What are facets?

By Zach Guy

CBAC Forecaster

If you've been reading our avalanche advisories lately, you've probably noticed we've been talking a lot about facets.  Last weekend, major winter storm slammed into our mountains, and we saw widespread avalanche activity, with facets being one of the major culprits.  So what is the deal with this snow grain type, how does it form, and why is it so problematic?

Facets are a sugary-like snow grain.  They glisten in the sun, bounce in your glove, and commonly make a noticeably soft or hollow layer in the snowpack.  If you look at them under a magnifying glass, they have many flat edges, hence the name facets.  

Courtesy Photo

There are several different ways that can cause snow grains to facet, but they all share the same underlying physical process.  Facetsform when water vapor moves quickly through the snowpack. Each particle gets handed off between grains, via sublimation and deposition, causing the grains to reconfigure into a more angular form.  Strong temperature differences within the snowpack cause this water vapor relay.  We can see facets form early season when the ground is quite warm, there is a shallow snowpack, and the air gets cold, especially at night.  This drives the water vapor upward through the snowpack causing it to facet, or rot out.  We also see a similar process occurring on top of the snowpack, called "near surface faceting."  When we get warm days and cold, clear nights, the snow surface undergoes wild temperature swings, causing the same rapid water vapor movement. This process is expedited if the surface is composed of soft, low density snow, rather than stiff, hard, compacted snow.  All of these ingredients have come together this winter.  We have seen basal facets form during the early season when our snowpack was shallow.  Then, on December 1st, we got about 4" of new, low density snow, which was followed by two weeks of warm days and cold, clear nights.  That surface snow metamorphosed into fragile, sugary grains, and we didn't get strong winds that can sometimes destroy that layer as it is forming.  On December 13th, that near surface facet layer was buried, and it is now under the weight of all the new snow that has fallen in the past 2 weeks.

Facets form a type of layer in the snow coined as a "persistent weak layer."  So once a layer of facets gets buried by a slab of new or windblown snow, it makes for a weakness in the snowpack on which avalanches can fail.  Worst of all, the weakness is long-lasting, so we can see avalanches fail on facet layers weeks or months after they are buried.  This means that we can see very large slabs of snow develop over the course of the winter before a facet layer might finally give out, causing a huge avalanche.  Facets have a few other nasty tricks up their sleeves.  If the layer is fairly continuous, it is so fragile that it can drive a failure very long distances across avalanche terrain.  I've seen avalanches fail almost a mile wide on facet layers!!  If you can get the layer to collapse on flat terrain, it can propagate up a slope and cause an avalanche to release above you.  So not only do these layers plague us for long periods of time, but they also behave in an unusual manner.  Keep this in mind in upcoming weeks and months, now that we have several facet layers buried in our snowpack.   For more information or to get daily avalanche advisories, visit www.cbavalanchecenter.org.

This is a photo of a shallow slab that propagated impressive distances on the December 13th facet layer, near Purple Palace.  Photo credit: Aaron Huckstep

Ian's Weather Resources

 Current Data

Surface:

    http://weather.rap.ucar.edu/surface/
    https://avalanche.state.co.us/obs_stns/stns.php
    http://mesowest.utah.edu/cgi-bin/droman/mesomap.cgi?state=CO&rawsflag=3
    Schofield SNOTEL: http://www.wcc.nrcs.usda.gov/nwcc/site?sitenum=737

Upper-air soundings:

    http://weather.rap.ucar.edu/upper/

Radar & Satellite:

    http://www.weathertap.com ($84/yr, great mobile website as well)

Discussions

    NWS: http://www.nws.noaa.gov/view/prodsByState.php?state=CO&prodtype=discussion
    OpenSnow: http://opensnow.com/dailysnow/colorado
    CAIC: http://avalanche.state.co.us/forecasts/weather/zone-forecast/
    http://wasatchweatherweenies.blogspot.com

