Steamboat & Flat Tops
AvalancheMax Strotbeck, Jason Konigsberg · Feb 18, 2024 · Steamboat & Flat Tops
Weather
We experienced overcast skies with very light to moderate snowfall throughout the day. Light to moderate winds from the southwest. Temperatures were in the low-to-mid twenties.
Snowpack
On west through north through northeast aspects observed below treeline in the Little Agnes area, we found height of snow to vary greatly, from 100 to 225cm, but averaging around 130cm on west aspects and 180 to 190cm on northerly aspects. Buried 40cm on a west aspect, below fist to four-finger hard snow, we found a mostly faceted melt-freeze crust (which we suspect was the layer buried February 3rd). An extended column test result showed failure and propagation on this weak layer with moderate taps (ECTP19). On north and northeast aspects observed, we found a right-side-up snowpack that was unreactive on the layer buried on February 3rd, which was 50 to 80 cm from the surface on these slopes. We did not observe surface hoar at this layer, which has been found in many other areas around the Park Range. Deeply buried on these aspects we found the same 2 mm ice lens, that the avalanche we saw today failed on, sandwiched by 4-finger hard facets. However, in these wind-sheltered areas where this layer is buried over 120cm deep, it is unlikely to be problematic.
Avalanche
We observed a large avalanche today (that we estimate released yesterday) on the leeside (north and northeast aspects) of the Little Agnes ridge in the Mount Zirkel Wilderness. Our best guess is that it ran naturally, but it could have been remotely triggered by snowmobilers who rode in the area, as we saw tracks near the left flank of the avalanche path. The avalanche failed on faceted snow immediately atop a 2mm ice lens that was observed in December and early January in the Zirkel Wilderness. Wind loading was the driving force of this avalanche. In some areas the avalanche failed directly on the ice lens and in others it failed within the wind drifted snow above and stepped-down to the ice lens layer. The crown ranged from an estimated 1.5-8 feet deep, with the average depth 3-4 feet. It was over 1000 feet wide and over 1500 feet long, with a vertical fall of about 700 feet. SS-U-R3-D2.5-O. We also observed another very small avalanche that likely released sympathetically when the large avalanche released.