View Full Version : BC Avalanche Conditions Dec 18

December 18th, 2009, 06:34 PM
December 18, 2009

Some thoughts on conditions as we head into the Christmas Holidays.

This year’s snowpack is turning out to be a pretty good one so far compared to the last couple of winters. The layers deep in the snowpack and near the ground are generally looking pretty good, the middle layers of the snowpack are pretty solid for the most part, and there’s some great powder on the surface providing wonderful riding conditions. This is not to say that it’s good to go everywhere all the time, but certainly this year looks a lot more normal or average than what we’ve experienced in the last couple of seasons. Let’s hope it keeps up.

I think the accidents and near misses we heard about earlier in the season were mostly related to an unsettled, immature snowpack that was building rapidly and not as solid or as well bonded as it appeared. This may have fooled some people into thinking it was late December, not late November! We’ve seen a significant drop in reports of incidents in recent weeks as the snowpack has settled and bonded. There are some lingering concerns about a crust layer that formed in mid-late October which has sugary facetted snow above it in some locations, but this problem seems to be restricted to areas where the ground cover is smooth and steep. This layer is something to keep in mind on steep slopes where the underlying surface is glacier ice, permanent year-round snow or ice, smooth grassy slopes, rock slabs, and shale or gravel slopes. In November the October crust layer (as it is now being referred to) was most active in the Purcell and Rocky Mountains. In recent weeks we’ve heard less about this layer and the most recent activity on it has been reports of the odd large avalanche that pops up in a variety of regions throughout the interior and rocky mountain ranges—nothing from the coast. A bit of research and extra caution is warranted if you know or suspect your trip will involve this kind of terrain.

A warm and stormy November produced some windslabs and thin crusts of various kinds in most regions but for the most part none of these appear to be a significant problem at this time. The South Coast (and possibly coastal areas in the Northwest) saw a strong crust form on the surface at the end of this period. We then experienced a clear spell in early December and some cold temperatures. It is the layer at or near the surface created by this cold, clear spell that is the current concern.

In the South Coast area (Sea to Sky Corridor, Pemberton, Duffy Lake/Cayoosh Pass, Goldbridge, Coquihalla, etc.) the early December cold snap left a layer of sugary facetted snow on top of a firm or crusty surface. This area has received a decent dump of snow in the last few days along with wind and warm temperatures. A slab of firmer, denser snow formed over the looser, sugary facets. As a result, avalanches are being triggered both naturally and by human activities. The Special Public Avalanche Warning for this region for December 18 - 21, 2009 was issued because we expect this problem to persist for at least several days after the weather clears and perhaps longer. Instabilities that persist for more than a day or two are unusual on the Coast and this is something people may not recognize, so we feel a special effort to warn people is required. We’ll reassess the situation after the weekend and extend the warning if we feel it’s warranted. We are especially concerned in this region because conditions have not been great for the last while and now, with the holidays starting and great new snow on the surface, people will be keen to hit the slopes and may forget to do their homework first. You South Coasters: keep your heads up and be more careful for longer than normal until we see how this situation plays out. Stay on lower angled slopes (less than 25 degrees) and simple terrain, stay out of start zones, and eliminate or minimize your exposure to terrain traps and large avalanche runout zones.

The North Shore region does not have this condition. The warm temperatures and rain have pretty much eliminated the any crust or facet problem there and the concerns there revolve mostly around warm, wet, slushy conditions when temperatures are high and very hard, icy conditions when it freezes—more or less the norm for this area.

In the north-west region, coastal areas appear to have a similar situation developing as what’s going on in the South Coast but there’s less storm snow on the weak layers, so the avalanche problem is not quite as touchy in the north as in the south—yet. I’d be watchful here; the next round of snow, wind, and/or warm temperatures might put this region over the top and create a condition like the one we are seeing right now on the South Coast.

In the Kootenay-Boundary region (the triangle on the US border from Greenwood to Nelson to Kimberley then south along the Kootenay River) we also had a pretty decent dump of snow, some winds, and warming temperatures in recent days. An avalanche cycle occurred in this region during the warmest temperatures on December 16th and 17th and included at least one large avalanche that put about 1200 cubic metres of debris up to five metres deep on the highway near Kootenay Pass on the 17th. A similar condition occurred in the South Rockies, especially in the Fernie area. These avalanches all appear to be failing on the surface facet layer that formed in early December but these regions don’t have the hard crust under the facets for the most part and temperatures didn’t warm as much as on the coast, so the problem seems not as widespread. I expect it will settle down a little quicker in the Kootenay-Boundary and South Rockies than on the coast. I’d still be pretty careful in the these southern regions for the next few days. Look for signs of instability such as whumphing, cracking, and avalanches occurring in the area. If you see these signs (or have the training to recognize “pops” or “drops” in compression or rutschblock tests), my advice to you is similar to what I suggest for the South Coasters.

In other areas of the province, we are seeing a less active avalanche problem. We’re concerned about windslabs where the wind has deposited layers of firm, dense snow that lie on softer, weaker layers. These windslabs are most likely to exist on east, northeast, and north aspects in the Alpine. You’ll probably also find pockets of slab on other aspects and at lower elevations where the wind has been swirling around and blowing across the slope. Be wary behind stands or lines of trees, in gullies, and in depressions. If the snow feels harder on the surface and softer below (you may need to step off your sled or skis to figure this one out) find a way to go around the problem or back off and choose a different, more sheltered route. This is not a dire situation or out of the ordinary for this time of year—just remember to keep your thinking cap on and use normal safety precautions like going one at a time up, down or across a slope; regrouping in safe places; and keeping an eye on your partners at all times when in avalanche terrain.

What happens next all depends on the weather. For the most part, my feeling is avalanche danger will rise with precipitation, wind, and warm temperatures and will fall after storms when the weather settles down. It looks like the southern Monashees (east of the Okanagan Valley) may be in line for the next storm on Monday/Tuesday with 50-70cm and strong winds. If the storm arrives as forecast, this will be the next place to watch. The biggest question for the long-term is whether the current weak layers will persist, especially in coastal areas where they lie on firm surfaces or crusts. Only time will tell that story.

Please keep in mind that this is a broad regional overview of the conditions as I see them on the afternoon of December 18th. There’s certain to be variation from one range or valley to the next. This information is also based on current conditions and will grow less applicable as time passes. If you are out in the mountains, we’d love to hear from you. Feel free to send a note to forecaster@avalanche.ca or call us at 250-837-2141 extension 230.

This year we are updating forecasts daily for the North Shore, South Coast, Northwest, North and South Columbia, Kootenay-Boundary, and South Rockies. Go to CAC | Bulletins | CAC Forecasts (http://www.avalanche.ca/cac/bulletins/cac-forecasts) to find more specific and more current information every day after about 4:00 pm PST. An abbreviated overview for each region can be heard by phone at 800-667-1105.

If you have questions or comments about anything in this report, please feel free to contact me directly by email.

Feel free to forward this or post to other forums.

Karl Klassen
Public Avalanche Bulletins Manager
Canadian Avalanche Centre
Revelstoke, BC