Forecast Models: Twisterdata.com

Top Center

NAM = 84hr 12km resolution

GFS = 384hr 27km resolution

RAP = 18hr 13km resolution

Left Side

700mb = orographic wind direction

500mb = vorticity

300mb = jet stream

Upper Right

Clickable map = one forecast image

Animated loop = all forecast images

Compare models = toggle between each model

dProg/dt = see the trend in the forecasts
Forecast Models: CAIC

    High-resolution version of the NAM model
    Graphics: http://avalanche.state.co.us/forecasts/weather/model-forecasts/
    Points: http://avalanche.state.co.us/forecasts/weather/point-forecasts/

Forecast Models: Weatherbell.com

    Access to European model graphics and many other models. Fantastic data. $185/year

New Website: Avalanche Problems

Zach Guy
Forecaster, CBAC 

By now, you've probably noticed the CBAC has had a bit of a make-over.  We have spent over a year designing, researching, redesigning, and coding our new website, which we launched this week.   We were operating on an old dinosaur of a web platform and it was time for a new site that matched the current state of the e-world.  This new site has improved graphics, higher resolution imagery, more user friendly from our end and yours, an improved observations platform, a format that is more consistent with avalanche centers nation-wide, and some additional forecasting tools which you can use to make safer decisions in the backcountry.  Bear with us as we work through the kinks and strive to improve the functionality of the site.  

Now let's jump into a key element of our daily forecasts: the avalanche problem.  The reason we put a lot of focus on avalanche problems is because the flavor of the avalanches we expect to encounter can be more influential in our terrain and risk management than a given danger rating.  For example, not all Moderat danger ratings are created equal.  I move through terrain and make snowpack assessments very differently for a Moderate danger when the only concern is wind slabs, versus a moderate danger involving deep persistent slabs.  The Utah Avalanche Center just published a new tutorial on avalanche problems; its worth a look.  http://utahavalanchecenter.org/avalanche-problems-tutorial.  The CAIC also defines each problem here


This idea of avalanche problems is nothing new to our CBAC users, but we're presenting it in a slightly different way. The four key elements to the avalanche problem is the avalanche character, its distribution, its likelihood, and its size.  Here's an example of how we present the problem. You can always click on the little blue "information" icons for more help or info.

The trickiest part of this is the distribution rose: where the problem is located across our terrain.  Imagine a conical shaped peak, and you are hovering above it in your private helicopter.  The inner-most rung is the highest elevation: above treeline, and the outer-most rung is the base of the cone below treeline.  Each triangular octet represents a compass direction, so imagine this conical peak is oriented the same way it would look on a map. Now here's the important part.  We shade the areas where the problem is most likely.  The problem distribution is never as black and white as it appears on this rose.  This gives you general guidance on where you are most likely to encounter the problem at the regional level, but it is still up to you to make assessments on individual slopes.  Here's an example: We get a mild snow event with strong westerly winds. Our advisory will probably shade the distribution of fresh wind slabs on leeward aspects (NE, E, SE) near and above treeline because windslabs will be fairly widespread on those slopes.  Given such an event, I can almost always find wind slabs on windward aspects (due West) too, if I seek out cross-loaded features or gullies.  There might be a few slopes below treeline that develop wind slabs as well.  So just because the wind slabs are prevalent on higher elevation, leeward aspects, it doesn't mean you shut off your snow senses if you're traveling elsewhere.  We will try to describe these nuances and subtleties in our text, so its in your best interest to read the whole advisory and not just look at the pretty pictures.  

The likelihood is fairly self explanatory.  This is the chance of triggering a slide if you are recreating in steep, avalanche terrain in the parts of the rose that we have shaded, where the problem is most prevalent.  Lastly, we describe the expected size.  Small avalanches are D1's: relatively harmless to people unless they push you into a terrain trap. Large avalanches are D2's: they could bury, injure or kill a person. Very large avalanches are D3's: these could bury cars, destroy a house, or break trees. Historic avalanches are nearing the maximum size a slope can produce. These don't happen every year.  

We'd like to acknowledge Brandon Clifford, our website designer, and the CAIC for contributing resources and forecasting elements.  We hope you find our new website useful in planning and making safe decisions in the backcountry.

Get ready for winter!!

The gold leaves and some snow in next week's forecast are both good reminders that winter is just around the corner!  Before the flakes start piling up, fall is a great time to freshen up on your avalanche education, or set aside some money and time to take an avalanche course.  I've put together a list of a few educational opportunities below.

