opportunity to observe the conditions leading up to the avalanche cycle that
occurred Friday afternoon and overnight into Saturday.
Wednesday: We flew in on the 18th with a break in the weather that gave
broken skies in the morning and a temperature of -12C upon reaching the hut.
Surprisingly little activity seen on the flight this morning despite the
good visibility. However it was obvious that things were quite stacked up at
ridge lines and start zones especially on the North and East aspects due to
the recent storm activity.
>From the onset we avoided exposure to overhead hazard on these aspects
believing them to hold the potential to produce slides that would run a
considerable distance.
It began to snow in the afternoon. 10 cm of snow fell and temps only climbed
as high as -6C with Calm winds. The resulting poor visibility and the above
limitations kept us skiing on South aspects to just above treeline and
sheltered East aspects immediately below the hut between 2400 and 1900m. In
the end this small piece of the tenure was the terrain we skied for the
entire trip. Skiing was excellent but required a considerable amount of care
to manage the hazards inherent with these locations.
The South aspects which presented two dominant crust layers proved to be of
the greatest concern for us:
The first of these was buried down 25-30 cm and would produce easy shears in
the decomposing fragments of precipitation particles (the broken arms and
remaining parts of snow flakes) above the 2 cm crust.
This layer was created previous to the 12th of March during a cold and dry
period with some sunny skies and temperatures as low as -25C. The cold temps
produced sugary facet crystals that do not adhere to each other. The Sun
produced the crust in areas that were reasonably steep and solar : SE- SW.
The crust becomes thicker with steepness of the slope (less reflection and
more heat absorption) and with aspects turned closer to South where the peak
of solar heating occurs.
We found that the 25-30 cm of snow on this layer to be soft with a limited
amount of cohesion however it would fail in soft chunks when cut from above
and below with a ski in the up track.
Of greater concern for us was the next deeper crust: the March 1 interface.
This was found to be 50- 65 cm deep and to be covered by the faceted snow
discussed above.
Produced during a mild period at the end of February this crust is now
beginning to degrade becoming several laminar layers instead of one clearly
defined crust. The main concern here however is the faceted snow that
overlies it.
Tests produced easy to moderate results that failed with a sudden collapse a
few cm above the crust. We experienced several settlements on these slopes
that can be attributed to the failure of this layer. The likely mechanism
for these events was the failure or cracking of the March 12 crust that
protected the deeper low density snow with the addition of a skier's weight.
The resulting whumph occurred as the faceted snow above the lower crust
collapsed under the slab above it. The character of these settlements on
this day were minor in nature this day. Generally they occurred often and
locally with the weight of one or two skiers that were close together.
We were concerned with the potential for this layer to propagate and
potentially create significant avalanches so we limited our exposure to
larger open slopes (where the magnitude of a failure would be greater) and
kept a good deal of time between each skier on the slopes we did ski to
minimize both the potential for triggering and involvement should things
slide. We chose slopes that were well supported and treated any convex rolls
with additional respect.
We experienced only minor results with ski-cutting on the March 12 crust in
steeper locations but even these results had us concerned with the potential
of a creating a small slide on the upper crust that could step down to the
deeper crust by focusing increased load on the weaker layer.
A third layer was also observed 75-90 cm deep as an interface with faceted
crystals (that were created during a sustained dry and cold period in the
middle of February) below firmer layers of more settled snow. Buried on Feb
22 these facets remain weaker than the snow that covers them. In most
locations we felt that the above mentioned crusts would prevent any
triggering on this layer. However these facets were found to be alarmingly
close to the surface in some areas that had been scoured by the wind.
Thursday: On the 19th with continued snow, -9C, and minimal wind, we now
found the march 12 layer down 35- 40 cm and the March 1 layer down 60-75 cm.
Temperatures increased from -9C in the AM to 3C for a brief period in the
afternoon with snow and imbedded fog all day except some brief windows of
clearing in the late afternoon.
We continued to experience settlements that increased in magnitude and
decreased in frequency as the day progressed. By the end of the day it took
2 or three people or more within 20 m of each other to produce these events.
We interpreted this to indicate that it may require more load to trigger an
avalanche but that the potential for larger propagation was increasing.
This day we explored a bit more below the hut(2150m) into more sun sheltered
Eastern glades and trees as low as 1900m. Here we found unsupported features
such as the mushrooms on boulders to fail easily as a soft slab. This
occurred on the March 12 interface mentioned above presenting here as
decomposing fragments overlying faceted crystals.
The Feb 22 layer was also evident buried 70- 80cm producing moderate results
that failed with a sudden planar character: a pop with the block moving
easily forward on the snow beneath the failure. As with the March 12 layer,
there were faceted crystals at this interface.
Neighboring huts reported two significant skier triggered events this day
both on North aspects in steep and unsupported terrain below 2000m, 50 -80cm
Deep that likely correspond to the March 1 or Feb 22 layers.
Friday: On the 20th we awoke to another 7cm of snow and a period of
reasonable visibility. We could see no wind at ridge top and the temperature
was a bit warmer than the previous morning at -7. The forecast called for
periods of snow and the winds to turn to the SW rising to 30 gusting to 50.
By the time we left the hut it began to snow and by the time we reached
ridge top we found winds to be out of the south east gusting to moderate
speeds.
1330 hrs: the winds had increased significantly and temperatures were above
zero degrees at 2000m. We began to notice the wind creating slab much more
rapidly with the warming.
1500 hrs: we could hear large avalanches coming down from the the large N
facing cliffs and slopes south of the hut.
1630 hrs: the temperatures had dropped but the wind and intensity of snow
increased covering our tracks in between laps.
Saturday: On the morning of the 21 we awoke to clear skies and -10C. The
winds had shifted in the night and had blown strong out of the West in our
Drainage. In open areas all evidence of our tracks had been erased and the
snow surface was wavy with wind affect.
A significant cycle to size 3.5 had occurred in open areas on all aspects,
many avalanches running full path: ridge top to valley bottom. Some observed
on the flight out crossed creeks or were channeled downstream a significant
distance. Many of these events had large chunks of failed cornice visible in
the debris. Most of these events started above tree line however in steep
locations on both north and south aspects there were large slides below
treeline in open areas that had seen increased loading with the wind.
In the end we had great skiing and were lucky that a steady stream of new
snow allowed us to heavily use the areas that we were comfortable with
instead of taking large risks in the name of a few good turns. Special
thanks to our great crew of skiers (and rider!) for a great few days in the
storm.
Tim Haggarty
ACMG Ski Guide and Assistant Alpine Guide
Paul Harwood
ACMG Assistant Ski Guide
_______________________________________________
These observations and opinions are those of the person who submitted them. The ACMG and its members take no responsibility for errors, omissions, or lapses in continuity. Conditions differ greatly over time and space due to the variable nature of mountain weather and terrain. Application of this information provides no guarantee of increased safety. Do not use the Mountain Conditions Report as the sole factor in planning trips or making decisions in the field.
Please check out http://acmg.ca/mcr for more information.