24-25

Still Wary of Persistent Weak Layers

Date

Sat, 03/01/2025 - 13:20

Activity

Snowmobiling

Another day of warm temps and clear skies allowed us to cover a lot of ground in the Southern Madisons. We rode into the Taylor Fork, up to the weather station, to the top of Carrot Basin, through Sage Basin, up and over into Cabin Creek, and all the way up to the head of Red Canyon.

We spotted a few old avalanches. One in Cabin Creek that was triggered two days ago by snowmobilers on a N facing aspect. At the top of Carrot Basin, we saw a small avalanche (R1-D1) on a N aspect that likely broke yesterday on buried weak layers. We also spotted one cornice-fall triggered (R2-D2) avalanche that broke earlier this week in Sage Basin and then an older wind slab in Sunlight Basin.

We dug a snowpit in Sunight Basin right next to where Alex and I dug last week. At that site on 2/21, we triggered whumpfing, shooting cracks and got propagation in our snowpit. Today we did not experience any of those signs of instability. Our snowpit showed poor structure with buried surface hoar and facets but yielded ECTN's on that weak layer.

There were a handful of small wet-loose avalanches on solar aspects that we noted throughout the day. While northerly aspects stayed cold, solar aspects became wet a couple inches down.

Despite no remarkable new avalanche activity and the stubborn reactivity in our snowpit, we remain wary of mid-elevation slopes steeper than 30 degrees. Over the last 12 days, we have received reports nearly every day of avalanches breaking on persistent weak layers buried 2-3' deep. Recent nearby avalanche activity in Teepee Basin and Lionhead is evidence that these weak layers should not be trusted quite yet.

The travel advice remains tricky, but you have a few options:

1. Before committing to steep slopes, dig down 2-3' and assess for layers of buried surface hoar and facets. Make observations as you step off your snowmobile or skis or board - are you staying on the surface or sinking all the way to the ground?

2. If you are choosing to ride steeper slopes, pick ones with less consequential terrain features like trees, cliffs or gullies.

3. The ol' reliable option: stick to riding slopes less than 30 degrees.

Region

Southern Madison

Location (from list)

Taylor Fork

Observer Name

H Darby

Natural Wet Loose seen on South Texas Shoulder & in Hourglass

Date

Sat, 03/01/2025 - 12:20

Observed multiple wet loose slides naturally triggering and running on south facing slopes beyond bradleys and on the south facing aspects of hourglass chute.

Region

Bridger Range

Location (from list)

Hourglass Chute

Observer Name

Tyler McGarry

ECTP 22 on the Fin

Date

Sat, 03/01/2025 - 13:45

Activity

Skiing

Snowpit attached from Mt. Republic today. East aspect, 10,000'.

HS 165, ECTP22 at 65.

No collapsing, no cracking, no avalanche activity to report.

40 deg F at Fisher Creek SNOTEL today (9100').

Region

Cooke City

Location (from list)

COOKE CITY

Observer Name

B. Fredlund

Beehive Windslab Avalanche

Date

Sat, 03/01/2025 - 12:45

Activity

Snowboarding

Kicked off a wind slab avalanche 25ft wide on the west ridge of beehive basin. The wind slab was around 8-10ft deep and the debris slid almost to the flat a few hundred feet down the slope. The slide occurred around 9100ft on the east aspect. Neither me nor my partner were carried or caught in the slide.

The windslab that broke off was about 8-10 feet tall. It was mostly a huge section of a cornice that broke and the slope it fell down onto seemed to propagate a few inches down as well. The majority of the debris was from the cornice.

Region

Northern Madison

Location (from list)

Beehive Basin

Observer Name

Reed Lando

GNFAC Avalanche Forecast for Sat Mar 1, 2025

Snowpack and Avalanche Discussion

Triggering a big Persistent Slab avalanche that breaks on the weak layers that formed in late January is the main concern today. Yesterday in the Lionhead area, Mark and Alex saw two Persistent Slab avalanches that were triggered by snowmobilers during the day (<a href="https://www.mtavalanche.com/node/34421"><span…;, <a href="https://www.youtube.com/watch?v=TWCNOrvNvLk&embeds_referring_euri=h…;). One of these slides broke up to 5 ft deep (<a href="https://www.mtavalanche.com/images/25/tracks-going"><…;). This is just the latest in a long string of avalanches on these weak layers over the last 10 days, including multiple close calls (<a href="https://www.mtavalanche.com/node/34413"><… in Cabin Creek</a>, <a href="https://www.mtavalanche.com/node/34368"><… in Cooke</a>, <a href="https://www.mtavalanche.com/node/34341"><… in Tepee Basin</a>, <a href="https://www.mtavalanche.com/node/34367"><… in Cooke</a>, <a href="https://www.mtavalanche.com/node/34301"><… in Tepee Basin</a>, <a href="https://www.mtavalanche.com/node/34327"><… in Taylor Fork</a>, <a href="https://www.youtube.com/watch?v=tAiSHSEDJhY"><s… last Wednesday</a>).

