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Avalanche Forecast

Archived

Mar 10th, 2017–Mar 11th, 2017

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

Regions

Lizard-Flathead.

Dangerous conditions are expected this weekend. New snow and warming will contribute to the storm slab, cornice, and deep slab problems.

Confidence

Moderate - Timing, track, or intensity of incoming weather system is uncertain

Weather Forecast

SATURDAY: Clear skies in the morning with increased cloud in the afternoon and light flurries, 30-50 km/h southwest winds, alpine temperature around -6 C.SUNDAY: Overnight snowfall of 10-15 cm easing off throughout the day, 30-40 km/h southwest winds, alpine temperatures around -4 CMONDAY: Cloudy with sunny breaks, 40-60 km/h southwest winds, freezing level spiking to 1800 m.

Avalanche Summary

A natural storm slab avalanche cycle occurred on Friday, mostly in the size 2-2.5 range. Storm snow was also reactive to skiers, producing several size 1-2 avalanches on a variety of terrain. Cornices are large and remain easy to trigger. A great image from Sunday illustrates just how surprising the results can be. Click here for details. A size 3.5 deep persistent slab avalanche likely occurred on Wednesday a northeast facing alpine slope on Mt Hosmer. It's thought to have been cornice-triggered and ran a distance of about 900m. This event highlights the isolated, yet destructive, nature of the deep persistent avalanche problem which should become more prevalent as we head into spring.Storm slabs will remain primed for human triggering on the weekend, while the warming trend will increase the likelihood of cornice failures and deep slab releases.

Snowpack Summary

Friday's storm delivered about 25 cm of heavy snow that settled into a touchy slab thanks to mild temperatures and moderate winds. The overall storm total since the start of the month now stands at roughly 70-120 cm. At times, strong southwest winds have extensively redistributed the snow forming large brittle cornices. An interface consisting of surface hoar, facets, stiff wind slabs, sun crusts, and a rain crust below 1900 m was buried in the middle of February. Little is known about the reactivity of this interface, but it may have the potential for large and destructive avalanches. Isolated basal facets still exist in shallow snowpack areas and can produce destructive full-depth avalanches. Likely triggers include intense storm loading, a cornice fall, or strong solar radiation.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.