Digging out the details of this storm. How big was it?

CB Avalanche CenterAvi-Off-Season

Zach Guy
CBAC Forecaster
Natural deep slab off of Afley Peak

Since January 29th, the mountains around Crested Butte have been walloped by heavy snowfall.  On February 10th, the storm finally cleared.  My back is nearly broken from endless shoveling, my roof has avalanched at least twice, and I’ve heard lots of folks saying this is the biggest storm they’ve ever seen.  How historic was this storm?
The Schofield Pass SNOTEL site is a remote weather station north of Gothic Townsite that measures snow depth and snow water equivalent at hourly intervals.  Snow water equivalent (SWE) is the amount of water that would melt out of the snow falling from the sky.  An inch of rain measures as an inch of SWE, and a foot of snow typically produces about an inch of SWE.  Between January 29th and February 10th, Schofield Pass received 9.5” of SWE. That’s easily 10 feet of snow!  Since Schofield’s weather station was installed in 1985, four major storms stand out, with a return interval of roughly 6 years:  February 1986 (13.5” SWE), February 1995 (9.7” SWE), January 2005 (8.0” SWE), and December 2010 (9.4” SWE).  All of these storms came in unusually warm, with abundant moisture streaming from the Pacific.  Three out of four of these storms resulted in Extreme avalanche danger, with widespread and long-running avalanches into valley bottoms.  

This storm, with 9.5” SWE, is comparable in precipitation levels to these previous historic storms, but it fell over nearly twice the time span.  At the avalanche center, we compared our current storm to previous storm patterns to help decide whether to upgrade the danger to extreme.  Extreme danger means very large and destructive avalanches are widespread; the type of danger where we might expect Red Lady Bowl to come across Kebler Pass Road.  It’s a very challenging forecast when the mountains are socked in clouds and we have very few observations to work off of.  During this storm, we rated the avalanche danger as high during 5 days of the storm.  After the clouds cleared, we observed widespread avalanches and a dozen or more very destructive avalanches.  One ran off of Gothic Mountain, crossed the East River, and buried a structure 20 feet deep! Did we hit extreme danger?  I’m not sure..we were dang close.  There are two reasons we didn’t bump the danger to extreme on February 10th:  The storm reached its maximum precipitation rate early in the storm rather than late, and it happened over a span of 13 days, while the other historic storms came in over 6 or 7 day periods.   All in all, it was a memorable storm and ranks as one of the larger storms we’ve seen in the past 30 years.  Whether it was at the ski resort, in the backcountry, or driving your car to work, I hope you found some of that fluffy white stuff flying in your face!

Thanks to Art Mears and Ben Pritchett for their contributions to this article.

Large natural avalanches off of Snodgrass Mountain snapped trees and crossed Gothic Road


What is surface hoar?

CB Avalanche CenterAvi-Off-Season

You may have heard us warning backcountry skiers and riders about buried surface hoar in our avalanche advisories this month.  So what is this lurking predator and how do we know where to find it? 
                Surface hoar, sometimes called “hoar frost”, is that beautiful, glimmering, feathery crystal you’ve probably admired while strolling around town.  When it gets buried on a steep slope, it can be an incredibly fragile and dangerous weak layer that can persist for weeks or months.  Surface hoar is the winter version of dew.  In order for it to form, we need 3 ingredients:  clear nighttime skies without canopy cover, calm winds, and some relative humidity in the air.  Of course, these feathery crystals aren’t a problem unless they are preserved below a slab of snow.  In Crested Butte, we frequently see strong winds prior to the arrival of the next storm, which helps destroy those fragile layers.  Sun-baked slopes can also cook the layer into submission.  The take-home point here is that there are a lot of factors at play that can make for variable and spotty distribution of buried surface hoar; some slopes may have it while others don’t.

                We had a significant surface hoar event around Thanksgiving that was buried on some slopes after our early December storms.  We’ve been finding it preserved on shaded and wind protected or leeward slopes, especially at lower elevations.  It has been the culprit for a handful of avalanches these past few weeks.  This layer has been fairly easy to identify in a snow pit: it looks like a thin grey stripe in the snow (this is not always the case). 

The obvious grey stripe in the middle of the pit is a layer of buried surface hoar found on Schuykill Ridge this week.

We are just exiting another high pressure weather pattern that is favorable for surface hoar growth.  When you’re out touring in avalanche terrain, look for surface hoar and what slopes it is forming on to give you a better idea of places to avoid if it gets buried and becomes problematic.  And don’t forget check our website for the most up-to-date avalanche conditions before you head out.  www.cbavalanchecenter.org

-Zach Guy

No Niño??

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For the past several years, much of the early season buzz about whether we were going to have a killer winter or a dud revolved (over. and over. and over…) about the pros and cons of an El Niño or La Niña   atmospheric and oceanic signal.  