Yesterday’s slides clearly show that these weak layers remain reactive. Stay alert even if you’re not riding up near ridgelines. Many of these slides have broken on mid-elevation slopes sheltered from the wind.

If you want to ride steep slopes today, take the time to do your homework. Dig down below the early February dust layer to look for weak layers and test the snowpack (<a href="https://www.youtube.com/watch?v=q0IGs0dKpQU"><s…’s video shows how to do this</a>). Make sure you’re riding one at a time on steep slopes and have a partner watching from a safe spot.

With temperatures a bit warmer this morning and forecasted to rise a bit higher than yesterday, watch out for triggering Wet Loose avalanches on sunny slopes this afternoon.

The avalanche danger is MODERATE.

This morning, after a solid overnight refreeze, avalanche conditions are generally safe and you’re unlikely to trigger a slide. The places where you could still trigger a wind slab are becoming fewer and fewer after days of warm temperatures.

As the day warms up and the snow surface starts to melt, Wet Loose avalanches will become a concern. With temperatures starting around freezing and forecast to rise 5 degrees warmer than yesterday (and maybe be the warmest so far this season) these could be a real problem today that deserve respect and consideration.

Wet Loose slides could start beneath your feet or sled, or as start as natural point releases, especially where there are exposed rocks above steep slopes. Look out for wet snow at low elevations and on sunny slopes, especially later in the afternoon when slopes have been baking in the sun all day (<a href="https://www.mtavalanche.com/node/34408"><… Creek road photo</a>). Be heads up if there are more than a couple inches of wet snow at the surface.

The potential for cornices breaking off is also worth considering before travelling above or below them (<a href="https://www.mtavalanche.com/node/34402"><… observation</a>).

The avalanche danger is LOW first thing this morning and will quickly rise to MODERATE as the day heats up.

Read more about GNFAC Avalanche Forecast for Sat Mar 1, 2025

Upcoming Avalanche Education and Events

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Weather and Avalanche Log for Sat Mar 1, 2025

Read more about Weather and Avalanche Log for Sat Mar 1, 2025

Read more about Targhee creek crown profile - 28 Feb 2025

Avalanche Details: Snowmobile triggered 3-5' deep hard slab at Lionhead

Lionhead Range, 2025-02-28

Access other image sizes:

Snowmobile triggered slide in Airplane Bowl

Airplane Bowl

Lionhead Range

2/28/2025

Code

HS-AMu-R2-D2-O

Elevation

9100

Aspect

Latitude

44.72500

Longitude

-111.32200

Notes

Also, on our way out we saw an avalanche in Airplane Bowl that had been triggered during the day today. This slide also broke on the persistent weak layers, 2-3' deep, 150' wide.

Number of slides

Number caught

Number buried

Avalanche Type

Hard slab avalanche

Trigger

Snowmobile

Trigger Modifier

u-An unintentional release

R size

D size

Bed Surface

O - Old snow

Problem Type

Persistent Slab

Read more about Snowmobile triggered slide in Airplane Bowl

Slab Thickness

30.0 inches

Vertical Fall

200ft

Slab Width

150.00ft

Images

Airplane at 4:00pm

Airplane at 10:30am

Snow Observation Source

Avalanches at Lionhead

Slab Thickness units

inches

Single / Multiple / Red Flag

Single Avalanche

Advisory Year

24-25

Snowmobile triggered 3-5' deep hard slab at Lionhead

Targhee Creek

Lionhead Range

2/28/2025

Code

HS-AMu-R4-D3-O

Elevation

9200

Aspect

Latitude

44.74130

Longitude

-111.37400

Notes

A rider triggered a huge avalanche in the uppermost reaches of Targhee creek. We discovered it on our way out. The best we could tell looking at their tracks was that they had no idea they triggered it, even though part of their tracks were swept away.

Number of slides

Number caught

Number buried

Avalanche Type

Hard slab avalanche

Trigger

Snowmobile

Trigger Modifier

u-An unintentional release

R size

D size

Bed Surface