fbpx North Sister, 3/26 and 3/30 | Central Oregon Avalanche Center (COAC)
Rapid temperature rise
Observation Date/Time:
Tue, Mar 30, 2021 - 9:00 AM
Three Sisters Area
Report Type:
Snow Conditions  
Travel Mode:
Ski or Snowboard  

Written Report:
Well it’s certainly an interesting time of year for ski mountaineering. Longer days, decent base, stuff mostly filled in up high, but still getting occasional winter storms, which then can be rapidly affected by intense sun… or not, depending on what the weather decides to do…

I headed out to North Sister on Fri 3/26 to check things out. TH was still about 2 miles out. We had a cold storm a couple days prior, and the next day some protected N-facing terrain in a nearby zone lower down held really good snow, so I thought things might be decent in protected chutes up high the next day. What I didn’t account for was the wind. I was intending to check out EMC, but the WNW wind was howling (wish I could post a vid, it was ridiculous). It was just sunrise at that point, so I figured I’d bail over to the Thayer side and see if it was a bit more protected. Got over there and saw another party already climbing the main Thayer line. I was curious who it was but didn’t want to disrupt their experience, so I climbed up a mellower line lookers left of that one figuring I’d drop in around the same time and say hi at the bottom. The wind was definitely better on that side, though it would occasionally whip up, in strange ways. In that cirque it seems like the wind gets turned around in weird eddies or something. It would be dead quiet, then it would suddenly blast straight down the chute, then quiet, then straight UP the chute, etc. During one of those downward blasts I got a great realtime demonstration of windslab formation. My skin track was maybe 6” deep, and when the wind blast came roaring down, I stopped and braced myself against it. I watched as snow blowing down the slope fell into the skin track on top of my ski. The wind blast was only like a minute or two long, and when it stopped I reached down and picked up a perfectly formed tiny SLAB on top of my ski like 2” thick (see pic). It was a totally solid piece of slab that literally formed in a minute before my eyes, like a time-lapse video. One thing that always confused me about windslab formation when I was first learning about it in avy classes was, I understood how wind could cause snow deposition at rates similar to a heavy snowfall, but I didn’t understand why the structure of the wind-deposited snow would be so much different (harder, i.e. windslab) vs. just falling out of the sky at the same rate. I later understood a key difference is that wind blows the snow grains along the surface from the windward side, and as they bounce around while getting blown, it alters the actual physical shape of the snow grains, such that they end up packing together much more tightly when they fall on the leeward side, giving birth to a windslab. And here was a great realtime in-field demonstration of that. Pretty cool.

The actual slope I was on was already smooth and most of the windblown snow was just rushing down to the bottom (unless encountering an hole like my ski track). That mostly had about a 1-2” light slab “skin” that was not that reactive, and was totally fine upon actual skiing. I dropped at the same time as the other party that was at the top of Thayer around 10:30. Met them at the bottom and turns out it was local legend Kevin and crew. They reported really good snow conditions in the line, great pow with what sounds like even less wind effect than I saw (the slope I was on was a bit more open). Stoked that they got that line in such great conditions. Another thing that is nice about this time of year is being able to ride all the way back to the TH w/o any dirt hiking.

Then we had the short but intense storm on Sunday, followed by really cold temps and some wind on Monday, with Tuesday shaping up to be a clear day w/o much wind, and temps rising. We decided to go back out there extra early, which is really the best way to approach that line in spring. Since much of it faces SE, as soon as the sun and warm temps hit it, stuff starts to move. The storm dropped less snow that forecast, so we weren’t sure what to expect. Skinning out there under the full moon was surreal, and quick travel since most of the approach didn’t have too much new snow. Getting up onto the apron of the line at sunrise we were pleasantly surprised to see good snow coverage in the chute itself, and not much if any evidence of slab formation. The only question mark was a hard crust in places, from the warmup over the weekend and subsequent refreeze prior to the new snow. This crust was generally down far enough that we were hitting it on the way up while booting, but would generally be above it on the way down on the boards. The new snow appeared to pretty well bonded to this crust, and it stayed consistent the entire way up. I should note that the large crown that was at the top of the line about a month ago is now basically completely filled in and gone.

We topped out at about 9 AM, and dropped at about 9:30, which was about perfect for that day. The temps had stayed really cold the entire approach in the dark (around 20 deg most of the time) and even climbing the chute after the sun was out the temps were still in the 30s. One in our crew actually dropped from the actual summit, which is doable at times, but even then only if you’re comfortable on super-steep snow over exposure with a mandatory straighline through rocks. First snowboard descent from the summit that I know of, know of 2 others on skis and maybe there have been others, regardless it was pretty impressive to witness. Snow in the chute was generally great, occasionally hitting the crust layer, but nothing too bad. No instabilities noted, there was the expected loose snow waterfall through the choke from our sluff, but otherwise surprisingly great. It’s always great with the conditions/timing/variables line up to make a line like that possible and actually fun!

Compared to some previous years, coverage is not actually that great this year, but there’s enough. Hopefully we get some more refreshes before it turns completely to spring corn season…


1) 1-minute slab piece that formed on my ski in the skin track
2-3) Summit snowfield
4) Great turns in the wide open top of the line