Regions
Northwest Coastal.
Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain for the entire period
Weather Forecast
A series of frontal systems is expected to affect the region throughout the forecast period with at least 20-30cm each day for Thursday, Friday, and Saturday, with Thursday looking like the warmest and wettest. Strong southerly winds on Thursday should shift to southwesterlies by Friday. Freezing levels around 1000m on Thursday should drop to or near valley bottoms by the weekend.
Avalanche Summary
Recent reports include more evidence of a large natural cycle from earlier in the week, but natural slab activity seems to have tapered off for the time being with only isolated relatively small events observed yesterday. Two slope-cut slab avalanches on a convex roll sliding fast on surface hoar down 40-50cm. Other slope cuts triggered Size 1 wind slabs in the immediate lee of ridgecrests and exposed rolls and explosive control in the Shames area triggered Size 1.5 to 2.0, 20-50cm thick slabs on east aspects. Several small natural dry sluffs involving the recent storm snow and surface facets were reported as travelling a fair distance for their size.
Snowpack Summary
Moderate amounts of new snow and strong southwesterly and down flow (katabatic) winds are keeping wind slabs fresh and weak on lee slopes and cornices looming from ridgecrests. It seems as though surface hoar buried at the beginning of March, now down 25-50cm under the recent storm snow, has a sufficiently cohesive slab for avalanches in some areas. The early February persistent weak layers down around 80-100cm at treeline seem to be bonding, but remain a concern at higher elevations with heavy triggers such as cornice falls. While below treeline these persistent slabs are reactive to human triggers on isolated sheltered steep terrain where buried surface hoar is preserved. Recent compression tests on a southwest aspect at 1000 m gave moderate sudden collapse results down 60cm where this weakness was found to be facets with an associated crust.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
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.