Recently formed wind slabs and touchy persistent weak layers are making for tricky conditions. Conservative terrain selection remains important.
Confidence
Moderate - Due to the number of field observations on Sunday
Weather Forecast
A ridge of high pressure off the coast will slowly move eastward on Monday and should be established over the interior by Tuesday. Mostly cloudy conditions and light scattered flurries are expected in the north of the region on Monday and a mix of sun and cloud is expected in the south. Alpine winds are expected to be light from the northwest in the morning but may become moderate in the afternoon. Freezing levels are expected to be below valley bottom during the over periods and stay below 1000m in the afternoons. A mix of sun and cloud is expected for Tuesday with light to moderate westerly winds in the alpine. Increasing cloudiness is expected for Wednesday in advance of a weak storm system that is currently forecast to arrive on Wednesday night.
Avalanche Summary
On Saturday, no natural activity was reported but explosives triggered five avalanches size 1.5-3 avalanches in the west of the region. These were all at around 2600m and occurred on a variety of aspects. The slab thickness were all 50-70cm and the failure plane was reported to be facets. Widespread natural avalanche activity up to size 3 was reported to have occurred during the storm Thursday overnight and Friday morning. Most of this natural activity was on wind loaded aspects (north through east) in the alpine and at treeline but a few avalanches were also reported on south and west aspects. Explosive use on Friday produced mainly size 2-3 storm slabs 20-50cm thick on a variety of aspects above 2300m. Some of the explosive results are assumed to be failing on persistent weak layers with slab depths up to 100cm. Also reported were a couple size 1-1.5 human-triggered storm slab avalanches. These were 25-30cm thick slabs. Recently formed wind slabs and a reactive persistent slab are both expected to remain sensitive to human-triggering on Monday.
Snowpack Summary
40-60cm of snowfall has typically accumulated over the past week and weaknesses may still exist within or beneath this snow. Southwesterly winds have recently build wind slabs on the leeward side of ridge crests and behind terrain features on cross-loaded slopes. The persistent weak layer of surface hoar buried in early-January is now typically down 60-90 cm and has recently produced some large, destructive avalanches in the region. The layer is slowly getting harder to human trigger but is still quite reactive in snowpack tests and has the potential for wide propagations. In general, the lower snowpack below this layer is well settled and strong.
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.