Northwest Inland Avalanche Forecast
Jan 9th, 2020 4:00PM
Lots of low density new snow in the forecast for Friday. Expect avalanche danger to increase over the day as depths increase and slabs begin to form. Increasing load on a buried weak layer makes conservative terrain selection especially important at treeline.
Thursday night: Cloudy with scattered flurries bringing 10-15 cm of new snow. Moderate south winds.
Friday: Cloudy with continuing flurries bringing approximately 20 cm of low density new snow and new snow totals to 25-35 cm by end of day. Light southeast or southwest winds. Alpine high temperatures around -10.
Saturday: Mainly cloudy with isolated flurries and a trace of new snow. Light southwest winds. Alpine high temperatures around -15.
Sunday: Sunny. Moderate northeast winds. Alpine high temperatures around -28.
Reports from the region are limited, however a widespread avalanche cycle is likely to have occurred throughout the southern part of the region on Friday and Saturday, when more than 40 cm of snow fell.
A report from Tuesday includes an observation of a large, audible natural avalanche in the Howson range. Details on the event are limited, but the natural trigger suggests touchy conditions still exist in parts of the region.
Before the last storm, reports of persistent slab avalanches associated with buried surface hoar were trickling in, from Ashman, Houston Telkwas, and the Howsons. They included natural and artificially triggered avalanches ranging in size from 1.5-3, and have occurred on all aspects, mainly at treeline. Loading from recent (and forecast) snowfall has (and will) increase the size and consequences of avalanches on these layers.
Looking forward, another round of snowfall through Friday is expected to build a widespread new storm slab problem to manage over the coming days.
Up to 35 cm of new snow is expected to accumulate in the region by the end of the day on Friday. The new snow will mainly bury wind-affected surfaces at alpine and upper treeline elevations. It may cover a new layer of surface hoar at sheltered lower elevations or a thin sun crust on steeper south-facing slopes.
Below the new snow interface, 40-60 cm of recent, wind-affected storm snow is expected to be forming a good bond with the interface below it.
A concerning pair of surface hoar layers are currently buried about 50-100 cm below the surface in sheltered areas around treeline. Reports of avalanche activity on these layers before the last storm suggest they may be active. Loading from forecast snowfall will increase this potential.
Terrain and Travel
- Storm slab size and sensitivity to triggering will likely increase through the day.
- Avoid freshly wind loaded terrain features.
- Be aware of the potential for large avalanches due to the presence of buried surface hoar.
Another day of heavy snowfall is expected to layer a new storm slab problem over the region on Friday. The forecast calls for low density snow, which means sheltered new accumulations may react as dry loose sluffs until snow settles into a more cohesive slab. Deeper, more reactive slabs will form much more quickly in wind-affected areas.
A couple layers of surface hoar are now buried 50-100 cm deep. There is uncertainty as to how the buried weak layers have reacted to the new load of storm snow, and Friday's forecast calls for yet another substantial snowfall. Human triggering of weak layers at this depth is definitely possible.
Valid until: Jan 10th, 2020 5:00PM