South Columbia

New snow, mild temperatures and strong wind are a recipe for HIGH avalanche danger. Minimize your exposure to avalanche terrain Monday.

South of Nakusp, where recent storm snow totals are less than 30 cm, avalanche danger may be a step lower treeline and below.

SUNDAY NIGHT: 10-25 cm of new snow. Freezing level 1500 m. Strong to extreme westerly ridgetop wind.

MONDAY: 5-15 cm of snow. Freezing level 1500 m. Strong west wind easing slightly.

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

WEDNESDAY: Sunny, freezing level 1000 m, light northwest wind.

Preliminary reports from Sunday include skier and explosive controlled storm slab avalanches showing wide propagation up to size 2. The beginning of a natural cycle was observed in neighboring Glacier National Park on Saturday, with storm slab avalanches up to size 3 running in steep alpine gully features. Otherwise, as of Sunday morning, recent avalanche activity within the storm snow has been limited to a few natural and skier triggered loose dry and soft storm slab up to size 1.5.

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.

5-15 cm of new snow is forecast to fall overnight, with another 5-15 arriving over the day Sunday. Recent snowfall amounts have been highest in the Monashees in the north of the region, where 40-70 cm has fallen over the last few days. Other areas have received as little as 20 cm since the cold snap. 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. At lower elevations, this recent low density snow is likely settling and gaining slab property in the warm temperatures. 

Below sits the old, facetted and/or extensively wind affected 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 ongoing loading.

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.

• Use increased caution at all elevations. Storm snow is forming touchy slabs.
• Storm slabs in motion may step down to deeper layers resulting in large avalanches.
• Stick to simple terrain features and be certain your location isn't threatened by overhead hazard.