Confidence
Fair - Intensity of incoming weather systems is uncertain on Thursday
Weather Forecast
The high pressure system of late weakens and gives way to a series of Pacific frontal systems.Tuesday: Mainly cloudy, no precipitation, alpine temperatures -8, moderate northwest easing to light west winds.Wednesday: A Pacific frontal system starts to affect the interior regions. Cloudy with light snowfall, moderate to strong west winds. Freezing level in the valley bottom.Thursday: Light snowfall, Alpine temperatures -7, moderate west winds.
Avalanche Summary
Reports continue to roll in about the extent of the avalanche cycle from last Thursday and Friday. The larger of the explosives triggered avalanches were mostly running on north through east facing slopes at upper elevations and involved the basal weaknesses near the ground up to size 3.5. More recent avalanche activity seems to be isolated to a few windslab releases up to size 2.5. These are running within the storm snow in the alpine and at treeline elevations.
Snowpack Summary
Last week's storm brought 25 - 60 cm of snow to the region. This snow lies on top of old wind slabs, a buried rain crust that exists below 1600m and a surface hoar or facet layer that is down 100-150cm deep. Snowpack depths vary, but in general 175 cm of snow can be found at treeline, with 125-300 cm in the alpine. In some places we're still dealing with a relatively thin snowpack (thanks to a windy early season). The basal facet/crust combo (weak sugary snow above and below a crust) near the ground was active in an avalanche cycle last Thursday and Friday. This weakness may be difficult to trigger but if triggered, will result in very large, destructive avalanches.
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.