Avalanche Forecast
Issued: Feb 11th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Wind Slabs and Cornices.
NWAC,
Northwest Avalanche Center
Light rainfall will renew the loose wet avalanche potential on all aspects of steeper slopes below treeline. New snowfall should bond well to a moist snow surface in most locations, but wind-driven snow may build unstable wind slab on lee aspects of higher terrain by late Friday afternoon.
Summary
Detailed Forecast
Mild snow levels should keep precipitation in liquid form below 6000 feet until a cold front passes through the Cascades Friday mid-afternoon with snow levels lowering to 4500 feet by late afternoon. Precipitation should be light, except moderate Friday afternoon with the frontal passage.
Light rainfall will renew the loose wet potential on all aspects of steeper slopes below treeline. New snowfall should bond well to a moist snow surface in most locations, but wind-driven snow may build unstable wind slab on lee aspects of higher terrain by late Friday afternoon.
Continue to watch for loose wet avalanche potential above terrain traps such as above cliffs, near gullies or where avalanche debris would deeply accumulate. These are the types of places that even small wet avalanches can have serious unintended consequences.
Avoid cornices along ridges and slopes below cornices since cornices may still be weaker due to the warm weather.
Snowpack Discussion
Weather and Snowpack
Two fair weather periods in January allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th, but are no longer considered a persistent slab threat after several warm and wet systems tested this layer and it has been unreactive where it can still be identified in recent snowpit tests.
A large upper ridge and warm air mass aloft over the West Coast from this past Sunday through Wednesday led to the warmest weather of the winter. During this stretch temperatures were generally above freezing. Freezing levels came down a few notches Thursday, but mild conditions continued with spotty light rain or snow increasing Thursday afternoon.
The warm temperatures and solar effects caused loose wet avalanches, snowpack consolidation, and melt-freeze surface crusts. This will have further stabilized the lower and mid snow pack and turned the most recent storm snow into moist to wet snow in most areas.
Recent Observations
Extensive observations in the Washington Pass area over the past few days indicated an active loose-wet avalanche cycle occurred mainly Monday afternoon.
NWAC pro-observer Jeff Ward was in the Washington Pass area in the 6000-7600 foot range on Tuesday and saw several size 1 natural loose wet avalanches on steep solar slopes.
NWAC pro-observer Tom Curtis in the Blewett Pass area in the 4000-5900 foot range Tuesday and also saw several small to large wet loose avalanches on solar slopes near and above tree line.
A professional observation from the central-east zone Thursday indicated that a facet/Jan 28th rain crust combo was reactive in snowpit tests, but no avalanches were observed down to this interface.
Problems
Loose Wet
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. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. 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.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Below Treeline.
Likelihood
Expected Size
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, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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 lips 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.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Feb 12th, 2016 10:00AM
