Persistent warm temperatures are penetrating and destabilizing the upper snowpack. Natural avalanche activity is likely. Pay attention to sunny slopes and avoid travel under avalanche paths especially slopes with cornices overhead.
Weather Forecast
Hot and sunny with freezing levels through the roof! Very little temperature driven overnight re-freeze is expected. TUESDAY: Mostly clear skies and freezing levels holding strong above 3100 m. Alpine temperatures near + 16.0 degrees with moderate southeast wind at ridgetop. Alpine temperatures may drop to + 4.0 degrees overnight.WEDNESDAY: Mix of sun and cloud with freezing levels above 3100 m. Alpine temperatures +15.0 degrees with light southeast ridgetop winds. THURSDAY: Cloudy periods. Alpine temperatures near + 10.0 degrees and freezing levels 2400 m during the day.
Avalanche Summary
On Sunday, a widespread natural wet loose avalanche cycle occurred and was reported up to size 1.0. As the heat continues to penetrate the snowpack we suspect widespread natural wet avalanches will continue through the forecast period. These will be large and small avalanches on all aspects and elevations.
Snowpack Summary
The heat wave has likely eliminated any trace of cold snow, but you may still find some on high elevation north features. Wind slabs are likely done at this point, zapped of their strength by time and warming. We're very concerned about the upper 20 to 30 cm of snow that sits on crust on steep south slopes and possibly weak surface hoar crystals on sheltered and shaded slopes.The even bigger questions are deeper in the snowpack. A layer of weak and sugary faceted grains sits on a melt-freeze crust about 50 to 120 cm deep. This layer is most prominent in the North Shore Mountains on north aspects. With little overnight re-freeze the warm temperatures will penetrate deeper allowing melt and a lot of water which will lubricate the upper snowpack. It also allows the upper snowpack to start creeping downhill at an accelerated rate. Its hard to say how many hot days and warm nights it will it take to wake up the more deeply buried weak layers. We're not sure, but now is probably a time to let the mountains do their thing from afar and check back in when the freezing levels return to seasonal norms which could happen by next weekend.
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.