19-20

Big natural avalanches on Flathead Pass

Flathead Pass
Bridger Range
Code
HS-N-R4-D3-O
Elevation
8000
Aspect
NE
Latitude
45.96840
Longitude
-111.02100
Notes

"Quick Obs from a tour up to the skiing just south of Flathead Pass [2/17/2020]. We headed that direction for some low-angle protected tree skiing and, instead, found evidence of a widespread natural avalanche cycle from 2~3 days ago.

There were 3 main avalanches with similar characteristics: HS - N - R4 - D3 - O. 

- Crowns topped out around 8000’ with large amounts of hanging snow (on steep slopes) above. Interestingly, the crowns connected down slope as low as 7800’ and jumped from tree to tree along convexities. The avalanches’ crowns ranged from 100’ wide to over 500’ wide.

- The avalanches failed on 4F- 1.5mm facets above the Thanksgiving crust, which was substantially harder than I have seen in the southern Bridgers.

- The avalanches stepped down to well-developed depth hoar (below the crust) in a few locations.

- The average crown depth was ~140cm where we felt safe looking (near bottom of crown) but was significantly more shallow near some trees.

- Conducted a quick crown profile and some tests. The standout results are PST 25/120 END x3 (we conducted the PST at a spot where the weak layer was buried 120cm deep) on the persistent weak layer with no notable results in two separate location ECTs and additional deep tapping…).

- The debris piles extended to previous old-growth maximums (extending the path of at least one avalanche) and had many snapped trees up to 10” in diameter deposited within."

 

Number of slides
3
Number caught
0
Number buried
0
Avalanche Type
Hard slab avalanche
Trigger
Natural trigger
R size
4
D size
3
Bed Surface
O - Old snow
Problem Type
Persistent Weak Layer
Slab Thickness
140.0 centimeters
Vertical Fall
500ft
Slab Width
500.00ft
Weak Layer Grain type
Faceted Crystals
Weak Layer grain size
1.50mm
Weak Layer Hardness
4F-
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Multiple Avalanches
Advisory Year

GNFAC Avalanche Forecast for Sun Feb 23, 2020

Snowpack and Avalanche Discussion

<p>The mountains last received snow four days ago. The snowpack has had time to adjust to the 4-6 feet of snow that fell since February 1<sup>st</sup>. This snow equaled 4.4-5.7” of <a href="https://www.nrcs.usda.gov/wps/portal/nrcs/detail/or/snow/?cid=nrcs142p2… water equivalent</a> (SWE) which increased the season’s snowpack water content by 25-45%&nbsp;and caused widespread natural and human triggered avalanches. Triggering a deep slab avalanche is getting more difficult, but still possible. It has been five days since a deep slab avalanche broke on weak sugary snow near the base of the snowpack (<u><strong><a href="https://www.mtavalanche.com/images/20/naturals-north-flathead-pass">pho…;, <u><strong><a href="https://www.mtavalanche.com/images/20/natural-avalanche-near-flathead-p…;, <u><strong><a href="https://www.mtavalanche.com/images/20/natural-avalanches-wilson">photo<…;), but we continue to get reports and take inventory of the recent carnage which warrants cautious travel.</p>

<p>Yesterday skiers near Emigrant peak in the Absarokas reported widespread old avalanche crowns (<strong><a href="https://www.mtavalanche.com/node/22202">details and photos</a></strong>). Doug visited the huge avalanche near Maid of the Mist in Hyalite and said, “it is as big as it gets” (<strong><a href="https://www.youtube.com/watch?v=a17oTfiLCyc&amp;feature=youtu.be">video…;, <strong><a href="https://www.mtavalanche.com/node/22177">details and photos</a></strong>). He measured an alpha angle of 16 degrees, the lowest he has seen in North America. (A lower alpha angle means a farther runout distance (<strong><a href="https://www.mtavalanche.com/node/22177">photo</a></strong&gt;).</p>

<p>Yesterday was a sunny Saturday without a report of a human triggered or fresh natural avalanche. This is a good sign, but I’m cautious of the deep buried weak layers and wouldn’t center punch any steep mountains (e.g., Saddle Peak, Mt. Blackmore). Browse the <u><strong><a href="https://www.mtavalanche.com/avalanche-activity">avalanche activity</a></strong></u> on our website to see where recent avalanches occurred and avoid similar terrain. Recent activity was widespread across all aspects and elevations. Deep slab avalanches broke in and below narrow rocky chutes, and through sparsely treed hillsides. I mention these features because we (skiers) sometimes feel safer and better about stability here which is a false sense of security. Carefully consider stability and consequences of an avalanche on any slope steeper than 30 degrees.</p>

