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.

South facing slopes were the go-to for many folks in town to find stable skiing and riding during our December storm cycle. The snow fell on dirt on many low elevation sunny slopes, rather than the problematic facets that were lurking on shaded slopes. Someone asked me recently, “When do south aspects become dangerous?” It depends, of course, but a lot of it has to do with crusts. With all the sun we’ve been getting in January, we’ve seen melt-freeze crusts form on most of our southerly slopes. So what does this mean for future stability and what happens when snow piles on above crusts?

There are a lot of variables at play, so I’ll illustrate with a few examples. The best case scenario is that we have a thick, stout crust that is still warm as a new storm arrives, and the new snow falls warm and wet. Instabilities at this crust/new snow interface will be short-lived because a good bond will form. This pattern is more often played out in the springtime, which is why we can sometimes find stable powder skiing after a spring storm drops snow on widespread crusty surfaces.

The worst case scenario is if we have facets form and stay preserved above a crust prior to a big snowfall. This can happen in a number of ways. Mid-winter, we will occasionally see crusts form on days when it is still pretty cold, but there is strong enough solar radiation to warm and melt the snow just below the surface. The actual surface of the snow stays cool because it radiates heat, so what we have is a thin crust forming below cold faceting snow at the surface. Suddenly we have a bed surface with a weak layer waiting for a slab to ruin someone’s day. A similar scenario is if we have a crust that has been forming for days or weeks, and then we get a dusting of snow on top followed by clear and cold weather. This gives us another scary bed surface with facets developing above it. Often these facets above crusts are difficult to spot with the naked eye after they are buried by a slab, but a stability test will usually give them away.

Now here’s the really tricky part. When a crust gets buried, it can have unusual behavior. Crusts act as vapor barriers to the normal, everyday movement of water vapor through the snowpack (Picture TSA security clogging the flow of people at an airport). This can lead to faceting above or below the crust, even if the bonds were originally strong. This type of problem is most common in cold, shallow snowpacks such as what we have in Crested Butte right now. We have already observed pronounced facets developing under our current crusts on south aspects. With a big enough load, that crust won’t be able to support the weight above it and the whole thing will come crashing down, failing on those facets below the crust.

So if I’ve lost you with all this technical jibber jabber, here’s the bottom line. Crusts have the potential to be a dangerous interface as more snow piles above them. Treat a buried crust as guilty until proven innocent. Watch for signs of instability and dig down to check how the snow is bonding to that crust before committing to steep terrain. And remember that south facing slopes that you were skiing or riding safely back in December may no longer be stable if we get another big storm cycle. For daily avalanche advisories and observations, visit cbavalanchecenter.org

CBAC Forecaster Zach Guy

Recent research in the snow and avalanche world is showing that how a compression test fails is just as or more important than the number of taps it takes for it to fail. Researchers have come up with a classification system to qualify and describe fractures in snowpack tests. This system is known as Fracture Character. Fracture Character does not necessarily replace Shear Quality—or the “Q scale.” It compliments it, and can even provide a bit more information about the relationship of the weak layer and the overlying slab. It breaks fracture type into 2 main categories—sudden fractures and non sudden fractures. This post will solely deal with sudden fractures.

Sudden fractures are what avalanche practitioners call “pops” and “drops” in snowpack tests. If the fracture is a pop, a thin crack will cross the column of snow with one loading step (or tap) and the block will slide off the weak layer easily. It could be your 5^th tap or your 20^th—the key is that one tap initiates a crack that crosses the entire snowpack column. This may not be as obvious as the old analogy of “it shot out like a cash register drawer.” On lower angle slopes you may have to grab the block and give it a slight tug. If it slides off without much effort you are still dealing with a pop. If it is a drop there will be an observable (and sometimes audible) collapse on the weak layer that occurs with one loading step, or tap. If you flip over the block it may not necessarily be smooth. Drops in depth hoar will commonly look rough when you examine the fracture.

