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Avalanche Forecast

Archived

Mar 26th, 2021–Mar 27th, 2021

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

North Rockies.

Another storm is barreling toward the region Friday night. Heavy snowfall and strong to extreme winds mean a natural avalanche cycle is likely to begin by the end of the day. Danger may be a step lower in lower snowfall areas where 20 cm or less accumulates.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Friday night: Cloudy with flurries bringing 5-15 cm of new snow. Winds becoming strong to extreme west.

Saturday: Cloudy with continuing snowfall bringing 10-20 cm of new snow, continuing overnight. Extreme southwest winds. Alpine high temperatures around -2 with freezing levels to 1700 metres.

Sunday: Cloudy with 10-20 cm from overnight and another 5-15 cm during the day bringing two day snow totals to 30-70 cm. Moderate to strong west winds easing and shifting north over the day. Alpine temperatures cooling to around -10 over the day as freezing levels return to valley bottom.

Monday: A mix of sun and cloud. Light to moderate north winds shifting west. Alpine high temperatures around -13.

Avalanche Summary

Explosives control in the Table River Valley on Thursday yielded many storm and wind slabs in the size 2-2.5 range, with one size 3 (very large) release. Evidence of Wednesday's widespread natural cycle was observed during the same mission, with many more size 3 avalanches noted on a greater range of terrain.

Looking forward, we have another storm of similar strength taking aim at the region Friday night through Saturday. Natural avalanche activity will likely follow a similar pattern, especially in the west of the region where the greatest accumulations are expected.

Snowpack Summary

Up to 35 cm of new snow is expected to accumulate along the west slope of the region over Friday night and Saturday, closer to 15 cm in typically lower snowfall areas on the east slope. Weather models call for this snowfall to be accompanied by extreme south and southwest winds. Rain is expected below about 1200 metres.

The new snow will bury melt freeze crusts at lower elevations and on solar aspects and otherwise add to 30 to 50+ cm of settling dry snow from this week's storm. Our new snow may be particularly reactive to triggers where it buries crust.

Cornices have also become very large along ridgelines and always have the possibility of failing naturally or from the weight of a human.

A weak layer of facets buried mid-February may now be found around 100 to 120 cm deep, or shallower in the east of the region and in thin snowpack areas. The most likely place to trigger this layer is where the snowpack is shallow in alpine terrain. Large loads, such a cornice falls or multiple sleds on the same slope often trigger slopes that a single rider couldn't trigger on their own.

Terrain and Travel

  • Use conservative route selection. Choose simple, low-angle, well-supported terrain with no overhead hazard.
  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Be alert to conditions that change with elevation and wind exposure.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.