Avalanche Training/Lectures

WMRT Avalanche Seminar.  7 pm, November 20th, 2014.  Western State Colorado University 

CBAC Avalanche Awareness Night.  December 6th, 2014. CB Center for the Arts
A night of avalanche talks, beer drinking, silent auction, raffle, and winter stoke, all  to support your local avalanche center

CBAC Beacon Brushup.  December 7th, 2014.  CB Town Park.
Learn how to conduct an avalanche search or tune-up your beacon skills.

AIARE Avalanche Courses 
Level 1, Level 2, and refresher courses offered locally through Crested Butte Mountain Guides or check AIARE's schedule for courses around the country.    

Colorado Snow and Avalanche Workshop. October 17th, 2014. Breckenridge, CO
A one-day seminar aimed at avalanche professionals, but with plenty of material for motivated recreationists.

 

Weather Forecasting Courses

Sick of your local weather experts blowing the forecast? 
Mountain Weather Workshop.  October 31st - November 2nd, 2014.  Silverton, CO
Winter Weather Forecasting.  October 10th-12th, 2014.  Jackson, WY

Recommended Readings

I put together a list of some of the classic avalanche literature out there. I've ordered this list from easiest to digest to most technical reading. Grab a book for the next rainy or snowy day!

Snow Sense: A Guide to Evaluating Snow Avalanche Hazard - 5th Edition.  By Doug Fesler and Jill Fredston.  

A great book for starters or if you need a refresher

 Staying Alive in Avalanche Terrain - 2nd Edition.  By Bruce Tremper

My favorite. Bruce does a great job of describing challenging concepts using analogies and examples

The Avalanche Handbook - 3rd Edition. By David McClung and Pete Schaerer

If you want to dive into the science behind basic concepts, this is a good one.

Snow, Weather, and Avalanches: Observation Guidelines - 2nd Edition. 

Learn how to make observations at a professional standard

International Snow Science Workshop Proceedings.  Free online here.

Science nerds rejoice! All of the proceedings from this bi-annual conference are now online. Heavy on science.

Have a great fall!


Zach Guy
Forecaster, CBAC

Oh no! Dust on snow

Yesterday, we got a brief and silty taste of the Dirty Thirties when a major dust storm blew over Crested Butte.  Scarp Ridge recorded wind speeds of 110 mph (the highest of the season), and these winds brought with them a sizeable chunk of Moab.

Dust-ageddon in Crested Butte on March 30, 2014.  Photo credit: Matt Hogan

How will this dust on snow event affect snow stability and avalanches?   Lets start with a key concept: albedo.   Albedo is how reflective a surface is.  White colors have a high albedo -- they reflect a high amount of radiation energy rather than absorb it. That is why you reach for a light colored T-shirt if you're going outside on a hot sunny day instead of a black T-shirt.  Fresh snow has a very high albedo and it reflects most incoming solar energy. (So thats why my mom always made me double up on sunscreen when I went skiing!)  On the other hand, the dark-colored dust more readily absorbs solar energy and heat, and also retains that heat longer.

Pristine snow reflects radiation more effectively than dusty snow.  Courtesy of Jeff Deems.

Dusty snow can absorb two or three times the solar energy of a clean snowpack.  Whenever dust is near the surface of the snow (even when its buried up to a foot deep), it amplifies the rate of surface warming, increasing the amount of snowmelt and weakening the snow around it.   This leads to more frequent loose wet avalanches.  Intense melt rates send freewater deeper into the snowpack, which can also compromise the strength of deeply buried weak layers, causing an increased likelihood of wet slab avalanches.  Both of these problems, with or without dust, can be avoided by monitoring how well the snowpack refreezes at night and getting off of slopes before they thaw too much.  Dust on the snow simply shortens that window of stable snow and may prevent good overnight refreezes in some situations.

Snirt.  Snow+Dirt.   Photo taken March 31, 2014 near Crested Butte.

