Regions
Northwest Inland.
A special warning has been issued for the substantial warming and sun this week. Deeply buried weaknesses within the snowpack are expected to wake-up and produce very large avalanches. It is best to avoid avalanche terrain during the heat of the day.
Weather Forecast
The ridge of high pressure is expected to break down on Friday as a weak frontal system reaches the coast. Friday should be sunny with alpine winds progressively becoming strong from the southwest and freezing levels progressively falling below 2000m. Cloudy conditions are expected for Saturday with moderate to strong southwest winds in the alpine and freezing levels dropping to around 1500m. A mix of sun and cloud is expected for Sunday with light to moderate southerly winds in the alpine and afternoon freezing levels around 1400m. A weak storm system is currently forecast to reach the region on Sunday night.
Avalanche Summary
On Wednesday, numerous slabs up to size 2.5 were reported on solar aspects in the far north of the region. Some of these releases were over 2m deep releasing on old weak layers. In the south, a natural icefall triggered a size 2.5 slab avalanche. Several loose wet avalanches were also reported. On Tuesday, widespread sluffing was reported at all elevations and well as some shallow solar triggered slabs up to size 1.5. Cornices and glide cracks are also reported to be very active in the north of the region. The continued warming, sun exposure, and limited overnight recovery means natural cornice releases, deep persistent slab releases, and loose sluffing are expected to continue on Friday. Very large avalanches will remain a serious concern until the region gets some sustained periods of cooling,
Snowpack Summary
A good overnight refreeze was reported in the south of the region on Thursday morning. This crust is expected to have quickly broken down during the day. In the north of the region, no surface crust formed overnight. On Wednesday, moist surface snow was being reported to mountain tops on all aspects with wet surface snow at treeline and below. A weak crust/surface hoar layer which was buried over a week ago is down 20-30cm in the north of the region. A widespread crust/facet layer buried in early February and a lingering surface hoar layer from January can now be found down a meter or more. Weak basal facets also exist in some areas and have been shown to be most reactive in the north of the region. All these old weak layers still have the potential to produce large avalanches, especially with a large trigger such as a cornice fall. Extra caution is required during this period of substantial warming and intense solar radiation, as this is when these layers have the biggest potential to wake up.
Problems
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.
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.
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.