Avalanche Forecast
Issued: Dec 21st, 2013 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Expect to find a wide variety of snow conditions depending on elevation on Sunday. Make new snow pack evaluations as you change elevation if you are in the back country on Sunday.
Summary
Detailed Forecast
Another warm front will move over the Northwest on Saturday night and Sunday morning. This should cause increasing crest level winds, increasing rain or snow and further rising snow levels. However this system should cause less precipitation in the south such as at Mt Hood compared to the last system.
The rising temperatures and increasing snow densities in the high alpine should help build some new areas of wind slab or storm slab of up to several inches in the high alpine on Saturday night and Sunday morning.
Rain near treeline and below treeline will also load and weaken recent snow in those areas and cause wet snow avalanche conditions. This should be most dangerous in areas that had greater amounts of new snow on Saturday morning.
Expect to find a wide variety of snow conditions depending on elevation on Sunday. Make new snow pack evaluations as you change elevation if you are in the back country on Sunday.
A break between systems should be seen Sunday afternoon with light rain or snow showers mainly near and west of the crest. This should allow slab layers in the high alpine and wet snow near and below treeline to partly stabilize.
A cold front should approach from over the Northwest coastal waters on Sunday night. This should cause increasing crest level winds, increasing snow and lowering snow levels Sunday night through Monday morning. This should cause an increasing avalanche danger Sunday night and Monday.
Snowpack Discussion
Warm wet weather last weekend created another crust layer in most areas. This seems to have capped and stabilized the older hoar frost and faceted snow under that crust and made those layers generally unreactive.
Cold weather, light amounts of low density snow and some surface hoar formation were generally seen mid-week.
This set the stage for a warm front storm that moved across the Northwest on Friday. This generally caused strong southwest winds, increasing moderate to heavy snow and greatly warming temperatures. The snow began at cooler temperatures and became wet and heavy as temperatures warmed through the day. Sites near and west of the crest ended up mostly with about 4-12 inches of very wet heavy snow with the most at the top of Alpental and at Paradise. This created unstable upside conditions with the higher density wet snow at the surface and likely avalanche conditions especially at higher elevations.
Reports are sparse for yesterday and today likely due in part to the poor ski conditions. The Mt Baker ski patrol reported sensitive ski triggered wind slabs up to about a foot by Friday mid-morning with these conditions increasing by midday. Some stabilizing had occurred by today when they reported only small areas of wind slab up to 14 inches. The Crystal Mountain ski patrol today reports to little new snow for slab formation. The Mt Hood Meadows ski patrol today reports a stable freezing rain crust in the near tree line and alpine but it is unknown if this condition extends very far around the mountain. Little new back country information is currently available.
Shallow snow cover continues to limit the avalanche danger at lower elevations in most areas.
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.
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
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: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Dec 22nd, 2013 10:00AM