The second avalanche concern is that dust can do weird things when its buried by a slab of snow, and it can behave like a persistent weak layer.  Because they absorb and retain heat longer, dust layers can cause wet grains above or below them to remain unfrozen and unstable longer. Dust can also cause tremendous temperature gradients in the surrounding layers, which causes the bordering snow around it to decay and facet. This is not always the case, but it is worth checking on how reactive dust layers are after they get buried by spring storms.  It will be the easiest layer you will ever identify in a snowpit!

The crown of a large slab avalanche that was remotely triggered from hundreds of yards away last spring.  It failed on a dust layer.

Sadly, dust on snow has negative impacts on the timing of spring runoff and water resources in the West.  The Center for Snow and Avalanche Studies in Silverton, CO tracks and studies the effects of dust-on-snow events, and is a great resource for more information on this topic.  On the bright side of things, your favorite mountain bike trails will melt out sooner now, and business is booming if you own a car-wash business.

Zach Guy
CBAC Forecaster

A Canary in the Elk Mountains?

Canaries were once used in coal mining as an early warning system for toxic gases leaking into the mine.  Signs of distress, or worse, a dead bird, would caution the miners that it was time to retreat from the mine.

Last week, a warm airmass brought our first real taste of spring to the Elk Mountains.  Nothing out of the ordinary as far as Crested Butte weather goes, but enough that pale white skin made its spring debut on Elk Street before hastily retreating to aloe vera treatments.  On Sunday and Monday (March 9 and 10), temperatures rose to just above freezing at 12,000 feet under an intense March sun.   Days later, following a few inches of snow and cooler temperatures, we observed the results of some very unnerving deep slab avalanches.  Sometime after Monday evening, a huge slab tore off of the south face of Mt. Owen.  This appeared to be triggered by a cornice falling onto a shallow part of the slope, but it propagated to parts of the slab that were 12 feet deep.   

Deep slab on Mt. Owen, first spotted 3/12/14

Looking down the crown

On Wednesday night, another monster ripped off of a southwest facing ridge near Avery Peak.  This one raised the hairs on the back of my neck.  Two deep slabs back to back, under relatively mild and stagnant weather.  The Avery slide did not appear to be cornice-fall triggered, and has raised a lot of questions and spurred a lot of speculation on the failure mechanics among local and statewide avalanche professionals.  Although meltwater on high elevation terrain has been pretty minimal to this point, it seems plausible that a hot spot on the slope, such as a sunbaked rock or simply an oven-like part of the slope, could have channeled heat and meltwater into a shallow part of the snowpack.  Once meltwater percolates to a buried weak layer, it can compromise the layer’s strength causing a wide and destructive failure. The timing of these events can be unpredictable.  Maybe warming had subtle effects on the slab properties that added up just enough on this slope.  We're still not sure.

 

And another.  This one near Avery Peak.

With just a tease of spring under our belt and much more warm and sunny weather inevitably to come, this begs the question:  Are we seeing the tip of the iceberg right now?  Did the canary just faceplant into the bottom of the cage?  Warming and meltwater will continue progressing into the snowpack, first on southerly slopes, and eventually around the compass to north.  We know weak layers are at the bottom of the snowpack lurking and they are proving to be reactive.  Cornice falls will become more frequent as these overhanging blocks of snow continue to thaw and sag from their own massive amounts of weight.   We are not out of the woods yet when it comes to deep slab problems.  These last two slides should serve as a healthy reminder to use an extra dose of caution this spring in your backcountry travels. Be diligent in your terrain selection and in the attention you give to weather, snowpack, and avalanche patterns in the upcoming months.

Zach Guy
CBAC Forecaster

Recapping a historic avalanche cycle

Zach Guy
CBAC Forecaster

The first two weeks of February were exciting (but sleepless) times to be an avalanche forecaster.  An unusually large storm was accompanied by unusual avalanches.  Around the central and northern mountains of Colorado, avalanches destroyed or buried buildings, closed roads, and extended trim lines of previous historic paths.  Sadly, four fatalities occurred as well.  

I put together a simplified summary of the storm and a timeline of natural avalanche activity, highlighting some of the largest and/or most unusual avalanches that occurred around our Crested Butte backcountry and surrounding roads.  I’m certain this is only a small percentage of the natural avalanches; our zone is too vast to get the full picture from our lookout points and observations.  I ordered and dated these avalanches according to the first date that they were observed or reported to us, but the exact failure dates are open for speculation.