Here is a post from one of my former meteorology professors at the University of Utah and total powder junkie himself, Jim Steenburgh.  He runs a great weather blog that discusses meteorology mostly around Utah, but a ton of his material is applicable to our locale as well.  Bookmark it and check it out! Infer what you can about how the lack of ENSO will affect our weather patterns…the overarching theme being….your guess is as good as mine.  A good perspective if nothing else.

http://wasatchweatherweenies.blogspot.com/2013/08/outlook-for-20132014-ski-season.html

My take is that the Arctic Oscillation is just as big of a contributing factor that is often over looked. “The positive phase of the Arctic Oscillation brings ocean storms farther north, making the weather wetter in Alaska, Scotland, and Scandinavia and drier in the western United States and the Mediterranean. In the negative phase of the Arctic Oscillation the patterns are reversed. A strongly negative phase of the Arctic Oscillation brings warm weather to high latitudes, and cold, stormy weather to the more temperate regions where people live. Over most of the past century, the Arctic Oscillation alternated between its positive and negative phase. For a period during the 1970s to mid-1990s, the Arctic Oscillation tended to stay in its positive phase. However, since then it has again alternated between positive and negative, with a record negative phase in the winter of 2009-2010″(National Snow and Ice Data Center).

What tilt are we in now you may ask??
These are very interesting (and confusing) but if you look at each graph, you have a neutral AO signal at zero, with the blue bars going up meaning positive, going down, negative. Makes you think a little what phase we have been in for extreme weather, like our early season snows, the Front Range floods in September, etc.
More info here:

I will probably steer some of my posts of this forecaster’s blog to Jim’s blog occasionally.  Enjoy this snowy Sunday…so far I like No Niño…

-Ian Havlick

TED Talk on Avalanche Hazard and Risk

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Grant Statham gave a great TED talk recently on risk in avalanche terrain that has been circulating around the avalanche community lately.  Grant has spent a lifetime climbing, skiing and guiding in extreme terrain around the world and he has spent the past 10 years or so working as a risk management specialist for Parks Canada.  Grant does a great job of explaining risk and hazard and how it relates to travel in avalanche terrain in this TED talk.

See Grant’s 12 minute talk by clicking HERE or on Grant’s photo below.


Grant Statham from Parks Canada

The components of avalanche risk from Bruce Tremper’s (Utah Avalanche Center Director) new book Avalanche Essentials, which illustrate the points Grant makes in his talk.

-Ian Havlick
CBAC Forecaster

Spring Skiing and Wet Slabs

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Spring is a great time to get into the backcountry and enjoy the expansive mountain ranges surrounding town.  You can safely pass through steep, avalanche terrain on most days, but there are still a few hazards that need to be on your radar, one of which is wet slabs. 
Presently, our high peaks either have a dry, winter snowpack (on north aspects), or are in some transitional stage as you move towards southerly aspects.   Many of these slopes have weak layers at the bottom of the snowpack that formed early this winter.   As the days get warmer and the sun angle gets higher, meltwater at the surface begins to percolate through the dry snow. We call this the “wetting front.”  As water moves through the snow, it changes the snow’s properties, causing it to weaken.  When it refreezes at night, frozen ice bonds form which make for a very strong and very stable snowpack.  When the wetting front advances to a weak layer, that layer can lose all of its strength and fail, resulting in a huge, wet slab avalanche. These are one of the most destructive types of avalanches we see.
Every winter we have weak layers near the ground, and every spring meltwater percolates through them.  Yet in most years, we only see a handful of wet slabs.  These are very hard to predict on a slope by slope basis, but you can eliminate your risk with a few spring skiing/riding practices. 
(1) Wet slabs are most common after several consecutive nights without a good refreeze.  Avoid avalanche terrain if it didn’t refreeze well overnight.  Clear, cold nights make for a good refreeze; cloudy, warm nights leave a poor refreeze.  Look at mountain temperature sensors (found at https://cbavalanchecenter.org/page.cfm?pageid=12570) and note the cloud cover before you leave home.  When you’re on slope, you can dig or probe to feel how thick that frozen block of snow is. On your hike up, if you are breaking through to your waist in wet grains, that’s a bad sign. 
(2) Wet slabs are most common during rapid or unusual warm-ups.  Plan to get off of slopes early in the day, when it is an inch of nice corn skiing rather than 8” of sloppy slush skiing.  This often means starting your hike before sunrise.  If you got a late start, go towards slopes that get sun later in the day, such as west or north aspects, rather than east or south aspects.  With your descent, anticipate that slopes lower in elevation or more easterly in aspect warmed up quicker; you might have to ski some frozen crust up high to play it safe down low.  Don’t go skiing if it is raining (duh).  If weather forecasters are talking about record high temperatures, go biking or drink beer on your deck. 
Thanks from the CBAC for another great season! We couldn’t do it without your support and observations.  Have a fun and safe spring ski season. 

-Zach Guy
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When do Persistent Slabs go away?

