20-21

Hyalite Peak

Date

Natural slide off lower Hyalite Peak.

Location (from list)
Hyalite Peak
Observer Name
Hart Van Denburg

Mt Bole/Twin Falls Basin

Date
Activity
Skiing

140-160 cm on the ground on a NNE aspect at 9600’. ECTP-14 on a FC layer 35 cm down. The snowpack was generally right side up but interspersed with at least two other thin persistent layers below the one we saw propagate. I was pleasantly surprised at the lack of major basal facets, at least in the spot where we dug. Despite the dramatic, planar ECT result there were no other signs of instability or recent avalanches, and I ski cut a number of heavily wind loaded small features with no result. The wind slabs were quite stubborn but our observations of the underlying snowpack definitely increases the spook factor for me today, at least in this range.

Region
Northern Gallatin
Observer Name
Henry Coppolillo

GNFAC Avalanche Forecast for Sat Dec 19, 2020

Snowpack and Avalanche Discussion

<p>Strong winds have grown existing drifts and also built new drifts of cohesive snow sitting on a weak foundation. Yesterday (before the winds picked up) we got reports of multiple slides triggered by snowmobiles, groomers, snowplows, and with explosives at the ski resorts (<a href="https://www.mtavalanche.com/images/20/groomer-triggered-road-cuts"><str…;, <a href="https://www.mtavalanche.com/images/20/small-snowmobile-triggered-slides…; </strong><a href="https://www.mtavalanche.com/images/20/snowplow-triggered-slide-near-big…;). These slides were generally small, but are a clear indication that the snow is unstable and that the conditions for larger slides exist. The very strong winds have likely built drifts in unusual places further below ridgelines than is typical. Continued strong winds today will continue to build new drifts, so stay on alert for changing conditions through the day. Avoid steep, windloaded slopes.&nbsp;</p>

<p>Human triggered avalanches are likely on wind-loaded slopes and the avalanche danger is CONSIDERABLE. On all other slopes the avalanche danger is MODERATE.</p>

<p>&nbsp;</p>

<p>There is less new snow available to drift in the Bridger and Northern Gallatin ranges. Drifts will generally be thinner and more isolated, but no less dangerous where there is a thick slab of wind drifted snow. Weak snow in the lower snowpack is widespread, so anywhere these drifts form they are likely to be unstable. Unfortunately, these weak layers will not stabilize quickly, so plan to stay on alert for them for the foreseeable future. Watch for and avoid heavily wind-drifted slopes.</p>

<p>The avalanche danger today is MODERATE on wind-loaded slopes and LOW on all others.&nbsp;</p>

<p>Wind drifts of new snow are also the primary concern around Cooke City, In general, drifts are not building on top of snow that is as weak as in other areas. However, there are spots with weaker snow, as evidenced by several natural slides that broke west of Cooke City earlier in the week (<a href="https://www.mtavalanche.com/node/23292"><strong><u>photo</u></strong></…;). Alex was in Cooke yesterday and recommended digging below the new snow to find out what it’s sitting on before riding steep slopes (<a href="https://www.youtube.com/watch?v=71V0bipTxfY"><strong><u>video</u></stro…;) Today, human triggered avalanches are possible and the avalanche danger is MODERATE on wind-loaded slopes 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>

Upcoming Avalanche Education and Events

See our education calendar for an up to date list of all local classes. Here are a few select upcoming events and opportunities to check out:

Snowplow triggered cut bank slide near Big Sky

NORTHERN MADISON RANGE
Northern Madison
Code
SS-AV-R1-D1
Latitude
45.29520
Longitude
-111.41000
Notes

Snow plow triggered slide near Big Sky on 12/18/20.

Number of slides
1
Number caught
0
Number buried
0
Avalanche Type
Soft slab avalanche
Trigger
Vehicle ( specify vehicle in comments)
R size
1
D size
1
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Single Avalanche
Advisory Year

Avalanches west of Cooke

COOKE CITY
Cooke City
Code
SS-N-R1-D1.5
Latitude
45.02020
Longitude
-109.93800
Notes

5 slab avalanches breaking on facets reported west of Cooke City. Likely broke on 12/16.  

From email on 12/18: "The avalanches looked to be about 36 hrs old, and were found on N/NE and SE aspects.  About 5 avalanches were observed in total.  Very minimal wind effect in regards to the avalanches, and the landscape in general.  The avalanches were all about 30-40cms deep, and a couple were over 100' wide.... The weak layer was a layer of 2mm facets that formed beneath a m/f crust."

Number of slides
1
Number caught
0
Number buried
0
Avalanche Type
Soft slab avalanche
Trigger
Natural trigger
R size
1
D size
1.5
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Single Avalanche
Advisory Year

From email on 12/18: "The avalanches looked to be about 36 hrs old, and were found on N/NE and SE aspects.  About 5 avalanches were observed in total.  Very minimal wind effect in regards to the avalanches, and the landscape in general.  The avalanches were all about 30-40cms deep, and a couple were over 100' wide.... The weak layer was a layer of 2mm facets that formed beneath a m/f crust."

Photo: B. Fredlund

Cooke City, 2020-12-19