The storm began January 30^th, with a quick and massive hit of roughly 4.0” of SWE in the favored zones and 2.0” of SWE near town in just 60 hours.  Natural avalanches were widespread, but confined to the new snow, and most common at low elevations where the surface snow was weakest.  We were a bit surprised not to see any deeper slides in the alpine when the clouds cleared for a day on February 2^nd.  Maybe our snowpack was tougher than we thought??

Anthracite Range. Observed February 2nd.  

Snowfall picked up again on February 3^rd: fluffy snow, modest accumulations, with moderate winds, and a lull in avalanche activity.  On the night of February 7^th, the Pacific river of moisture found a direct path to Crested Butte.  Wind assisted snow transport had been steady and relentless for several days, and Gothic was the first to shed in a big way.  The NE face ran full track, crossed the East River, and ran uphill burying (and perhaps destroying?) a newly installed outhouse structure in the Judd Falls parking area.  Oh crap! Several observers called this one a D4, about as big as they get in Colorado.  

Gothic Mountain runout.  The pipe is to the top of the outhouse structure that was buried 10-20 feet deep. Observed February 8th. 

The river of moisture was relentless until February 10^th, and the favored mountains picked up another whopping 3.5” of SWE while Mt. CB got 2.5” over the 4-day period.  Schofield Pass hit 9.5" of SWE over the storm period.  Winds continued at moderate speeds, with the usual stronger gusts.  We had no visibility and limited avalanche observations during this time.  Then the clouds lifted on February 10th and 11th, and revealed a warzone.  Almost everything steep near town had slid, from small road cuts, to river banks, to more sizeable paths.  Almost all of Snodgrass had slid, breaking trees and covering Gothic Road in feet upon feet of debris.  Debris ran across Peanut Lake Road and onto the nordic tracks.  Unusual crowns were all over the mountains closer to CB South.  Deep slabs pulled out of the Ruby Range.  Slides that hadn't run in 20 years crossed Cement Creek Road. At least 5 paths crossed Taylor Canyon Road, and several structures near Almont and in Taylor Canyon were impacted.  At least 4 paths crossed Spring Creek Road (not sure on the timing of these last few slides...could have been a few days later).

This slide crossed Peanut Lake Road around 8:30 a.m. February 10th.  

An estimated 2/3 of Snodgrass Mountain slid to near the ground.  Observed February 10th.

Unusual crowns on Double Top Mountain. Observed February 10th

Large slides above town from Gibson Ridge.  Observed February 10th.

Crown lines running across the entire width of Cement Mountain.  Observed February 10th.

Slides near Meridian Lake.  Observed February 11th.

Peeler Basin. Estimated half mile wide. Observed Februay 11th. 

Deep slab on Purple Mountain, estimated up to 20+ ft deep. Observed February 11th.  

Deep slab on Afley.  Observed February 11th.

Small doses of snow fell and wind speeds increased on February 12th through the 15th.  Temperatures started to climb, eventually hitting the high 40's in Crested Butte. We even saw short bouts of rain at all elevations.  The natural cycle was far from over.  Another natural came down to Cement Creek Road.    A few more huge ones ripped off of the peaks. 

Near White Mountain; this path ran similar to the notorious skier triggered slide from 2008.  Observed February 14th.

Several historic slides crossed Spring Creek Road.  Observed February 14th.

Slides across Cement Creek.  Observed February 14th and 15th.

Slides impacted structures in Taylor Canyon and near Almont. Observed February 15th.

Slab failing near the ground on Whetstone.  Observed February 16th.

Another D4 off of Peeler Mountain.  Observed February 17th.

Near WSC Peak.  Observed February 17th

This memorable cycle was spooky, challenging, exciting, and stressful all at once.  I wish I had a helicopter to fly around the range now to get a full view of the impacts of this cycle.   

Thank you to everyone for submitting observations the past few weeks.  A special thanks to Ben Pritchett, Ian Havlick, Pete Sowar, Briant Wiles, Andrew Breibart, Jayson Simons-Jones, and Gary Dotzler for their photos. 

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

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