Danger will be highest during periods of strong solar radiation, affecting sun-exposed slopes and cornices.
Watch for wind slabs at upper elevations and adjust your travel plans according to the changing conditions of elevation, aspect, and time of day.
10-20 cm of recent snowfall and southwest winds are expected to form reactive wind slabs in lee features in the alpine and treeline.
Cornices have grown significantly in the past two weeks. Give them a wide berth when travelling on ridge-lines and minimize overhead exposure to large cornices, especially when the temperature is warm or solar radiation is strong.
Last week's 30 to 60 cm of new snow has shown evidence of forming a persistent slab above weak layers. There have been reports of a poor bond to underlying sun crust layers on south slopes and surface hoar crystals on north slopes.
At this point, we are uncertain about how long this problem will take to heal and suggest a more conservative approach to terrain to handle this uncertainty.
If the sun comes out on Sunday, daytime warming compounded by strong solar radiation may cause the surface snow on sun-affected slopes to loose cohesion rapidly and form wet loose avalanches out of steep terrain.
If triggered, wet loose avalanches have the potential to step down to deeper weak layers creating large avalanches.
SATURDAY NIGHT: Snowing, trace to 15 cm of accumulation. Moderate to strong westerly winds. Clearing in the early morning, 1500 m freezing level dropping to 500 m.
SUNDAY: Partially cloudy with the potential for scattered flurries. Light northwesterly winds. Freezing level rising to 1300 m in the afternoon.
MONDAY: Cloudy with light flurries. Light to moderate westerly winds. Freezing level rising to 1500 m in the afternoon.
TUESDAY: Cloudy with light flurries. Moderate to strong westerly winds. Freezing level rising to 2500 m in the afternoon.
We have heightened concern about a developing persistent slab problem in some parts of the Purcells (read more in this blog).
Several notable human-triggered slab avalanches occurred in the past week, suggesting that the recent snow is bonding poorly to the underlying layers. These occurred on a range of aspects above 2000 m. Most slabs were small in the top 20 to 30 cm of snow (size 1), but one larger 70 cm thick slab was remotely triggered from below by a group of skiers (size 2). This avalanche occurred on a south aspect at 2250 m.
5-15 cm of new snow will add to last week's highly variable snowfall totals, with 20 cm in the eastern Purcells to 60 cm in the western Purcells. Snow depths also taper significantly with elevation, with moist and crusty surfaces below 1800 m and on south aspects into the alpine.
In some areas, last week's snow is well bonded to old crust layers, while in other areas the snow has settled into a reactive slab above weak layers around the crusts. At this point, it appears persistent slab problems could be developing in the deeper snowpack areas along the western side of the range, while the eastern side more likely has thinner wind slabs. The lower snowpack is generally well bonded.