Why is this important? Researchers have correlated sudden fractures with skier triggering avalanches. Recent studies have shown that “Fracture Character is a valuable addition to the compression test score since most failure layers of slab avalanches produce sudden fractures in compression tests” (van Herwijnen and Jamieson 2004a). Paying attention to whether your compression test “pops” or “drops” can give you a better indication of whether or not a specific slope has the potential to avalanche. This is also great information to include when you send in your observations to the CBAC.

Remember that performing snowpack tests is just one part assessing the snowpack.  No one test can tell you if a slope is stable. If you want more information on this topic check out Alec van Herwijnen and Bruce Jamieson’s 2004 paper titled Fracture Character in Compression Tests. The chart below compares Fracture Character with shear quality and provides definitions for Fracture Character. 

Works Cited

van Herwijnen and Jamieson 2004a. Fracture Character in Compression Tests.” ISSW 2004.

 

                  With the arrival of some real snowfall and wintery conditions, the ski conditions have drastically improved.   But with the new snow came plenty of avalanches in the backcountry around Crested Butte.  If you are just now digging out your winter sticks and brushing the dust off your beacon, here is some recent news and important reminders about avalanche beacons.

                  John Barkhausen of Prescott College recently published his research about whether electronic devices interfere with avalanche beacons.  You’ve probably heard rumors that having your phone turned on could affect your beacon’s performance.  John tested this by systematically checking the effective searching range of a beacon and with different electronic devices turned on, such as cell phones, iPods, GPS units, radios, SPOT locators, and digital cameras.  The good news is that these devices have very little effect on a sending beacon (i.e. the buried victim’s beacon).  The bad news is that a number of electronics significantly reduce the ability of a searching beacon to pick up a signal.  IPod’s and cameras were especially bad.  However, when a searching beacon is held more than 40 cm away from the various devices tested, interference is essentially gone.  This is about arms length away.   So the bottom line is hold your beacon away from your body if you have any electronic devices on.  It’s probably a good idea to turn off your cell phone and reconsider using a chest-mounted GoPro.  If you’re listening to your IPod, then you probably didn’t hear the rumbling collapse that would have warned you of dangerous avalanche conditions in the first place.

                  Another recent study in Europe showed that novice/average users failed to find the last burial victim in a triple burial scenario in roughly 30% of rescue scenarios.  This was attributed to failure of the marking function that most modern beacons are equipped with.  While multiple-burial marking functions are a great addition to beacons and can save precious time, it is important to remember that they are not fail proof.  The bottom line is that you need to practice backup search strategies too.  For more information about these techniques, check out http://beaconreviews.com/transceivers/multipleburials.asp  If beacon searches are a foreign skill to you, check out one of the many avalanche classes offered in the valley.

                  And lastly, some friendly reminders.  Don’t use rechargeable batteries or lithium batteries in your beacon; they could cause unexpected loss of power while you’re out in the backcountry.  Make sure to change your batteries when they get near 50% power; lower power can reduce effective search ranges.  Make beacon checking part of your daily routine, and most importantly, Practice! Practice! Practice! 

Zach Guy

 

This article will be published in this week’s Crested Butte News.

By John MacKinnon

CBAC Forecaster

The long-range weather models are beginning to show those dark red and purple bulls’ eyes of precipitation stacking up over Colorado in the coming week. This is great news because it means winter is finally on its way to Crested Butte; but it is also important to remember that any significant snowfall will bring a return to dangerous avalanche conditions.

Many people are already comparing our delayed start to winter to last year. What both years have in common is a couple of early season storms between late October and mid-November which have been followed by long dry spells. Long dry spells with cold clear nights and mild sunny days have provided the perfect ingredients for our existing snowpack to morph from snow-flakes into large angular snow crystals known as facets. During both years, much of the early season snowpack turned into weak sugary snow during the first part of winter.

