Wednesday January 9, 2019
Wind Slab, Loose Wet, Cornice Fall
Travel & Terrain Advice
Avoid travel in avalanche terrain during the peak of the next storm system. Avoid wind loaded features and convex rolls, especially at ridge top. Give growing cornices a wide berth when travelling both above and below them.
Widespread small human triggered loose dry avalanches observed on Tuesday afternoon. These avalanches were triggered from all sufficiently steep terrain at treeline and below treeline.
Sustained strong to extreme winds associated with the incoming storm system will very likely create reactive wind slabs in the alpine and exposed treeline terrain on NW aspects. These avalanches will have the potential to be large in size (size 2-3) and may be easily triggered by a light load.
Rising freezing levels will likely produce small (size 1) loose wet avalanches below treeline on all aspects. These small avalanches may have large consequences if terrain traps are involved.
Cornices have grown substantially recently, and will continue to do so with the addition of large amounts of precipitation and sustained strong winds. When freezing levels rise, these fragile cornices may possibly fail and could have the potential to trigger deeper weak layers creating a very large avalanche (size 3+). Expect the greatest danger in the alpine and upper treeline on NW aspects.
A brief lull in the storm parade produced widespread surface hoar development on many aspects at all elevations Sunday and Monday. These weak feathery crystals formed on top of settling recent storm snow. With the incoming storm system, this weak layer of snow will be buried in sheltered terrain at all elevations. Below these layers, old storm snow is beginning to settle and bond well to adjacent layers which include a thick strong crust. Some weak sugary facet crystals have formed around this crust, but have not shown evidence of reactivity. With incoming snow load and rising freezing levels, a very large trigger on a large alpine feature could potentially awaken deeper instabilities and produce very large avalanches.
|Surface||20-40 cm of new low density storm snow|
|Upper||A thick crust and weak buried surface hoar layers|
|Mid||Settling old snow and some unreactive crusts|
|Lower||Well settled and dense|
Clear and calm conditions on Monday across the forecast region followed by the arrival of the next vigorous Pacific frontal system Tuesday morning. Outflow winds were strong from the NE veering to strong to extreme from the SE with the onset of precipitation. Approximately 20-30 cm of new low density snow fell through the day Tuesday and was transported into lee terrain by strong SE winds.
WEDNESDAY - 50-80 mm of precipitation with freezing levels rising to around 1600 m. Strong winds from the SE.
THURSDAY - 30-60 mm of precipitation with freezing levels around 1500 m. Strong winds from the SE.
FRIDAY - 10-15 mm of precipitation with freezing levels around 1650 m. Winds strong easing to moderate from the SE.
Posted on Wednesday January 9, 2019 by Dan Goodwin