As we approach spring, remember that the sun can have a very rapid warming effect on snow. Cornices, wind slabs and loose surface snow can easily fail when exposed to rapid warming.
Weather Forecast
Mostly cloudy with convective cells moving through. Only trace amounts expected. Light winds at ridge top, gusting to 45km/hr. Alpine high of -8. Valley bottom freezing level. We are at the time of year when any solar exposure can have a dramatic and sudden effect on the snow surface. It is also very difficult to forecast for these sunny periods. Watch for sun exposed places, and treat accordingly.
Avalanche Summary
A natural cornice sz 2.5, 2600m, NE aspect, initiated in steep cliffs and entrained snow below. There was widespread pin wheeling and loose wet avalanches from yesterday afternoon's heat. Average size was 1, with a few scattered 1.5's.
Snowpack Summary
Despite yesterday's high of 5 degrees, the snow held in there. We escaped another temperature crust and are left with a new crust on all solar aspects. This new sun crust extends well into the alpine. Valley bottom travel is reasonable with the older breakable crust down only a few cm's. At treeline, northerly aspects still have dry snow with some wind effect in open areas. The alpine has been hit with winds and is mostly breakable windslab on lee aspects (NE-S).
Problems
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.
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.