Lingering storm instabilities will persist for several days, especially in the alpine. Human-triggering remains likely and conservative decision-making remains critical.
Confidence
Fair - Timing or intensity of solar radiation is uncertain
Weather Forecast
A ridge of high pressure will build over the interior on Tuesday. Mostly cloudy conditions are expected but sunny breaks are possible in the afternoon. Light scattered flurries are possible in the morning but things should dry out by the afternoon. Alpine winds should remain light. Freezing levels on Tuesday are expected to be around 1500m but should fall to near valley bottom Tuesday overnight. On Wednesday, mainly dry conditions are expected with a mix of sun and cloud. Freezing levels are forecast to climb as high as 2000m by the end of Wednesday. There is some uncertainty for Thursday with light scattered flurries possible or partly sunny conditions. This will depend on the northern position of the ridge. Freezing levels are expected to climb to around 2500m on Thursday.
Avalanche Summary
A widespread natural cycle to size 3.5 has been reported on Friday through Sunday. This includes both storm slabs and persistent slabs. Avalanches are expected to be starting dry and ending wet, potentially running to valley bottom. On Tuesday, natural avalanches remain possible at upper elevations but are generally not expected. Human-triggering remains likely and travel in avalanche terrain is not recommended until the storm slab has had a chance to stabilize.
Snowpack Summary
60-90cm of rapidly settling storm snow sits over the late-Jan weak layer at higher elevations. Rain has soaked the snow surface up to around 1700m and moist snow is reported to at least 2000m. Strong SW winds have formed wind slabs in exposed leeward terrain features. The storm slab sits on an old rain crust at lower elevations, variable surface hoar, and/or wind affected surfaces at higher elevations. The mid-January surface hoar is typically down 80-120cm and is still reactive in some areas. The mid-December weak layer is down over 120cm but has generally been unreactive.
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.
Wet Slabs
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.