Avalanche: Birthday Chutes

Observer Name
Observation Date
Saturday, November 29, 2014
Avalanche Date
Saturday, November 29, 2014
Salt Lake » White Pine Dup » Birthday Chutes
Location Name or Route
Birthday Chutes
Slope Angle
Trigger: additional info
Remotely Triggered
Avalanche Type
Hard Slab
Avalanche Problem
Persistent Weak Layer
Weak Layer
Depth Hoar
I was expecting fairly poor riding conditions today so I decided it would be fun to get up to a high summit to get a view and maybe get blasted by the wind. This time of year I enjoy walking up the west ridge of the Birthday Chutes to the summit of Red Top. If need-be, a relatively safe descent can be made following the ridge and skinner back to the White Pine road. There are a couple short, steep pitches one has to navigate before you get on the ridge that I thought would be manageable in the current danger. We broke trail from the road and the shady snow was good. Higher, we were skinning up the ridge on generally wind-hammered snow when, at the location of the X in the illustration, I had a crack shoot out from my skis. I could see that the whole slope to my left had spiderwebbed, but not moved. As we (party of two) were marveling at the collapse, we saw a powder cloud shoot out from the bottom of the westernmost Bday chute. It had slid wall-to-wall, to the ground, basically removing the entirely of the snowpack from the length of the chute. No other chutes slid to our knowledge. The collapse must have propagated 150-200 feet to cause this. The debris piles were large, with big chunks of windboard in it. As usual, the strong SW winds were loading this slope and I suspect this made the danger locally higher. In the illustration the blue line is our skinner, the red line outlines the collapsed area, and the red hash indicates areas that slid. A large area that was probably near 30 deg didn't slide. One thing I have been thinking about since the seriously unstable snow last weekend is whether or not one can look at a spiderwebbed slope as being as safe as an avalanched one. If the fracture has already propagated across it, can you cause an avalanche by then riding it? My intuition says 99% of the time this is true. But not 100%. Is this proof that the slope is not steep enough to slide? I am guessing that with these slides failing on the ground, the friction of the bed surface is relatively high, and just because the slope didn't totally fail this time, doesn't mean that with a smooth suncrust as a bed surface it might not slide then. We took the most conservative route down to avoid testing these questions. Even though we had no intention to ski the Bday chutes, it was still a good scare at the beginning of the season to keep one honest. The phone was dying so we didn't get but one poor picture that I didn't attach. Flat light made visibility difficult anyway.