CB Avalanche CenterAvi-Off-Season

Steve Banks
Executive Director
CBAC

You may have noticed that the Crested Butte Avalanche Center’s daily bulletin has been talking about the persistent slab problem for most of the winter now. We have also had some other avalanche problems such as storm slabs, wind slabs and even wet loose avalanche problems. These problems seem to come and go, while persistent slabs stay in the bulletin day after day. So when will the persistent slabs go away?
Persistent slabs is the name we give to the avalanche problem when we have a weak layer that is made up of faceted snow grains, depth hoar or surface hoar with a thick denser layer above. These types of weak snow grains tend to change very slowly and often linger throughout the entire winter season. Really it is a persistent weak layer that is the problem, but since it is the slab that could potentially kill you, we call it a persistent slab avalanche problem. These particular weak layers need a lot of time to morph into a better type of snow grain, and often we get into full on spring corn cycles before these layers begin to look any better.
So it would seem that if persistent slabs are really a problem, we must be triggering a lot of avalanches, right? Well, not really. These weak layers can have a certain amount of strength to them. The trick is to be able to recognize where and when they are strong enough to support the overlying snow as well as the additional weight of a backcountry rider. This is why we perform snowpack test like compression test and extended column tests. We are trying to determine how strong the weak layers may or may not be. Given the idea that stronger layers over weaker layers in the snowpack create avalanches, merely looking at the structure of the snowpack would indicate dangerous conditions. However snowpack tests are beginning to show harder results leading us to believe that the snowpack is gaining strength.
Now comes the tricky part. If the weak layers creating the persistent slab avalanche problem are becoming stronger, how do we know where and when they are strong enough to trust? It is always difficult to assess where an avalanche will occur, but it is especially difficult during times of relatively benign weather. This is when forecasters will be talking about low likely hood and high consequences avalanches. While it is difficult to initiate the slide, the resulting failure could entrain a lot of snow and have a life threatening outcome. So where would you be more likely to trigger an avalanche? The best way to know the answer is to dig in to the snow to check out the layering and perform some tests to see just how strong the slab is and just how weak the weak layers are.  In general we have a weaker snowpack closer to the town of CB where the snowpack is shallower. Stronger snowpacks can be found deeper in the mountains where the snowpack is deeper. When in doubt, err on the side of caution, and always remember to check the current avalanche bulletin at www.cbavalanchecenter.org.

Interpreting Snowpack Layers and Hardness

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When the word avalanche is mentioned, images of curtains of snow cascading down the mountain come to mind. Specific conditions create avalanches. One of the basic elements, of course, is snow. The snowpack (the seasons snow on the ground) is a complex, but a key to understanding it is identifying layers and differences between them. Layers within the snowpack are a record of the winter’s weather. Like tree rings or strata of rock, layers can be traced to dates and conditions that formed them.
 One of the most important characteristics of a layer is its hardness. Harder snow is stronger and cohesive, while softer snow is weaker. For a slab avalanche we need a strong layer over a weak layer. We actually use a “Hand Hardness” scale by pushing our hand into each layer to determine the hardness. “Fist” hard snow is the softest, then “four finger”, then “one finger” snow being harder, and so on. No, there’s no “middle finger” on this scale. The greater the hardness difference of neighboring layers, the more likely we are to see avalanches at that interface.
The recent snowpack history from the Crested Butte backcountry is a great example of layer hardness relating to human triggered avalanches. Consider these three scenarios:
1) The last week in January brought up to 50” of snow over seven days. While this storm caused some natural avalanches, riders initially weren’t triggering many slab avalanches. Most of the snow came in light and soft. Digging in sheltered and shaded areas during the middle of the storm, you’d find very soft (fist hard) new snow on top of some slightly harder old snow.
2) The storm ended on Thursday January 31st. By then the wind had formed stiff slabs at all elevations but especially near and above treeline. Digging on some leeward slopes near treeline you could find one finger hard wind-loaded snow sitting over four finger hard facets. Many reports of natural and skier triggered slides came in from wind-loaded areas.
3) By Monday, February 4th, the storm had ended four days prior. Mild daytime temperatures had created settlement in the height of the storm snow and an increase in its hardness. I was skiing on sheltered easterly aspects below treeline.  These slopes had seen very little wind in the past week. Throughout the day, we felt many rumbling collapses and we remotely triggered two sizable avalanches. On these sheltered slopes we found storm snow that had stiffened to a hardness of four finger sitting over fist hard facets.
This storm and avalanche cycle in the late January to early February shows how layer hardness often relates to skier triggering. The most consistent skier-triggering occurred where we found the most change in hardness in adjacent strong over weak layer. If this talk of layers seems daunting, remember that you don’t have to figure it out on your own. The daily avalanche forecast is best place for current conditions. You can check out the “Observations” page to read what other backcountry travelers are seeing. You could even look at the “Snow Profiles” page for technical graphs of recent snow pits. From there you can investigate the snow as much or as little as you want. But most of all have fun in the mountains, make sound decisions, and stay safe doing it. Check out the daily forecast at www.cbavalanchecenter.org
CBAC Forecaster Josh Hirshberg