Regions
Northwest Coastal.
Stormy conditions are elevating the avalanche danger to HIGH. Strong winds, rising freezing levels and heavy precipitation may trigger very large avalanches.
Confidence
Moderate - Intensity of incoming weather systems is uncertain on Sunday
Weather Forecast
On Sunday and Monday another wet, warm and windy storm will impact the region. Expect between 30-50mm of precipitation per 24 hour period with freezing levels climbing from 500m on Sunday morning to 1800m by Monday afternoon. On Tuesday, expect a mix of sun and cloud with the freezing level shooting up to 2500m. Ridgetop winds should remain strong to extreme from the southwest for the forecast period.
Avalanche Summary
Avalanche observations have been quite limited in recent days, mostly due to stormy weather. However, in the mountains near Stewart fairly widespread storm slab activity (mostly size 2 with a few to size 3) was reported on Friday. Looking forward, a juicy storm on Sunday and Monday is forecast to promote widespread storm slab avalanches. New snow will also increase the likelihood of destructive persistent slab avalanches. At elevations where rain falls loose wet and wet slab avalanches are also likely.
Snowpack Summary
A warm, wet and windy storm forecast for Sunday will create deep new storm slabs throughout the day adding to an ongoing storm slab problem. Freezing levels are expected to rise dramatically throughout Sunday leaving surfaces at treeline and below saturated by the afternoon. Recent storm accumulations may be especially touchy as they are sitting on a new surface hoar layer that was buried on February 3rd. The January 9th surface hoar/facet layer is down 70-150 cm in most places, and remains a concern on all aspects and elevations, especially with heavy and/or thin-spot triggers. Around the same depth you may also find a surface hoar/facet interface from around New Years. There continues to be a lot of uncertainty regarding the reactivity and distribution of this destructive persistent avalanche problem; however, warming and forecast heavy storm loading will increase the likelihood of triggering these layers where they still exist. A conservative approach to mountain travel is still required.
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.
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.