Freezing levels are expected to rise to 3000 m for the next two days. Treeline temperatures steady near +6 accompanied by intense solar radiation and limited re-freeze. Expect danger ratings to remain elevated through the forecast period.
Confidence
Fair - Intensity of incoming weather is uncertain on Thursday
Weather Forecast
The beautiful weather we've been seeing over the past few days will start to change as a dominating upper ridge begins to break down, and turn to a warm Southerly flow. A series of systems caught in this flow will affect the region through to Friday. Tuesday: Continued sunny, cloudless skies with light ridgetop winds from the SW. Freezing levels could reach 3000 m. Treeline temperatures steady near +6 C. Wednesday: A mix of sun and cloud with possible afternoon rain showers. Treeline temperatures near +10, with freezing levels 3000 m. Thursday: Possible 20-30 mm of precipitation (mix rain and snow) through the day, with freezing levels falling to 2000 m. Ridgetop winds moderate from the West.
Avalanche Summary
On Sunday a natural cornice fall triggered a large size 3.5 avalanche. This occurred on a North aspect at 2800 m. The width of the avalanche was 100 m, running 1100 m in length. This ran on facets to ground. This may be an indicator of more to come; especially with an increase of solar radiation and high freezing levels. One other natural cornice fall triggered a size 1.5 on the slope below. This cornice chunk triggered the March 27th layer on an East aspect around 2700 m. Last Friday, control work in the region produce 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.
Snowpack Summary
Mild temperatures and intermittent solar radiation have really settled out last weeks 25-40 cm of storm snow. The recent storms have added up to about 125 cm of snow which rests on the March 27th interface. This interface consists of 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. This will be a layer to watch over the next couple of days with rising freezing levels, little re-freeze. 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.
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.