Avalanche Forecast

A fatal size 3 (very large) deep persistent slab avalanche occurred yesterday near Invermere, which is directly adjacent to this forecast region. This persistent and very high consequence problem affects the central Columbias as well.

The remainder of reports from Wednesday showed a solid mix of new snow instabilities with storm slab, wind slab, and dry loose avalanches triggered both naturally and with rider traffic and explosives. This activity likely stepped up a notch again on Thursday as moderate to heavy snowfall moved into the region.

Almost all operators in the region reported some variation of the theme of a natural and skier-triggered avalanche cycle early this week as 40-70 cm of new snow weekend settled into storm slabs in some areas and produced dry loose releases with rider traffic in areas where slabs hadn't quite formed. The cycle was generally observed to have produced avalanches to size 2.5 (large!) that for the most part were confined to the depth of new snow. A few operators noted the cycle being less widespread than expected.

Clear skies last week allowed for observations of many deep persistent slab avalanches as well as explosive control missions, which produced large to very large (size 2 to 3.5) avalanches. Another one was human-triggered in Glacier National Park on Monday and another size 3 (very large) triggered by cornice fall near the southeast corner of the Park on Wednesday. These avalanches have been occurring on all aspects and generally between 2200 m and 2800 m in elevation. Although many of these avalanches have occurred in unrideable terrain, they are evidence that the weak basal snowpack is still triggerable. Riders should absolutely continue to avoid thin snowpack areas, which are often found near ridgelines.

Looking forward, we expect a mix of gradually diminishing storm slab reactivity, a shift toward wind slabs being the most reactive slab problem, and continued dry loose hazard in sheltered areas where snow remains low density. The possibility for very large deep persistent slab avalanches should not be ruled out.

Another round of stormy weather mid-week has so far brought about 15-30 cm of new snow, greatest in the north of the region, with more to come overnight Thursday. For the most part the new snow adds to 40-70 cm of low density storm snow from the weekend, but it may also have buried recent wind slabs in more exposed areas and likely a thin sun crust on sun-exposed aspects.

The interface below the snow from the weekend storm includes small surface hoar in lower elevation sheltered areas and more widespread faceted (sugary) snow and heavy wind effect. Now roughly 50-90 cm deep, this interface remains in question with storm slabs having been generally slow to form over it in recent days.

Around 70 to 130 cm of snow may now rest on a small layer of surface hoar crystals that was buried mid-February. This layer is most likely found around treeline elevations in areas sheltered from the wind. It appears to be dormant in most of the region but it has been identified in a few small persistent slab avalanches in the adjacent Kootenay Boundary where lesser recent storm totals may have only recently brought it to a critical load. This makes it a layer to factor into your terrain selection in the southernmost Columbias. The remainder of the mid-snowpack is generally strong.

A layer of large and weak facets that formed in November is deeply buried, found near the base of the snowpack. The likelihood of human triggering this layer is currently low given its depth. However, alarming reports like this one from Glacier National Park continue to trickle in, confirming it is still best to avoid steep, thin, rocky slopes near ridges at alpine and upper treeline elevations.

Thursday night

Cloudy with continuing convective flurries bringing a trace to 10 cm of new snow. Light to moderate southwest winds.

Friday

Cloudy with continuing convective flurries bringing up to about 5 cm of new snow and storm totals to 20-40 cm. Light southwest winds. Treeline high temperatures around -8.

Saturday

Mainly cloudy with isolated flurries. Light east winds. Treeline high temperatures around -7.

Sunday

Mainly sunny. Light east winds. Treeline high temperatures around - 7.

More details can be found in the Mountain Weather Forecast.

• Use increased caution at all elevations. Storm snow is forming touchy slabs.
• Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
• Be aware of the potential for loose avalanches in steep terrain where snow hasn't formed a slab.
• Avoid shallow, rocky areas where the snowpack transitions from thick to thin.