Avalanche Forecast
Issued: Feb 8th, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
A heightened avalanche danger will mainly be limited to steep open terrain or lee slopes above treeline.
Summary
Detailed Forecast
Warming is expected Monday with periods of light to moderate rain and snow with generally light to moderate winds. Rain near and below treeline should maintain some wet surface snow conditions, making triggered small loose wet avalanches possible on steeper slopes.
At the highest elevations, generally above 6000 feet, some building storm or wind slab is expected. These areas of storm or wind slab may build on isolated terrain features, such as lee aspects near ridges, mainly NW through NE facing.
Do note as a result of the unseasonably low snowpack, especially at lower elevations, numerous terrain hazards such as open creeks, partially covered rocks and vegetation are widespread.
Snowpack Discussion
Periods of light snowfall occurred this week east of the crest at higher elevations with more snowfall at higher elevations in the northeast and central east zones. The snow level stayed about 5500 feet in the northeast Cascades for most of Thursday night with snowfall accumulation likely in the 6-10 inch range in the Washington Pass area before likely mixing with light rain Friday. Much less new snow was reported in the Stuart range and Mission Ridge area above 6500 feet Friday. Slight cooling with a frontal system Saturday deposited an additional about 2-4 inches of snow above about 4-5000 feet as of Sunday. Warm daytime temperatures and some sunshine Sunday likely settled or melted recent new snow.
NWAC pro-obs and North Cascade Guides, report last week in the northeast and central east zones have the January 15th facet/crust layers at 40 cm-1 m below the surface with the facets starting to turn to rounded grains and not reactive or not releasing in pit tests. It thus appears the January 15th facet/crust layers are strengthening and stabilizing. This problem has been removed from the central east zone forecast, but retained in the northeast zone until more information becomes available following this storm cycle.
Jeff Ward was in the Stuart Range Friday and found small but sensitive, loose wet avalanches naturally and skier triggered above about 6500 ft. Below this elevation, there was not enough new snow for wet loose danger and the most recent crust had not significantly softened. In the area traveled, the rest of the pack was consolidated and the Jan 15th layer was non-reactive in snowpit tests.
Problems
Wind Slabs
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: North, North East, East, North West.
Elevations: Alpine.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: Alpine.
Likelihood
Expected Size
Persistent Slabs
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 9th, 2015 10:00AM
