Avalanche Forecast

Archived

Apr 8th, 2012–Apr 9th, 2012

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

Purcells.

Confidence

Good

Weather Forecast

Monday is the tail end of the beautiful weather pattern we've been in for the past few days. The upper flow switches from northerly to a more mild southerly on Tuesday which will push freezing levels above 3000 m. Monday: Freezing level starts near the surface, topping out around 2000m. A few clouds build into the region in the afternoon. The current forecast shows that Monday night is the last good overnight refreeze until next weekend. Tuesday: Temperatures should already be near 2000 m at dawn and are forecasted to continue to climb to 3000 m by lunch time. Cloud cover increases to 50% as winds blow at light values out of the east below treeline. At ridge top expect moderate southeasterlies. Temps remain high through the night. Wednesday: Freezing level stays around 3000 all day. Ridge top winds moderate SE diminishing in speed at lower elevations.

Avalanche Summary

Very little avalanche activity on Saturday. The only reported avalanche involved an upper elevation W facing slope in the central portion of the region that produced several solar induced avalanches to size 2. On Friday control work in the region produced avalanches to size 3 on N through E aspects in the Dogtooth Range. Natural avalanches were observed in the central & southern portions of the region on both shady and sunny slopes at upper elevations to size 3. In two separate instances cornice fall triggered large avalanches on NE facing slopes in the alpine, something that I suspect will continue through the holiday weekend.

Snowpack Summary

Mild temperatures and intermittent solar radiation between breaks in convective snow flurries have really settled out last weeks storm snow. Wednesday's system produced around 40 cm of new snow in the north & 25 cm in the south. The recent storms have added up to about 125 cm of snow which rests on the March 27th interface. This layer displays as a sun crust on southerly aspects and a temperature crust on more northerly slopes. A thin layer of facets and or surface hoar can be found on this crust on all aspects. Cornices are reported to be very large and exist on most ridge lines. The mid-pack is well settled and strong. There is a weak layer of facets or depth hoar in shallower snowpack areas that will likely fail once the region starts seeing warm days without overnight refreeze. There is also a weak layer of facets or depth hoar above a crust at higher elevations where there was already snow in October.

Problems

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.

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.