Highly sensitive storm slabs overlying surface hoar are reactive in all elevation bands. Conservative terrain selection is essential for safe travel.If the sun is out in full strength on Wednesday, use extra caution on steep south facing slopes.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain on Thursday
Weather Forecast
A weak ridge of high pressure should resulting in mainly dry and sunny conditions for Wednesday. Freezing levels are expected to be 1000-1200m and alpine winds should remain light from the southwest. A fairly substantial storm system is expected to arrive Wednesday overnight or Tuesday morning. Models are currently showing 30-40cm for the region by Thursday night. Alpine winds are expected to be strong from the southwest during the storm. Freezing levels during the storm are uncertain and could be anywhere from 1700 to 2300m on Thursday afternoon. The storm will likely continue on Friday morning but is expected to clear out by the end of Friday.
Avalanche Summary
On Monday, explosives triggered several size 1.5-2 slabs which included avalanches running within the storm snow as well as on the early-January weak layer. Natural avalanches were also reported to be releasing below treeline on the early January layer as a result of warming temperatures. On Sunday in the upper Lillooet, numerous recent natural dry loose avalanches up to Size 1.5 were observed below treeline. Subsequent avalanche control produced 25-45cm thick storm slab avalanches up to Size 2, running on the early January surface hoar. On Saturday, three human triggered avalanches were reported from north of the region. A ski cut on a northwest aspect at 1700m caused sluffing which subsequently triggered a size 1.5 slab avalanche which released on the early-January surface hoar. Two 30cm thick soft slab avalanches were ski cut in a northeast facing cross-loaded gulley feature at treeline.
Snowpack Summary
20-25cm of fresh storm snow has buried the recently formed mid-January surface hoar layer. Below that are the two early-January interfaces down 40-60cm which are separated by around 5-10cm of snow and both consist of a sun crust on steep solar aspects and a layer of surface hoar on sheltered and shady slopes. Strong southeast through southwest winds have been loading lee features at treeline and in the alpine. The mid and lower snowpack is generally strong, with the exception of shallow snowpack areas that may be more faceted.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.