Buried surface hoar starting to show reactivity in the Yuba Pass area.

We went up to Yuba Pass today to take a look at conditions where the largest buried surface hoar we know of currently within the forecast area sits underneath the most snow accumulation from the last storm. We headed to an out of the way spot without previous tracks. Breaking trail to this location we observed cracking up to 4 feet long around our skis. The recent storm snow had settled enough to become a 5 inch thick cohesive slab of 4F- hardness. Lifting up cracked sections of the snow surface, large 25 to 30 mm surface hoar (SH) was the obvious failure layer. This is the first area where we have seen a slab on top of the Jan 29 SH layer that had the necessary characteristics to indicate some potential for instability and propagation. At 6,920' in an open area below treeline on a N aspect we performed a PST on the Jan 29 SH layer. The result was PST 18/100 SF on the Jan 29 SH weak layer. Not quite the right slab characteristics just yet to fuel long propagation.

To further test this layer pre-storm, we went to work building an artificial slab by adding 10 cm worth of disaggregated snow to the surface. We then left the test to sinter for a couple of hours while we went up to the ridgetop to make wind slab observations. Upon our return, someone had followed our track to this out of the way open area and their curiosity had led them to damage our test sample. Unfortunately, it was no longer usable and we did not have time to build a second sample and give it the appropriate time to sinter. I thought this spot was far enough out of the way that no one would pass by and bother it. I was wrong...quite the bummer to lose this data.

While we were up on the ridgetop, we noted light amounts of drifting snow over the ridgetops that had deposited as wind slabs up to 6 inches thick in areas adjacent to cornice features. The ridgetop had held 2 to 6 inches of recent storm snow earlier in the week and was now scoured back down to rain crust. Ski cuts on NE aspect near treeline wind slab test slopes revealed no signs of cracking. There was no buried SH in this wind slab area. As we descended back below treeline, we noted cracking around our ski tracks again as we moved back into terrain where the surface hoar was present, but smaller at 8 to 12 mm in size. I also saw areas where snowmobile tracks had also caused cracking of the snow surface with 8 to 10 mm SH below the surface slab.