Friday March 9, 2018
Cornice Fall, Loose Wet, Wind Slab
Travel & Terrain Advice
Thursdays new snow and then strong winds from the SW-SE will redistribute and create wind slabs on lee aspects. Avoid travel in zones that have been loaded like zones directly below ridge tops and on crossloaded features. Even though snowfall amounts from the storm are not great the wind can magnify depth and increase hazard significantly. Sunday we will see a significant warm up. Really the first true big warm up since our cold snap began over a month ago. These temps will certainly play a roll in avalanche activity and triggerability. Windslabs will be more reactive, Cornices will become weaker and even smaller avalanches will have more force to push us into danger areas like terrain traps. Avoid exposure and stay out of avalanche terrain when thing begin to warm up. Large rock faces (covered with ice and snow) may begin to shed their loads as things warm up. Avoid travel under these features as you maybe in for a nasty bump on the head if you do not.
No new avalanche activity reported over the past forecast period. Some minor sloughing of Thursdays new snow off steep smooth planar surfaces with ski cuts.
Large cornices still exist near ridgetops in the alpine and treeline. These cornices over hang W-N aspects. With Sunday’s warm temps and high freezing levels these cornice will become significantly weaker. We may see natural triggering and certainly human activity on top of these monsters could very well trigger large failures up to size 2. Avoid travel on and under these dangerous hazards during these coming warm period.
Sundays significant warm up and high freezing levels will very likely produce naturally triggered loose wet activity and will almost certainly be created with human activity up to size one. Avoid exposure above terrain traps during this warm unstable time. These loose wet sloughs will be on all aspects at treeline and below treeline and on solar aspects in the alpine. They will mainly be associated with start points from trees and rocky outcrops.
Thursday’s moderate new moist snowfall will potentially be transported by fallowing strong west winds. Avoid exposure to loaded lee slopes slopes in the alpine and treeline. These slabs may possibly trigger naturally and are likely to trigger with human activity. They may only be size 1 to 2 but as they will be more moist/dense than usual they will have some force to them. Be very wary if you are exposed to terrain traps below in wind loaded terrain. Note these slabs will very likely become more active/easier to trigger with the warm temps on Sunday!
New snow Thursday falls on a relatively stable mid snowpack. However, some surface weak layers were noted before this storm arrived. These layers (buried preserved persistent surface hoar, graupel and thin crusts on solar aspects) may be the weakness that prompts avalanche activity strictly in the new accumulations.
|Surface||New snow with accumulations around 10-15 cm falling with warm temperatures|
|Upper||Numerous weak layers built up from the previous weeks.|
|Mid||Well settled crusts and bonded snow|
Prior convective activity put down a deep layer of graupel in the east and west zones (none to the north). The past three days have seen clear cold nights creating surface hoar and light winds with small accumulations of new snow. Clear warm days creating moist surfaces on solar aspects and the formation of thin sun crusts.
Accumulations Thursday of around 10 cm fallowed by strong winds from the SW-SE (wind transport most likely by Friday). A very significant warm up starting Saturday night and into Sunday will drastically change the snowpack and potentially prompt avalanche activity.
Thursday - 10-15 cm new snow east and west, possible mixed precip to the north.
Friday - trace to no new snow for east and west, 3mm of rain and snow mixed for the north, Strong W winds for all regions, temps -3 to -7, freezing levels 350 to 980 m
Saturday - 0 to 2 cm of new snow, moderate winds from the SW to SE, temps 0 to -4, freezing levels 50 to 900 m
Sunday - no new snow, moderate SE winds, 0 warming to POSITIVE 4, freezing levels 650 to a high 2000 m.
Posted on Thursday March 8, 2018 by Bill Phipps