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Author Topic: February 16th, Bluwett Pass Windslab Overnight  (Read 3727 times)
avajane
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February 16th, Bluwett Pass Windslab Overnight
« on: 02/16/14, 07:49 PM »

Yesterday I posted a photo and trip report showing off some light 3-4" of powder over a firm and smooth, easily skiable crust. More loading overnight would make the slope dangerous - but since there was only about one inch of new - I went back up to the same spot. Today the parking access was inexplicably blocked by a single black Jeep, so I had to risk parking right on the edge of hiway 97. During my approach this morning, I noticed that despite only an inch of new snow, my tracks from yesterday afternoon were almost completely gone. (That should have been a clue) Everything looked about the same, until I came to my 3,600 foot highpoint on an exposed and lightly treed ridge.

Instead of 3" of powder over crust, there was a solid foot of windslab over the light powder creating what looked like a dangerous situation to me. Perhaps this happens often, but I was shocked to see just how much slab could blow in - in 20 hours! I took a couple of pictures and threw some on the slab to show off it's stiffness. I turned around and had an uneventful run thru the trees.

I have skied the backcountry quite a bit over the years but I am essentially still a resort skier. From those of you more experienced than I...Does this happen very often? And was I correct in assuming that this much windslab over 3" of powder on top of a firm crust was to be avoided?



* IMG_0787_edited-1.jpg (145.34 KB, 777x800 - viewed 1522 times.)

* IMG_0788_edited-1.jpg (174.92 KB, 694x800 - viewed 1527 times.)
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Brian Izdepski, Facebook TAY
Charlie Hagedorn
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Re: February 16th, Bluwett Pass Windslab Overnight
« Reply #1 on: 02/16/14, 08:54 PM »

Wind can move a lot of snow quickly. The FSAvalanche page claims deposition rates as high as 10 times that of natural snowfall.

http://www.fsavalanche.org/Encyclopedia/wind_loading.htm

At Alpental yesterday, a 10" windlip leading into the lower entrance to Upper Nash rarely showed more than two tracks, even though skiers were skiing through it at least once every minute or two. When conditions are right, you can see far more loading than 1' in 20 hr.

In the situation you describe, your decision sounds astute.  A decision to select a safer option in the face of uncertain, unexpected, and potentially-dangerous conditions is laudable. A foot-thick slab atop a potential weak layer deserves respect.
« Last Edit: 02/16/14, 09:01 PM by Charlie Hagedorn » Logged

JoshK
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Re: February 16th, Bluwett Pass Windslab Overnight
« Reply #2 on: 02/16/14, 10:32 PM »

Yeah, wind can move an amazing amount of snow very quickly. An area like Blewett (dry side of crest, lighter snow, shallower snowpack) can showcase this well. My GF and I were up there on Friday and it was quite windy. We played around on the ridges and in the trees and I dug a big pit for her for the purpose of education. I'd say average snow depth was 2 1/2 feet, give or take,but within a fairly small area we saw everything from bare ground to roughly 6' of snowpack where I dug the pit.

You were right to be concerned about a wind slab of that thickness, especially if it was quite hard. You are more likely to see that in open areas or cross loaded terrain. Heading for the trees was a wise call as they provide somewhat of a natural windbreak and discourage formation.

Wind is the single worst weather phenomenon in the winter in my book. It ruins snow, makes things dangerous, makes you cold...it pretty much just sucks. And as you saw, it can do all of this very quickly!

We were hoping to go back up there tomorrow but based on what you saw, perhaps we will try elsewhere...
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aaron_wright
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Re: February 16th, Bluwett Pass Windslab Overnight
« Reply #3 on: 02/17/14, 06:48 AM »

Were you near King Creek avajane? We were up there yesterday on the other side of the highway between 5 Mile and Tronsen. I was surprised at the variability in snow depth. Roughly 2 feet on the level in protected areas and drifting on the lee of ridge tops to 4 feet or more. In exposed areas and under the canopy just a thin layer over dirt and pine needles.