<p>Today it is possible to trigger avalanches many feet deep on weak sugary snow near the bottom of the snowpack. Choose routes that minimize exposure to avalanche terrain (<u><strong><a href="https://www.youtube.com/watch?v=ZmDJHoXSKvg&amp;list=PLXu5151nmAvSbcbVf…;). Avalanche danger is <strong>MODERATE</strong>.</p>

<p>The southern Madison Range, southern Gallatin Range and Lionhead area near West Yellowstone received no snow in the last five days, and since February 1<sup>st</sup> got 3-4 feet of snow equal to 2.8-4.1” of SWE. The snowpack handled it better than the rest of the advisory area. The last reported avalanche was a week ago in Taylor Fork, triggered by a snowbiker and broke on sugary snow near the ground (<u><strong><a href="https://www.mtavalanche.com/node/22074">details</a></strong></u&gt;). Doug and Ian rode into Cabin Creek last Wednesday under clear skies and didn’t see any avalanches. They dug multiple pits and found a generally stable snowpack (<u><strong><a href="https://www.youtube.com/watch?v=2IzNvRo0R3w&amp;list=PLXu5151nmAvSbcbVf…;). Today avalanches are unlikely and danger is LOW.</p>

<p>If you get out, please send us your observations no matter how brief. You can fill out an&nbsp;<u><strong><a href="https://mtavalanche.us17.list-manage.com/track/click?u=6653a830e4819c9e…; target="_blank">observation form</a></strong></u>, email us (<u><strong><a href="mailto:mtavalanche@gmail.com&quot; target="_blank">mtavalanche@gmail.com</a></strong></u>), leave a VM at 406-587-6984, or Instagram (#gnfacobs).</p>

Upcoming Avalanche Education and Events

Our education calendar is full of awareness lectures and field courses. Check it out and plan to attend one or two: Events and Education Calendar.

COOKE CITY

Every Friday and Saturday, Snowpack Update and Rescue Training. Friday, 6:30-7:30 p.m. at the Soda Butte Lodge. Saturday anytime between 10-2 @ Round Lake.

BOZEMAN

Natural on Wolverine Peak Cooke City

Wolverine Peak
Cooke City
Code
Latitude
45.05300
Longitude
-110.01000
Notes

"While sled skiing a bowl over, we noticed the second half of a natural slide. We forgot to go over and take photos, but it seemed like it was either a cornice break or a point release, and slid somewhere around 1,000 feet."

Number of slides
1
Number caught
0
Number buried
0
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Single Avalanche
Advisory Year

Natural avalanche Pine Creek

Pine Creek
Out of Advisory Area
Code
HS-N-R3-D2-O
Elevation
9700
Latitude
45.49000
Longitude
-110.49500
Number of slides
1
Number caught
0
Number buried
0
Avalanche Type
Hard slab avalanche
Trigger
Natural trigger
R size
3
D size
2
Bed Surface
O - Old snow
Problem Type
Persistent Weak Layer
Slab Thickness units
centimeters
Single / Multiple / Red Flag
Single Avalanche
Advisory Year

Natural Avalanche in Absarokas

Other place
Out of Advisory Area
Code
HS-N-R3-D3-O
Elevation
9700
Aspect
NE
Latitude
45.65530
Longitude
-110.55800
Notes

From IG #gnfacobs post: "An avalanche from early Tuesday (02/18) morning caused by a natural cornice fall. Note the step down partway down the path. ENE @ 9700. Ran 1300’." 

Number of slides
1
Number caught
0
Number buried
0
Avalanche Type
Hard slab avalanche
Trigger
Natural trigger
R size
3
D size
3
Bed Surface
O - Old snow
Problem Type
Wind-Drifted Snow
Slab Thickness
50.0 inches
Vertical Fall
1300ft
Slab Width
75.00ft
Slab Thickness units
inches
Single / Multiple / Red Flag
Single Avalanche
Advisory Year

Many large, destructive slides on Emigrant Peak

Emigrant Peak
Out of Advisory Area
Code
HS-N-R4-D3-O
Elevation
9800
Aspect Range
All
Latitude
45.26300
Longitude
-110.70700
Notes

[date of activity roughly estimated] From email on 2/22/2020: "The mountains around Emigrant Peak have certainly reached their tipping point. Skinning up in the morning we witnessed the debris of more than a dozen avalanches on all aspects, more than I have ever seen in a single day. Many of these avalanches were extremely large and powerful deep slabs that broke towards the ground, likely to have released within the past week. Slides were running to the end of their runout zones and carried with them many trees that were torn to shreds."

Number of slides
7
Number caught
0
Number buried
0
Avalanche Type
Hard slab avalanche
Trigger
Natural trigger
R size
4
D size
3
Bed Surface
O - Old snow
Problem Type
Persistent Weak Layer
Slab Thickness
60.0 inches
Vertical Fall
1000ft
Slab Width
1000.00ft
Slab Thickness units
inches
Single / Multiple / Red Flag
Multiple Avalanches
Advisory Year