So this brings us to the question of what is going to happen when it actually starts snowing. If Monday’s storm (12/3/12) is any proxy for what will happen when we get a significant load of new snow, the results following the upcoming forecasted storms could be impressive. The storm from earlier this week dropped between 1-5” of snow onto our fragile snowpack. As winds pushed this snow around and drifted it on to specific slopes, many small avalanches occurred—most human triggered but some occurred naturally. The weak faceted snow crystals could not support even the smallest of loads.

It is especially important to remember the effects of Monday’s storm if we get a good 12” of snow in the next couple days. If Monday’s storm was akin to throwing a couch cushion onto 50 fluted champagne glasses, imagine what would happen if you began stacking mattresses on those same glasses. Our weak base of old snow would quickly and dramatically fail if loaded with a foot or more of new snow. A mix of patience, caution and respect could be the best tools for dealing with backcountry riding for some time.  

The Crested Butte Avalanche Center is now open for the 2012-2013 season! While the first year the CBAC was in operation was 2001, this is our 10^thanniversary of becoming “officially” incorporated as a non-profit entity.  Since we opened ten years ago the CBAC had grown into a professional forecasting office with a staff of 4 forecasters and a board of directors of 10 local professionals and backcountry enthusiasts with diverse backgrounds. New this year we will be introducing CBAC Ambassadors who will be the extended face of the CBAC out in the backcountry and here in town as well.

Look for the CBAC to be making some changes to our website product this year. The avalanche forecasting community is making a move to become more consistent with what the avalanche danger means and how to determine what the avalanche problem of the day is. Once the main avalanche problem is determined, it is a bit easier to figure out how to manage the risk for that particular day. Not all avalanche problems are created equal, and not all problems should be dealt with in the same way. Hopefully our new format will make it more evident to backcountry users as to what their considerations for the day should be. Of course we will still have days when our complex snowpack dictates that there is more than just one avalanche problem out there!

While things are slow to get rolling with snowfall in the backcountry so far this year it is important to remember that if there is enough snow to play in, there is also enough to create an avalanche.  While the current avalanche danger is limited to specific areas (where there is actually accumulated snow!) we have already seen some large and impressive slides earlier this month. In your quest to get your season on track, remember to take the time to pay attention to the snowpack. Not just for today’s avalanche conditions, but for what might evolve for the rest of the season. When we get early season snow, and then long dry periods, the snow tends to “rot” or facet. This faceted snow will create a weak layer near the ground, which will be the supporting snow layer for additional snows to (hopefully) come. With a weak supporting structure we can expect to have an active avalanche season! The good news is that with a lot of bare ground showing out there right now, there is no weak base structure yet in some areas. These might be the best places to play in the coming winter.

With not much quality skiing or riding out there just yet, this is a great time to brush up on your avalanche knowledge. The CBAC, in conjunction with Crested Butte Mountain Guides and the Alpineer will be hosting a “Beacon Brush Up” day on Saturday December 1^st. Come on out from 11-3 in the Town Park to learn or practice your beacon skills, demo new avalanche beacons and airbags and get tips from the pros. The Alpineer will host a gathering afterwards from 3-6pm with food and beer as well as special discounts on avalanche gear.

For more information on avalanche danger ratings or avalanche problems, to get the daily avalanche bulletin in the Crested Butte area, to submit your backcountry observations, or to become a sponsor of the Crested Butte Avalanche Center, check out our website www.cbavalanchecenter.org.

