Turnagain Pass

• There was a large human triggered avalanche on the backside of Seattle Ridge in -1 /Warmup Bowl yesterday. We have limited details but it was triggered on a slope that already had tracks on it and was in hard wind affected snow with wide propagation at the crown.
• There was a natural avalanche cycle that occurred during the storm. Evidence of this was observed across the forecast area yesterday.

Triggering a large dangerous avalanche like the one yesterday in -1 /Warmup Bowl is still very possible today and remember it might not be the first person out on the slope that triggers the avalanche. Tracks are not necessarily a sign of stability. Steep slopes that were wind-loaded during the storm require extra caution, especially unsupported ones. Yesterday the wind effect from the hurricane force winds was notable and variable depending on terrain orientation. When the storm eased off early Wednesday morning there was a window of snow falling without wind. This set-up may be a bit misleading with the terrain that was wind affected having soft surface snow. It could be easy to get well out onto a slope and actually be on a hard wind slab, just covered by a bit of soft snow and have the slab fail. This is often a characteristic of this avalanche problem. As mentioned yesterday very strong winds can have unusual loading patterns with wind slabs forming lower off of ridgelines and significant cross-loading. These lurking wind slabs could be anywhere from 2-5′ deep, possibly even deeper. In addition, the winds are forecast to increase today and with snow available for transport new shallow, 6″-1′ wind slabs could form.  Watch for blowing snow. Pay attention to how far you sink in on your skis, board or machine. Are you actually traveling on hard snow under a little soft snow. Prior to this storm there were a few different surfaces including old wind affected snow, soft settled powder and exposed melt-freeze crust. Part of the current issue after the storm is how well the new snow bonds to the old surfaces.  Observers found that the interface between the new snow and old snow to be reactive yesterday in wind affected terrain. Below 2500′ we are also still tracking the December 1st rain crust under the December snow.

If you venture out today be sure to look for signs of instability and choose terrain wisely.

Red flags to watch for:
–  Recent avalanches.
–  Whumpfing (collapsing) of the snowpack, sure sign to avoid avalanche terrain period.
–  Shooting cracks, likely to be seen near ridgelines and on slopes where the wind has formed wind slabs.

This crown on Wolverine observed yesterday is in steep wind-loaded terrain. This is the type of terrain that you could trigger an avalanche today. Photo taken from Tincan. 12.23.20

New wind affected snow failing at the new/old interface yesterday at 3200′ on Tincan. 12.23.20

We did not see evidence of avalanches breaking down to the buried facets at the base of the snowpack above 2500′ during this storm cycle. As this point this really is just a concern south of the forecast area in Summit Lake where the overall snowpack is much thinner. Even in this zone observers have noted this to be generally unlikely as the facets have shown less and less reactivity over time and are starting to round.