Hi All,
I went of a 2 hour flight today to check out a couple of operations for which we are forecasting avalanche hazard. The flight took us from Squamish southeast over to the Fraser River just north of Hope up to the south end of the Stein then back across just south of Whislter over to the Squamish Elaho confluence and south to Squamish.
The weather was generally good with a high overcast. A layer of marine cloud was crawling north from the lower mainland at about 5500-6500 feet but was broken and ended at the north end of the big fjordal lakes (Pitt, Stave, Harrison etc.). East of the Lillooet drainage there was a 30-50km NW wind that was transporting a lot of snow and made for a queazy flight. At 5000' it was -10deg and at 8000' it was 0 deg.
Below are some observations cut from the report I wrote about the flight for the clients:
GOAL:
The aim of this flight was to assess the snowpack that has
developed over the last month. A secondary but equally important part of the flight
was to see how much snow fell in the last weather event and see if any of the
weaker layers that have been observed throughout the industry are becoming
reactive.
FINDINGS:
- There was a significant inversion. It was -10ºC
at 1500 m but 0ºC at 2500 m
- There was less snow from the recent storm in the Garibaldi zone. There was more storm snow west of the Sea to Sky and the area between the Lillooet River and Fraser River.
- The freezing levels during the storm hovered at
or just above 500m
- A large avalanche cycle was observed. The cycle
occurred for the most part toward the end of the last snow event, which ended
on Monday (Mar 05) morning. The majority of the slides occurred around
treeline, sometimes at quite low angles.
- The deposits of the slides reached around the
middle of the runouts and appeared as though they were very cold a dry when
they slid.
- There was significantly more wind effect in the
alpine. In the alpine there were only avalanches in a few isolated areas that appeared protected from the wind.
- West of the Pitt River there were less observed
avalanches and these seemed to propagate shorter distances. Crowns averaged
40-80cm.
- East of the Pitt River the avalanches were much
larger in size even though the crowns were not as deep. Entire ridge lines up
to 2000-3000m long slid during the cycle producing up to size 4.5 avalanches. Some
of these ran several kilometers and over 2000 m vertical. Most of these slides were only 30-50cm deep
(March 4 SH?), but in many places stepped down (Feb 16 SH/FC?) another 40-80cm.
- In all areas 10-20cm of low density snow was
sluffing on solar aspects.
- It appeared that there was another avalanche
event about a week or two ago, but crowns and deposits were quite covered so it
is hard to estimate the timeframe of this event.
SUMMARY
The snowpack finally reached a tipping point. Two buried
instabilities, the Feb 16 and Mar 03 became reactive with this last storm. Cold
temperatures since have slowed the stabilization process and preserved these layers. If the warm temps tomorrow aren't enough to trigger a number slides then the
added load of storms forecast this coming week will likely bring a large avalanche
cycle both in areas that have not slid yet as well as reloaded bed surfaces.
It is unclear if the mentioned surface hoar layers were blown away
before further loading in the alpine, but the snow appears to be slightly more
stable above the treeline. If the weak layers persist and wind slabs bridged
the instabilities during this last snow load then large full path avalanches
could be expected with this coming storm system.
Conny Amelunxen
MG