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 buried surface hoar.
By Saturday morning, up to 50 cm of relatively unconsolidated snow will sit over the January 30 surface hoar. 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 5-10 cm, moderate SW wind, treeline temperature around -5 °C.
Saturday: A mix of sun and cloud, light to moderate W-NW wind, freezing level reaching as high as 1500 m.
Sunday: Mainly sunny, moderate W wind, freezing level reaching as high as 1800 m.
Monday: A mix of sun and cloud, moderate W wind, freezing level as high as 1800 m.
On Thursday, three natural size 1-1.5 wind slabs were reported on east through south aspects at 2200 m elevation. This MIN report describes a snowmobile-triggered size 1 slab on an east aspect below treeline failing down 30 cm on the January 30 surface hoar.
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.
Prior to Friday's storm, up to 40 cm of snow from last Sunday buried the January 30 interface which consists of a melt freeze crust at lower elevations and on solar aspects extending into the alpine and widespread surface hoar in sheltered areas at and below treeline which is typically up to around 20 mm in size and very weak in some places. 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 weak surface hoar and crust. 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 widespread January 18 rain crust is now around 40-50 cm deep with weak faceted snow above. This layer has not produced any recent avalanche activity but will get tested this weekend with warming and sun.
The early December crust/facet persistent weak layer is now 100-200 cm deep. This layer produced numerous 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.
