Avalanche
(This test pit video was taken 30 minutes prior to the human triggered Skyscraper avalanche. The test pit was in a similar area.” – HPAC)
So many close calls in North America over the weekend (including a 12-15 foot deep avalanche in CO). Lets all learn from these and stay safe out there. It’s still low tide all over North America, just look at the rocks in the above photo. Bouncing down those rocks at speed is not good for your life.
“Nov 22, 2015 – Human Triggered Avalanche, Skyscraper, East Face, 4500′. Airbag deployed, no injuries. Powder cloud went through parking lot, reaching the rescue cache, and covering the light poles. Debris reached the creek. (HS-ARu-R3 D2.5-O)
Crown at top of snow coulior about 3 feet deep, visible on entrance to run – viewer’s left. This slope was heavily cross-loaded from Southerly winds.
Avalanche danger was rated at “High” at the time of this avalanche. No other details are available at this time.
HPAC Avalanche Discussion, Saturday, Nov. 21st, 2015:
High avalanche danger. Up to 2 feet of new snow overlying weaker, older snow, will be very reactive. Natural and human triggered avalanches are likely. Large avalanches in many areas, or very large avalanches in specific areas.
Travel in avalanche terrain is NOT recommended. If you do decide to get out an play in the snow, stick to slopes 25º or less and out of the runnout of steeper slopes.
High Avalanche Danger at mid (2500′-3500′) to upper (3500’+) elevations on slopes 30° and steeper or in the runnout of these slopes. The hazard will be greatest on wind loaded and cross-loaded slopes and features on Northeast to West aspects.
Moderate Avalanche Hazard at lower elevations (below 2500′) for loose snow avalanches. If you are using the ROADRUN use the buddy system and be cautious on steep rollovers with terrain traps. It will be possible to trigger small avalanches that could bury a person in isolated locations.
Click here for the North American Public Avalanche Danger Scale.
RECENT ACTIVITY and CONDITIONS
Many human triggered avalanches occurred over last weekend due to a buried weak layer of faceted snow overlying a firm melt-freeze base. This layer was very reactive after it was buried by a rapid load on November 9th and stability was very slow to improve. With Thursday, Friday’s and today’s new snow load, this layer will have a chance of becoming active again.
This week was cold, clear and calm, perfect conditions for preserving powder, but also perfect conditions for the formation of weak, persistent grains like surface hoar and near surface facets. These weak snow grains are now buried by up to 2 feet of new snow and will be very reactive.
While this is a great start for the season in terms of snow amounts, it is a very rough start to the winter season in terms of the avalanche hazard and the human factor. Recognize that most of us are eager to “get after it”, however it will be prudent to wait and allow the snowpack to adjust to this new load before we will be able to play in it safely.
Widespread shooting cracks, natural and human triggered avalanches and extremely sensitive test pit results were observed on test slopes on Thursday and Friday. These observations indicate an extremely sensitive snowpack. Recent poor visibility and storm conditions have limited the ability to gather observations at upper elevations, however, the indicators are clear, bulls-eye clues for avalanche problems.
A winter weather advisory is in effect until this afternoon.
Since Thursday, HP has received at least 22″ of new snow at 3000′. Expect more at upper elevations.
Winds speeds at 4500′ on Thursday and into Friday were Southeast to South, 16-31mph with gusts to 54mph. These winds began tapering off and shifting Easterly as of yesterday evening.
Colder temperatures from earlier in the week warmed into the low 20°F’s at 4500′ with the onset of Thursday’s storm. Today at 5:00 am at 4500′ the temperature is 20°F.
AVALANCHE PROBLEM 1
After many days of clear and cold weather this week, the old snow surface weakened, forming widespread, persistent grain types including surface hoar and near surface facets. About 2 feet of new storm snow now sits on this very sensitive weak layer.
Thursday’s snow storm came in cold and warmed into the 20ºF’s. This snow will be “upside-down” and unstable. The combination of upside down snow and weaker buried snow increases the probability of avalanches.
New storm snow avalanches will have the chance of stepping down into the deeper October facets and producing much deeper and more dangerous avalanches up to 4 feet deep.
Natural and human triggered avalanches are likely on this very sensitive layer on all aspects at mid-upper elevations on slopes 30° and steeper.
AVALANCHE PROBLEM 2
Winds and significant gusting on the night of the 19th and into the 20th were strong enough to transport snow and built wind slabs mostly at ridgeline and upper elevations on North to West aspects. Visual clues in identifying these slabs will be difficult because they are now buried by Friday night’s new blanket of snow. These slabs will be 2-3 feet deep, very touchy, and easily triggered on the old/new snow interface of surface hoar and near surface facets.
Triggering a wind slab also carries the larger consequence of stepping down to the October facets and producing a much deeper persistent slab avalanche (the same problem layer as last week’s significant and prolonged avalanche cycle). The greatest hazard for this will be on previously wind loaded and cross-loaded features on North to West aspects above 3000′. With this hazard, slab avalanches could be up to 4 feet deep in specific areas.
Click here Avalanche problems in forecasting explained
TREND
The avalanche hazard will persist through the weekend and be slow to improve.
