A complex upper snowpack exists, especially in the northern areas of this region. Avalanche danger ratings may be a step lower in the south.
Confidence
Fair - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
A Pacific frontal system reaches the coast today and affects the Interior regions tonight through the forecast period. Warm air aloft will invade the region until Friday night with associated freezing levels near 2800 m. A trailing cold front will bring light precipitations and strong ridgetop winds. On Thursday, light precipitation amounts up to 5 mm expected, ridgetop winds strong from the SW and freezing levels rising to 2800 m. On Friday, solar radiation could come into play with a mix of sun, cloud and freezing levels steady at 2900 m. Unsettled conditions on Saturday will bring light precipitation amounts 5-10 mm, ridgetop winds moderate from the NW and freezing levels dropping to 2000 m.
Avalanche Summary
On Tuesday we received two separate reports of a skier triggered storm slab size 2 and a remotely triggered slab size 3 which occurred on Monday. A solar induced natural avalanche cycle occurred tuesday afternoon up to size 2, and explosive controlled size 2-3 slab avalanches were initiated from steep N-NE aspects above 2300 m. On Thursday, wind slabs are expected to be touchy and reactive to human-triggering. Where enough new snow exists, it may be possible to trigger storm slab avalanches up to size 3.
Snowpack Summary
At upper elevations recent storm snow amounts vary. The NW part of the region received up to 60 cm while the Dogtooth received around 30 cm and the far south received close to zero. This dense storm snow sits over the mid-March interface which consists of a series of crusts, wind affected surfaces, and/or old wind slabs. Snowpack tests are showing moderate- hard results with a sudden planar fracture characteristic. In exposed terrain, strong SW winds had redistributed new snow into wind slabs on leeward terrain features and slopes. On Sunday, the snow surface was reported to be wet to around 1800m and moist to around 2200m. Snow surface are moist up to 2200 m with no significant re-freeze expected. Digging deeper, 20-50 cm below the surface is the mid-February facet/crust interface. This interface has not been reactive in the Purcells but has been very reactive in the neighboring North Columbia region. Deeper persistent weak layers in the snowpack that have been dormant for several weeks and are not currently a problem but still have the potential to wake-up with major warming or heavy loading. Persistent weak layers may be suspect in shallower snowpack areas.
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.
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.
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.