Avalanche Forecast
Issued: Dec 24th, 2013 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Summary
Detailed Forecast
Mostly sunny days and fair nights are expected except some low clouds or fog in the lower elevation valleys. Winds should remain light with mild temperatures at higher terrain Christmas Day. This should allow for some surface snow melt on sun exposed terrain. Some shallow wet snow conditions may develop by late morning or afternoon on some of these steep sun facing slopes where caution should be exercised. Previously formed wind slab near ridges should also continue to slowly stabilize.
Snowpack Discussion
Warm wet weather December 13-14th followed by dry weather with high freezing levels helped to form crust layers and stabilize earlier persistent buried weak layers from earlier in December. Most recently a series of fronts over the past four days caused additional precipitation, mostly at moderate freezing levels causing rain or wet heavy snow at mid and lower elevations while depositing 30-60 cm (1-2 ft) of storm snow over most areas along the west slopes and volcanic peaks. This storm cycle while not powerful for northwest standards as caused some recent avalanche activity ranging from wet slabs, loose wet, storm slabs and wind slab releases.
Most recently on Monday 12/23 widespread natural and triggered wind slab releases where seen in the Mt Baker area. The slab releases ranged mostly from 40-60 cm (18-24 inches) and ran long distances with some sympathetic releases triggered remotely. These size 1.5-2 slides released on wind loaded northerly facing slopes below ridges but were extensive.
A wet slab was reported below Chair Peak in the Alpental Valley Sunday near midday and ran a far distance.
Fluctuating freezing levels over the past several days was followed by a cold front late Monday that brought significant cooling by Monday night and early Tuesday. This cooling has likely helped to strengthen and consolidate previous wet snow or unstable storm slab layers helping to lower the current overall danger.
Watch for areas where a locally deeper snow pack may exist. The most likely areas to find unstable conditions would be on steep generally northerly to east facing terrain below ridges and near or above tree line. These would be the areas most likely to have formed wind slab.
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, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 25th, 2013 10:00AM
