South Columbia

Conditions will deteriorate Sunday as temperatures rise and winds pick up. Storm slabs are expected to become touchier through the day.

South of Nakusp, where recent storm snow totals are less than 20 cm, avalanche danger may be a step lower at all elevations.

A warm front will bring rising temperatures, wind and steady snowfall to the region on Sunday and Monday. Heaviest precip amounts will be in the Monashees.

SATURDAY NIGHT: 5-10 cm of new snow, freezing level 1000 m, moderate west wind at most elevations with strong west wind in the higher alpine terrain.

SUNDAY: 5-15 cm of new snow at upper elevations with rain in the valleys, freezing level rising to 1500 m, moderate southwest wind at treeline with strong to extreme southwest wind in the alpine.

MONDAY: 10-20 cm of new snow at upper elevations with rain in the valleys, freezing level holding at 1500 m, strong to extreme southwest wind.

TUESDAY: 5-10 cm of new snow, freezing level 1000 m, light northwest wind.

Limited reactivity has been observed in the recent low density snow as it fell cold and unconsolidated, with storm slab activity isolated to areas kissed by the wind. As temperatures rise and winds ramp up Sunday, increased storm slab activity is expected at all elevations. 

Last week, older winds slabs in cross and reverse-loaded terrain features surprised a number of skiers, triggering avalanches up to size 2.5. This MIN report describes one such event on a southeast aspect in the alpine that stepped-down to deeper persistent weak layers to produce a very large avalanche that ran full path and destroyed mature timber. 

Persistent slab avalanche activity has dwindled since the beginning of February but a conservative approach is critical as this storm begins to add weight to it. This old MIN report from Joss Mountain is a helpful example of the lower elevation slopes where the surface hoar has been preserved and may reactivate this weekend.

Recent snowfall amounts have been in the 20-40 cm range in most areas, and highest in the Monashees in the north of the region, where 40-60 cm has fallen over the last few days. This recent low density snow will likely experience rapid settlement Sunday as temperatures rise and 10-20 cm of heavier snow falls overtop. At upper elevations, all of this recent snow will be easily blown around by strong to extreme westerly winds, forming thick, reactive slabs in lee features.

Below sits the old, extensively wind affected and faceted snow that sat on the surface during the extended cold, windy drought period. There is uncertainty around the strength of this interface with the new snow, which will surely be tested by the changes brought on by this active weather.

We've now got 70 to 130 cm of snow above the late January persistent weak layer which is composed of surface hoar. This layer is most prevalent at treeline and in "treeline-like" features such as cutblocks. On steep south-facing terrain, this layer may consist of facets on a sun crust, which has produced very large low probability/high consequence avalanches.

• Watch for changing conditions today, storm slabs may become increasingly reactive.
• Storm slab size and sensitivity to triggering will likely increase through the day.
• Storm slabs in motion may step down to deeper layers resulting in large avalanches.
• As the storm slab problem gets trickier, the easy solution is to choose more conservative terrain.