Avalanche Forecast
Issued: Dec 25th, 2015 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 Dry.
NWAC,
Northwest Avalanche Center
A consolidating and stabilizing trend will continue on Saturday. Remember to discuss plans and maintain visual contact with your partners.
Summary
Detailed Forecast
An upper ridge will shift over the Northwest on Saturday. This should cause light winds and fair weather on Saturday with freezing levels staying low in the Olympics and Cascades. Some high clouds should be seen by Saturday afternoon from a weak system that will approach Saturday night.
This weather will bring further consolidating and further stabilizing on Saturday. Cool temperatures may somewhat slow the stabilizing but the avalanche danger will be less than the previous few days.
Recent wind slab should remain possible on north to east slopes in the above tree line east of the crest. Wind and storm slab avalanches should stay within recent storm layers but could still move fast. Test for inverted strong over weak storm snow and give cornices a wide margin.
Beware of loose dry avalanches in steep non-wind affected terrain that could have unintended consequences such as knocking you off your feet and into a terrain trap.
Remember to discuss plans and maintain visual contact with your partners.
Snowpack Discussion
We have had about a week of heavy snow and cool temperatures in the Olympics and Cascades. Sites east of the crest have had 2-4 FEET of snowfall during this time!
A report via the NWAC Observations page indicates 2 skiers were caught in a wind slab avalanche, carried a short distance but then were able to dig themselves out on Driveway Butte on Tuesday in the Methow River area. The crown at the trigger point was 6 inches but ranged up to 2 feet. This was on a north slope at about 5840 feet.
A snow pit report from 5700-6500 foot range in Stemilt Basin near Mission Ridge also on Tuesday indicated a buried surface hoar at 45 cm. This layer gave some cracking and whumping.
NWAC pro observer Tom Curtis was at Iron Mountain a bit west of Blewett Pass on Wednesday and found some wind slab on steep open slopes even to below tree line and often above terrain traps like creeks or trees. A storm layer was seen at 35 cm with wind and storm slabs reacting to tests to this interface. He noted wind slab releases of 20-35 cm on northeast to east slopes near tree line and storm slab releases near and below treeline on steep terrain features of varied aspect and on small steep slopes above Forest Service roads. He also found some difficult trail breaking in deep powder.
A report from the North Cascade Mountain guides near Washington Pass on Wednesday indicated a storm layer at 15-25 cm but ski cuts on test slopes did not give any results. Another report from the guides near Washington Pass on Thursday indicated a storm layer at 30 cm with isolated small ski triggered storm slab and some loose dry releases and good stability. A couple natural small wind and storm slab near ridges and dry loose avalanches were noted. Quite a bit of wind effect was seen on some nearby slopes. Natural cornices drops did not cause any avalanches.
The southeast zone should have a shallower snow pack. However, we have no recent observations from the southeast zone.
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, Treeline.
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, Treeline.
Likelihood
Expected Size
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 26th, 2015 10:00AM
