Stevens Pass Avalanche Forecast

Issued: Dec 16th, 2013 7:47AM

Mon Dec 16th Current Conditions
Alpine Moderate Treeline Moderate Below Treeline Low
Tue Dec 17th 2 Day Outlook
Alpine Moderate Treeline Moderate Below Treeline Low

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Loose Wet and Wind Slabs.

Summary

Detailed Forecast

Mild or warm temperatures should generally persist until Tuesday morning. Then the south part of a greatly weakening front will begin to reach the Olympics and Cascades Tuesday afternoon and night. This should cause light rain or snow and cooling to spread from the Olympics and north Cascades Tuesday afternoon to south Cascades Tuesday night.

The main effects of this weather Tuesday should be to cause the snow pack to further consolidate and further stabilize. Mostly small wet loose avalanches should still be the main concern Tuesday morning and this should be mainly but not exclusively on steep solar aspects. Watch for roller balls, pinwheels, wet surface snow deeper than a few inches and natural avalanches. This problem should decrease as cloud cover increases and temperatures begin to cool by Tuesday afternoon.

Isolated small areas of wind slab may also linger at the highest alpine elevations of the north Cascades where temperatures have been cooler and will have slowed the stabilizing of these layers.

New rain or snow should only reach the Olympics and north Cascades Tuesday afternoon before spreading to the rest of the area Tuesday night. Overall new snow amounts should be light and should not cause an extensive new avalanche danger by Wednesday morning.   The cooling trend should help to create favorable shallow stable new snow profiles. Small areas of shallow new storm slab or wind slab might be possible if any area gets more than a few inches of snow by Wednesday morning.

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. Little avalanche activity was seen following the storm due to the cooling, good bonds to the newly formed (early December) crust, lack of wind and generally cohesionless new snow.

Dry, cold and locally windy weather followed during the first week of December. This produced local wind slab and scoured surfaces in exposed areas, and hoar frost and near surface faceted snow in sheltered areas.

Several hours of strong westerly winds and a warming trend hit the north Cascades last Thursday. This 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 late Thursday to Friday morning 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 and some shallow storm slab such as at Mt Hood Meadows.

The most important weather factor for most areas should be the warming that began Friday and continues Monday on both sides of the Cascade crest. This should have mostly eliminated the temperature gradient that was in the snowpack in most areas. This will be causing rounding of old hoar frost and faceted snow and will be consolidating and stabilizing previous snow and wind slab layers in most areas.  A variety of mostly minor wet snow conditions are reported the past couple days such as small shallow wet loose avalanches, roller balls, pinwheels and shallow wet snow conditions such as from Hurricane, the Mt Baker ski area, Stevens, Snoqualmie and Mt Hood.

Shallow snow cover is limiting the avalanche danger at lower elevations.

Problems

Loose Wet

An icon showing Loose Wet

Likelihood

possible

Expected Size

1 - 1

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.

Wind Slabs

An icon showing Wind Slabs

Likelihood

unlikely

Expected Size

1 - 1

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.

Valid until: Dec 17th, 2013 7:47AM

Forecast Trend

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