tuesday illustrates this instability (<a href="https://www.mtavalanche.com/images/21/natural-avalanche-sheep-mountain"…;). <a href="https://youtu.be/QOBIciPigDY"><strong><u>Dave’s video from Monday</u></strong></a> outlines his concern with wind-drifted snow. On some slopes the snowpack has a weak layer of faceted, sugary snow in it’s lower third which will become more sensitive to avalanching as the snow piles up. For today, be mindful of the increased likelihood of triggering slides which must be balanced with our stoke of new snow. The avalanche danger is rising and rated CONSIDERABLE on wind-loaded slopes and MODERATE elsewhere.</p>
<p>Ian and I rode into Tepee Basin in the southern Madison Range yesterday and saw many small avalanches that slid last week during the High danger. These failed on sugary, faceted snow near the ground (<a href="https://www.mtavalanche.com/images/21/teepee-basin-avalanche-1-0"><stro…;, <a href="https://www.mtavalanche.com/images/21/tepee-basin-avalanche-2"><strong>…;). We dug in the flanks of an avalanche and confirmed the poor structure (<a href="https://youtu.be/2rjpm0iLm_I"><strong><u>video</u></strong></a>, <a href="https://www.mtavalanche.com/images/21/tepee-basin-profile-22-dec-2021">… profile</u></strong></a>) which resembled the snowpack at Lionhead (<a href="https://youtu.be/7EEn6f8-4fA"><strong><u>video</u></strong></a>). This area got 1-2” of new snow yesterday which will not adversely affect the stability. However, it remains possible to trigger avalanches on steep slopes that either have a wind-drift or weak facets underlying the snowpack. For today the avalanche danger is rated MODERATE on all slopes.</p>
<p>From the Bridger Range to Big Sky the snowpack is 2-3 feet deep and harbors a layer of faceted snow near the ground that is not very reactive. In the last 48 hours we have been to Buck Ridge (<a href="https://youtu.be/cf-qqv2Ssjw"><strong><u>video</u></strong></a>), Beehive Basin (<a href="https://youtu.be/ePMCJs3qAs0"><strong><u>video</u></strong></a>) and Mt. Blackmore (<a href="https://www.mtavalanche.com/images/21/snowpack-wind-loaded-slope-mt-bla…;, <a href="https://www.mtavalanche.com/images/21/scoured-thin-snowpack-n-face-mt-b…;). The story is the same in each: in areas where previous wind-loading created stiff, dense drifts, we can occasionally get them to break on this faceted (sugary) layer in our stability tests. For today, it is possible to trigger a small slide on these isolated drifts which are found on many aspects and elevations. The avalanche danger is rated MODERATE on slopes with older wind drifts and LOW on all others.</p>
<p>If you get out, please send us your observations no matter how brief. You can submit them via our <a href="https://www.mtavalanche.com/node/add/snow_observation"><strong><u>websi…;, email (<a href="mailto:mtavalanche@gmail.com"><strong><u>mtavalanche@gmail.com</u></str…;), phone (406-587-6984), or Instagram (#gnfacobs).</p>
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Pit from 12/22/21 at 9,850' on the NE face/shoulder of Mt. Blackmore. Snow depth was 111cm, first ECT was ECTN10 below the wind slab then ECTN22 on facets at 70cm. A second test was ECTN17 on that facet layer, then ECTP20 on depth hoar at 35cm. A third ECT was ECTP17 on the facets at 70cm, then a fourth ECT was another ECTP17....
North face of Mt. Blackmore on 12/22/21. South-southwest winds were moderate-strong and scouring more than loading it seemed like, but isolated slabs and whales of snow could be found. We did not get any drifts to crack or collapse. Fresh wind slabs were 2-4" thick, 1f-P hard, and not very continuous. Photo: GNFAC
Mt. Blackmore
I skied Mt. Blackmore this morning. The trail was in much better condition than last week. Good, not great, ski-in/ski-out coverage.
South-southwest winds were moderate-strong and scouring more than loading it seemed like, but isolated slabs and whales of snow could be found. I did not get any drifts to crack or collapse. Fresh wind slabs were 2-4" thick, 1f-P hard, and not very continuous...
I dug a pit high on the NE face/shoulder. As I moved from the ridge to a loaded slope of the face snow depth went from 12" to 36" over about 6 feet. I dug where it was 111cm and my first ECT was ECTN10 below the wind slab then ECTN22 on facets at 70cm. A second test gave ECTN17 on that facet layer, then ECTP20 on depth hoar at 35cm, so I did a third ECT which gave ECTP17 on the facets at 70cm, then a fourth ECT repeated that ECTP17... pit attached. I was moving into shallower snow as I got more unstable results, down to about 90cm HS. Anyway, I didn't think I would trigger an avalanche on that layer, but it did give me pause to be any further out or higher on that face where it was steep. I didn't intend to go higher or further out anyway, mainly due to not wanting to deal with the winds any higher up on the ridge/summit. Even though I feel danger for that PWL is low, the ECTPs were enough to ensure I stayed conservative.
For the long term I think the snowpack is not great, especially the higher you go. We will need to stay conservative with these facets mid-pack (30-70cm) (throughout the area). Since they are not hair-trigger or showing obvious signs of instability with these small recent storms I fear we (or someone) will get surprised with a cumulative loading or big wind event, or who knows... As usual, it comes down to: the snowpack has layers, it could avalanche [at some point, somewhere], be careful out there.
*I will add my pit and photo(s) to the website, I just wanted to test the obs submission form.
Many small avalanches in Tepee Basin
There were many natural avalanches that were breaking on facets near the ground (4-6mm). The 3 feet of snow that fell created unstable conditions and avalanched during the HIGH danger. Most slides were less than 100 feet wide. By GNFAC.