Yes snow can collect a lot faster than that. At Mission I've seen foot thick wind pillows form in less than an hour on Windy Ridge.
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avajane
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Re: February 16th, Bluwett Pass Windslab Overnight
« Reply #4 on: 02/17/14, 08:58 AM »

Yes Aaron, that was the area. Only been able to ski there recently. After the first snow I went up but it was a disaster as there was 18" of dry powder and the bottom was rock and dirt.
The crust that formed during the warm spell allowed this weekends skiing. 
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Brian Izdepski, Facebook TAY
kerwinl
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Re: February 16th, Bluwett Pass Windslab Overnight
« Reply #5 on: 02/17/14, 04:46 PM »

The windslab in general will be denser as the wind breaks down the snow crystals and allows them to pack in more tightly, windslab (dense snow) over light powder is upside down in terms of ideal snowpack (where you would want the most dense layers on the bottom). It is advisable to avoid windslabs over weak snow (upside-down snow), as the wind-slabs are most likely not bonding to anything underneath them, and thus relying on the strength of the slab to prevent release.
« Last Edit: 02/17/14, 04:53 PM by kerwinl » Logged
avajane
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Re: February 16th, Bluwett Pass Windslab Overnight
« Reply #6 on: 02/17/14, 06:27 PM »

The windslab in general will be denser as the wind breaks down the snow crystals and allows them to pack in more tightly, windslab (dense snow) over light powder is upside down in terms of ideal snowpack (where you would want the most dense layers on the bottom). It is advisable to avoid windslabs over weak snow (upside-down snow), as the wind-slabs are most likely not bonding to anything underneath them, and thus relying on the strength of the slab to prevent release.
I knew that intuitively - but I hadn't put that together with the often used term "upside down snow". This condition looks to be there for some time now, and buried under a bunch more weight.
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Brian Izdepski, Facebook TAY
JoshK
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Re: February 16th, Bluwett Pass Windslab Overnight
« Reply #7 on: 02/17/14, 09:06 PM »

We were up at Blewett today. It's amazing to see how much snow is there now compared to when we visited on Friday. It looks like mid-Winter now instead of late fall!! The snow was great, but we just did some of the XC trails and low angle trees for exercise before going to ski at Snoqualmie. Whatever layers were of concern that avajane mentioned are now buried under an impressive amount of new snow. I'd be curious to hear what others who go up there this week find in terms of snow stability and any findings from a deep pit.

Oh, and as a bonus the WSDOT was so busy keeping the roads clear they couldn't dig out the Sno-park so only a couple of sno-mos were around making noise. Smiley
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bc_skier
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Re: February 16th, Bluwett Pass Windslab Overnight
« Reply #8 on: 02/18/14, 06:10 AM »

An 'upside down snowpack' is also referred to when a storm comes in cold and goes out warm. You get low-density snow (cold dry) on the bottom, then when the temperatures warm up the top layer will be high-density (warm heavy) snow. Upside down snowpack. This would be an unstable scenario. Also can be referred to as 'storm slab'.

A 'right side up snowpack' refers to a storm coming in wet and going out cold, which is a good scenario. The wet snow typically will bond better to the older surface snow and when the air/snow temp cools down the snow bonds.

All bets are off if the winds pick up creating wind slab.

All conditions and stability should be confirmed in the field.

This is going to be a tricky week in the backcountry with more than average amounts of snowfall, wind and fluctuating temperatures.
« Last Edit: 02/18/14, 06:23 AM by bc_skier » Logged
avajane
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Re: February 16th, Bluwett Pass Windslab Overnight
« Reply #9 on: 02/18/14, 09:34 AM »

If there is another foot or two of snow on top of the hard crust near the bottom of the pack, I doubt that I would even go up the logging road as the areas between the switchbacks are very steep and not anchored by trees in many areas.
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Brian Izdepski, Facebook TAY
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