Avalanche Forecast
Issued: Dec 15th, 2013 10:59AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
Warm weather will cause stabilizing of recent snow. Watch mainly for loose wet avalanche conditions. Old wind slab may be possible but only in the highest alpine and mainly in the north Cascades.
Summary
Detailed Forecast
An upper ridge should amplify over the US west coast on Monday. This should cause mostly dry and even warmer weather on Monday. Plenty of sun should be seen in the central and especially the south Cascades. But the sun is weak at this time of year.
The main effects of this weather should be to cause recent snow to further consolidate and further stabilize. Wet loose avalanches should be the main consideration and this should be mainly but not exclusively on steep solar aspects. Watch for increasing roller balls, pinwheels, wet surface snow deeper than a few inches and natural avalanches.
Isolated pockets of wind slab may also linger at the highest alpine elevations of the north Cascades where temperatures have been cooler and delayed the stabilizing of these layers.
Snowpack Discussion
The last major storm was at the start of the month. This storm produced heavy rain followed by Arctic cooling that deposited 1-2 feet of low density snow at sites near and west of the crest. Profiles following this storm cycle were generally lower density surface snow which was well bonded to a strong and generally thick rain crust. Little avalanche activity was seen following the storm due to the cooling, lack of wind and lack of slab in the new snow.
Dry and very cold weather followed with periods of both strong easterly and strong west to northwesterly winds a week ago. This scoured some slopes and built small, isolated wind slabs on a variety of lee slopes at higher elevations. The dry cold stretch also produced widespread, thick surface hoar frost and near surface faceted snow at or near the surface of the snow pack in sheltered areas.
Several hours of strong westerly winds and a warming trend hit on Thursday such as was recorded by the NWAC weather station at the Mt Baker ski area. This transported loose surface snow and built pockets of shallow wind slab that produced some skier triggered releases at the Mt Baker ski area. Weak layers were provided by some of the recently buried hoar frost or near surface faceted snow.
About 2-9 inches of wet snow was seen later Thursday and early Friday with the most at Paradise. This produced some more 6-12 inch skier triggered storm slab releases at the Mt Baker ski area on Friday. Shallow ski triggered storm slab was reported elsewhere such as at Mt Hood Meadows. With warming temperatures some small shallow wet loose avalanches also began to be reported.
Warmer weather is continuing to destroy old weak layers and is causing consolidation and stabilizing and a variety of minor wet snow conditions continue Saturday and today. Small shallow wet loose avalanches, roller balls, pinwheels and shallow wet snow conditions are reported from many areas including Hurricane, the Mt Baker ski area, Snoqualmie Pass, Paradise and Mt Hood. Shallow snow cover is limiting the avalanche danger at lower elevations.
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.
Aspects: All aspects.
Elevations: All elevations.
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.
Elevations: Alpine.
Likelihood
Expected Size
Valid until: Dec 16th, 2013 10:59AM
