PWL: Rain Crust Observations

Wanted to post this thread to start a discussion regarding observations of rain crusts.

After a BC session around Crystal on Saturday (12/, signs of this PWL were very apparent... Looks as though we may be setting up for a rather dangerous avalanche season with such an early PWL...

Curious to hear about other's field observations.

I'm curious to know the thickness.

I'm curious to know the thickness.

I observed the same thing as Link...
I did not get down and measure but I noticed that it was a veneer large grain ice layer probably 3-4 mm thick and crumbled with any aggressive finger swipe.
I know that does not conform to the agreed standard of data input.
So call it Med/Hard surface; Granular

I have seen at least two of these similar layers on Hasty pits due to the warm ups and then new snow or transport. Some of the rain has been limited at higher elevation (5500ft and above) and the pack did not get quite the soaking as the lower elevations.
IMO the recent warming Sunday 12/9 will do the same thing since it was followed by colder temps. I need to follow up to see the results, but my gut tells me that the bridge will be weak in that one too.

Is this the rather thick crust from the significant rain event on or around Nov 23rd, or a more superficial layer?

Is this the rather thick crust from the significant rain event on or around Nov 23rd, or a more superficial layer?

I think its the one from around December 4-5. We were able to find it at Crustal last Friday underneath the most recent storm snow. It wasn't there on 12/2 or we didn't notice it in the fresh stuff we were skinning up. On 12/7 it seemed like it was holding okay to the rain crust but that's just based on some crude hand shear tests. The snow wasn't real consolidated so it wasn't the best observation setting.

Like Chuck's info...I did not dig that far down, so I am referring to the upper layers that have been effected by the recent temp fluctuations and loadings sandwiched between them.

On Monday 12/10 out of white pass I hiked to 6500 ft elevation and skied a NE aspect that was rain crusted to a half inch thickness and very difficult to ski. The rain crust was everywhere and my hopes are that it has settled some or the new snow in the forecasts will not be so much as to be crowns instead of sloughs. My thoughts are wondering if this crust was eventful in the Baker area.

We skied in the backcountry near Steven Pass yesterday and found a variable rain crust under about 6" of light and dry snow all the way to the ridge at 5500'. It was widespread but seemed more pronounced on N aspects at higher elevations. The new snow is not well bonded to the crust, which is about 2mm thick and easily breakable by hand or ski (no significant effect on skiing). We saw no obvious signs of instability at this point. Other than the thin breakable crust, the snowpack looks great up there with a near perfect density profile.

Good input. Yes, I'm speaking particularly of the most recent rain crust resulting from last weeks warm up... (12/4...)

I skied 12/2 around Silver Basin. It was my first day up this season, so my only input on the snowpack (prior to getting on the slopes) was studying the 10-day telemetry and peeking at a Thanks Giving Norse Peak TR that was posted. I was leery of the Post-Turkey Day crust with lot's of new on top of it, but the dragon that day was heavy windloading near the ridges; we saw some naturals go but without much run out. The Post T-Day crust didn't seem to be much of a factor on the N-NW slopes we were skiing...

The most recent crust seems to be quite the bed surface, or lack thereof. Cohesion seemed to be scant upon skiing around Blue Bell Pass 12/8 . The snow definitely wanted to move. It just looks like a nasty PWL, for the sake of term. I'm a bit concerned about the sandwiching going on, and with little bonding to the most recent crust. Kinda hoping for a warm storm to roll through for an early cleanse.

Skied Jim Hill Mtn on 12/13. The thin crust is present there too, all the way to at least 6400ft. It's thin enough that it didn't affect skiing quality though. There was about 3 inches of cohesionless snow on top of it at lower elevations, and variable amounts at higher elevations where things were more wind-affected. Lots of surface hoar.

The telemetry seems to indicate minimal snow accumulations since last weekend, and a high of 32-degrees on Sunday at 6830' from surveying the Crystal Mt data. Seems like it's been depositing right-side up since then but in obviously small accumulations.

NWAC speaks of a hoarfrost layer from last week. The density of last weeks rain crust seems like a pertinent question. Perhaps there is a level of bridging happening over what snow accumulated up to 12/8, assuming it is thick enough to hold, which then shifts present concerns to the mentioned hoar layer. But if the rain crust is weak, then it seems like we have a potential danger-sandwich on the cutting board- rain crust-> low-density snow-> hoar layer-> this weekends storm.

I'm not up in the hills enough thus far, so much of what I'm typing is speculative; I'm thinking aloud for the sake of recognition and to share my input for discussion purposes. I may not be giving enough credit to time and settlement of the snowpack, but can't help to acknowledge the existing dangers.

A ton of rather cold, low-density powder appears to be on the way flowing out the west. Be careful out there.