At higher elevations, Friday's storm is expected to form touchy wind slabs.
There is a lot of uncertainty about how buried weak layers are going to react to sun and warming this weekend. Use a conservative approach to terrain selection and watch for signs of instability.
Ongoing SW through NW winds have formed reactive wind slabs over the past week. The storm on Friday is expected to form new wind slabs or continue to develop existing slabs in exposed terrain at higher elevations. These slabs are particularly reactive where they overlie a crust or buried surface hoar.
By Saturday morning, up to 45 cm of relatively unconsolidated snow will sit over the January 30 crust/surface hoar/facet layer in wind-sheltered terrain. With warming and sun in the forecast, this snow may quickly settle into a reactive slab this weekend. There is a lot of uncertainty regarding the timing and spatial extent of this scenario so extra caution is recommended, especially around open slopes at and below treeline where the buried surface hoar is most prominent.
A ridge of high pressure builds on Saturday bringing dry and sunny conditions with mild temperatures.
Friday Night: Snowfall up to 5 cm, moderate to strong SW wind, treeline temperature around -6 °C.
Saturday: A mix of sun and cloud, moderate W-NW wind, freezing level reaching as high as 1500 m.
Sunday: Mainly sunny, moderate W wind, freezing level reaching as high as 2000 m.
Monday: A mix of sun and cloud, moderate to strong W wind, freezing level as high as 2000 m.
On Thursday, three natural size 1-1.5 wind slabs were reported on east through south aspects at 2200 m elevation and explosives triggered one size 1 wind slab. Loose dry activity up to size 1.5 has been reported over the last few days.
Earlier in the week, shooting cracks and whumpfing had been reported which indicates the buried surface hoar layer is very weak. So far, this layer has primarily produced avalanches in wind loaded terrain but if warming causes a slab to form, a more widespread persistent slab problem should be expected. The surface hoar distribution is spotty and not found at some locations in the region.
Prior to Friday's storm, 20-40 cm of snow from last Sunday buried the January 30 interface which consists of a melt freeze crust on all aspects to around 2000 m and on solar aspects extending well into the alpine, spotty surface hoar in sheltered areas at and below treeline which may be large in some places, and/or small facets. Prior to Friday's storm, the older storm snow had not formed a slab in most places except for where it was wind loaded. With warming and sun this weekend, rapid settlement is possible and this may result in the formation of a more widespread reactive slab over the potentially weak surface hoar/crust/facet interface. Ongoing wind over the past week has formed reactive wind slabs in exposed terrain at higher elevations and new wind slabs may build during Friday's storm.
The early December crust/facet persistent weak layer is now 100-200 cm deep. This layer produced numerous very large avalanches in January but is now considered dormant. We continue to track this layer of concern and expect it will wake up again with sustained warming or a large storm event. This recent forecaster blog goes into more details on the layer.
