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Avalanche Forecast

Archived

Dec 26th, 2013–Dec 27th, 2013

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.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Northwest Coastal.

Confidence

Fair - Due to the number of field observations

Weather Forecast

Friday: Heavier precipitation should ease off by morning, then cloudy with flurries (5-10 cm). The freezing level lowers to around 800 m. Winds are light to moderate from the W-NW. Saturday: Cloudy with flurries or periods of snow 5-10 cm. The freezing level is near 500 m and winds are light to moderate from the southwest. Sunday: Periods of snow 10-20 cm. The freezing level should climb to 1500 m.

Avalanche Summary

It is likely that a natural avalanche cycle is ongoing Thursday morning with continued heavy precipitation, mild temperatures and strong winds. On Wednesday, several natural and explosive controlled avalanches up to size 2 were reported, but observations were limited. Natural avalanche activity should taper off on Friday as temperatures cool slightly and precipitation eases off; however, rider triggering remains likely.

Snowpack Summary

The stormy weather pattern continues with another 15-30 cm accumulating by Thursday morning (and more expected through the day). Temperatures have been mild with rain up to treeline in parts of the region. Winds have also been strong from the W-SW resulting in continued wind slab formation in exposed lee terrain. The December 22nd surface hoar layer is now over 100cm deep in southern sections and around 50 cm further north. This layer may have been flushed out in many areas, but remains a concern on slopes that have not released or where less snow has accumulated. The mid pack features several persistent weak layers but these have likely gained strength at this point. In the north of the region a basal facet and crust combo lingers near the ground and is most prevalent on NW - E facing aspects at the upper TL and alpine elevations.

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.

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.