Figure 1

First off, let’s get some defintions out there: According to SWAG, avalanche danger, or avalanche hazard, is defined as “the potential for an avalanche to cause damage to something of value [people or property].” A leading authority on avalanche forecasting describes an avalanche danger rating as “a ranking of avalanche danger for a period of time over a specific region.” (Stratham et al 2010) 

The CBAC issues an avalanche danger rating of LOW, MODERATE, CONSIDERABLE, HIGH or EXTREME for specific aspects and elevations within our forecast area–the greater Crested Butte backcountry which roughly covers 100 square miles centered around the town of Crested Butte. This creates the Avalanche Danger Rose (Fig. 2):

Figure 2

So what goes into the danger rating? It all starts with the forecasters assessment of snow stability and how the current and future backcountry conditions fit with the avalanche danger definitions. The forecaster looks at the likelihood of both human-triggered and natural avalanches as well as the size and distribution of avalanches. Each level of avalanche danger has a specific definition for those categories (Fig. 1). 

Easy enough, right? If it could only be so simple! It has been very challenging here at the CBAC to attach an avalanche danger rating to our snowpack since the first major storm of 2012 arrived on January 16th. We have been reading (and re-reading) all the qualifying and defining traits of each danger level and assigning danger based on those definitions. Each morning, we look at the probability of both natural and human triggered avalanches and then think about how big avalanches will be and where they will be occurring in the terrain. The resulting product has been a strict interpretation of MODERATE or CONSIDERALE danger that appropriately sums up the avalanche conditions but does not necessarily connote how “dangerous” it is out there. This is where the “SCARY” modifiers have been coming into play this season. 

While not an official danger rating, forecasters often attach or imply the “SCARY” pre-fix during times when the danger is at the upper end of either MODERATE or CONSIDERABLE. Often times during SCARY MODERATE, there is a lower probability of human-triggering an avalanche but the resulting avalanche has the potential to be very large, and deadly. 

We dealt with SCARY CONSIDERABLE after the January 16th storm. 1.3″ of water fell on a mostly shallow and weak snowpack. The danger did not spike all the way to HIGH because the size and distribution of avalanche activity did not fully meet the HIGH requirements. We did not see “large avalanches in many areas”– a clause that helps define HIGH danger. Instead we observed “small avalanches in many areas and large avalanches in specific areas”–two of the defining traits of CONSIDERABLE danger. It was SCARY CONSIDERABLE because we were pushing the limits of the definition, but the way the storm and subsequent avalanche cycle played out fit into CONSIDERABLE danger. During the peak of the storm–the night of 1/16–the probability of human triggered avalanches may have jumped to “Very Likely”–a defining trait of HIGH–but the natural activity observed over the next several days all pointed to the fact it was CONSIDERABLE danger. 

The storm that arrived the following week–the January 22nd storm–dropped over 2″ of water onto our weak, top-heavy snowpack. The danger jumped to HIGH post storm due to the combination of a larger load of snow, winds, and our increasingly unstable snowpack. The result of that storm was a spike in danger to HIGH for 24 hours. More larger avalanches were observed after the 1/22 storm than the 1/16 storm and HIGH danger was warranted. 

The danger then hung out at CONSIDERABLE for the next 2 weeks on many aspects. Why? Naturals weren’t occurring, but human triggered avalanches were “LIKELY” on most slopes. Going back to the definitions (Fig. 1), “likely” triggers are enough to keep the danger at CONSIDERABLE. Also, all the reports of cracking and collapsing hinted that remote triggered avalanches were a real possibility as were “large avalanches in specific areas and very large avalanches in isolated areas.” 

Currently we are at SCARY MODERATE. Human-triggered avalanches are no longer “likely,” but they are “possible” and even very possible on certain slopes. Why is the “Scary” pre-fix still being tossed around the forecasting community? We use the term SCARY MODERATE during times when the probability for triggering an avalanche is slowly decreasing, but the consequences remain severe and even deadly. 

The Bottom Line: Our snowpack remains dangerous despite the fact it is getting harder to trigger an avalanche. Don’t let decreasing signs of instability lure you into exposed and consequential terrain. Many fatal avalanches or close calls (think Lucky Boy Slide on Whetstone) occur during SCARY MODERATE danger. Do not let your guard down and treat all slopes over 35 degrees with extra caution. 

John MacKinnon

Forecaster