24-25

Isolated shooting cracks and windslabs above treeline south of Cooke City today. Strong winds from the west. Lots of wind transport going on up high. The vis was good this morning and I could see far for the first time in a while. I saw no new avalanches. 

From email: "Skies were clear at 8am, but clouds rolled in quickly and we had S-1 snowfall starting at 1pm. Winds were L in the basin and L-M at ridge tops out of the W-NW. Moderate snow transport on high elevation peaks. West aspects near ridgetops were scoured, as they usually are. We found a 2-3cm thick MF crust under 5-10cm of new snow on E, W, and S aspects near ridgetops and in the basin. No cr, co.

Students dug pits on E-SE aspects at 9200', as well as on E and W aspects near the Prayer Flags. HS ranged from 100-145cm. Two of ten pits had propagation. ECTP1 on small facets above the MF crust and under a wind slab. ECTP30 on 2mm facets 30cm up from the ground. Hand pits had planar results below the MF crust where it was capped by a wind slab." 

1 meter deep snowpack showing the obvious facets in the bottom third

We went to Lionhead seeking some of the thinnest and weakest snow. We found relatively thin snow - the snowpack was mostly a 1 meter deep (3.2 ft). It was supportive for sleds and you could walk on top with about 6-8 out of  every 10 steps not punching through to the ground. This is because there is a very cohesive slab on top of the early December facets.

What surprised us was how stubborn that weak layer was in our tests. We dug in 4 different places on E, E, E, and NW aspects between 8000 and 9200 feet. ECT's either wouldn't even break or would propagate on the weak layer after mid to high 20's for taps.

We observed some new facets near the snow surface that formed last weekend during very cold weather. On Lionhead Ridge we found these facets capped by a hard but thin (~4 inch thick) wind slab. 
 

Summary

• The early December faceted layer seems mostly dormant for now.
• Wind slabs are the main problem and they will be extra sensitive as more snow comes Friday providing winds more ammo to make the wind slabs deeper. Because these wind slabs may be resting on facets, they could stay a problem for sometime.
• We did not get a chance to map how widespread or isolated this wind slab/facet combo is.

Top of Bridger Lift storm total 16" snow/0.25" swe (1.5% density!)

Winds have worked over many slopes near the Throne. We found some slopes stripped nearly to dirt with the snow blown off to who knows where, and others had wind-sculpted sastrugi. Trees were broken off, and debris littered the snow surface. Photo: GNFAC

We rode to the motorized boundary and toured up the shoulder of the Throne, poked out to the north-facing runs at the top, and then moved to the south-aspect gully from the upper saddle. 

Winds have worked over many slopes near the Throne. We found some slopes stripped nearly to dirt with the snow blown off to who knows where, and others had wind-sculpted sastrugi. Trees were broken off, and debris littered the snow surface. We probed for snow depths. On the east face, depths ranged from 20 cm to 100 cm on the shoulder (we may have missed deeper spots). At the upper portion of the north-facing run, we found 50-75 cm depths. The south face had a 115 cm depth.

There was some isolated wind-loading mid-slope. We saw one old crown that was nearly drifted in on a steep break over at mid-elevation on the east face. 

Somehow, we found a slope sheltered from the wind's effects. Because the danger is low on non-wind-loaded slopes, we considered traveling down through avalanche terrain. Before we did, we assessed stability to give us one last chance to turn around, and we followed safe travel practices, exposing one skier at a time to potentially hazardous terrain.