Spring Avalanche ClosuresPosted: April 15, 2009
Okay – I’m back at Lowdown HQ after some time off and a busy Easter weekend…
We’re a few weeks into April now, and while we’ve not been getting total cooperation from Mother Nature as far as a warm spring goes, we have had enough warm weather lately to create a few melt/freeze cycles, and also for the avalanche control team to put up their one big spring avalanche closure line.
We’ve had warm (+0C) days throughout this season, but there’s one main difference that sets apart a warm day in January and one in April, and that is the temperature during the night following a warm day. In January, it’s highly unlikely that above-freezing temperatures will continue through the night, meaning that whatever melting that occurred during the day will have a chance to recover overnight once the temperatures dip back to below 0C. This freezing will bring back whatever snowpack stability may have been lost the previ0us day, and the next day starts on a clean slate.
In April, there’s a much higher chance that warm temps will continue through the night. The snowpack will have recovered less, and will start the next day already behind the stability 8-ball, so to speak. Especially in areas that receive a lot of direct sun, unsafe conditions can arise quickly. These unsafe conditions come in the form of an isothermal snowpack, which means the entire pack is the same temperature from top to bottom, rather than having a gradual change as you move through the various layers. Isothermal snow loses all cohesion, and the snow becomes completely unsupportive. On steeper slopes, isothermal snow is unable to adhere to the slope or surrounding snow, and will often avalanche on its own, without any natural or human trigger required.
On warm days the avalanche control team continually monitors all avalanche terrain, ready to close it if conditions become too hazardous. Usually, only the top of the snowpack will experience recovery, while the rest remains unsupportive. This is a type of bridging, where the solid top layer will support the weight of skiers or riders only so long as the temperatures and exposure to sun permit. After a long, cold night, there may have been significant bridging, and the crust stays strong well into the next day. If overnight freezing temperatures were only brief, then recovery will be minimal and the top layer will quickly succumb to more warmth or sunlight. The avalanche control teams constantly monitor snowpack temperatures in order to be able to act proactively in closing terrain.
S0 – what weather makes for great spring skiing from a stability point of view? The key is cold, clear nights. Clouds act as an insulating layer, so if a particular day was warm, and then the clouds move in for the night, then all that warmth from the day will be trapped and the snowpack won’t have a chance to recover. A cloudless night allows accumulated heat to escape, giving the snowpack a better chance. During the daytime, clouds can be a good thing, since they’ll lessen the effect of direct sunlight. For quality of skiing, this can be good or bad, since some heat is good in order to soften the snow enough to make for good turns.
Anyone at Lake Louise on Easter weekend for the annual Big Mountain Challenge may have noted that the venue for the finals on Saturday was changed from Upper ER 5 to Er 3, since Friday was a hot day and parts of Upper ER 5 became isothermal. Without knowing how it would recover that night, the avalanche forecaster decided to close that area and move the finals down the ridge a bit. In Upper ER 5, the conditions were variable. For example, in the gullies dividing Upper and Lower ER 5, the skier’s right sides were solid and almost crusty, since they were protected from the sun. The skier’s left sides, on the other hand, directly faced the sun, and turned to slop in no time at all. With that kind of variability, the slope had to be closed until it could have a good chance at recovery.
Warmer weather and longer days also means that areas normally not a part of an avalanche closure become so. These areas are on the frontside of the mountain and sit in the sun almost all day long, and include Mirkwood, Grizzly Gully, Grizzly Bowl, and Kernahan’s. In order to protect these areas behind a closure, a long fence gets set up every spring. It starts at the top of the Flight cat road above Home Run, and follows Home Run as it rounds Grizzly Gully towards the top of the old Olympic chair. It then continues down the skier’s left side of Wrong Turn so that people aren’t able to do a long traverse from that run over to Kernahan’s. Ski patrol can then easily close the entire area if conditions dictate.
This avalanche closure is like any other on the mountain – it can open or close at any time, and it’s important that those entering these areas are certain that the area is open before ducking a rope. Just because it was open on the last lap doesn’t mean it will be on the next.