Avalanche Forecast

Issued: Dec 23rd, 2015 12:06PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Wind Slabs, Storm Slabs and Loose Dry.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Further wind and storm storm slab and wind slab are likely Wednesday but also watch for where a small loose dry avalanche could have unintended consequences such as knocking you off your feet and into a terrain trap like creeks or trees.

Summary

Detailed Forecast

A cool upper trough will continue to sink south over the Pacific Northwest on Thursday. West winds should begin to decrease Thursday initially in the north Cascades. Snow showers Thursday should bring another few inches mainly to the central and south part of the east slopes.

Dangerous avalanche conditions should persist Thursday especially in the near and above treeline zones where new or recent storm and wind slab should be likely. Storm and wind slab avalanches should remain within recent storm layers but could move fast. Watch for evidence of wind deposited snow or pillows. Test for inverted strong over weak storm snow and give cornices a wide margin.

Loose dry avalanches in non-wind affected terrain will be difficult to manage on steep slopes. Even a small loose dry avalanche could have unintended consequences such as knocking you off your feet and into a terrain trap like creeks or trees.

Snowpack Discussion

The Christmas snow globe keeps giving here in the PNW! The east slopes are getting in on the action as well! It's already well beyond anything we achieved last year and getting deeper by the day. By Thursday morning the east slope areas will have received 2-4 feet of snow in less than a week.

A report via the NWAC Observations page indicates 2 skiers were caught in a storm or wind slab avalanche and carried a short distance 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.

NWAC pro observer Tom Curtis was at Iron Mountain a bit west of Blewett Pass on Wednesday and found 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.

The southeast zone should have a shallower snow pack. However, we have no recent observations from the southeast zone.

Problems

Wind Slabs

An icon showing 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

Likely

Expected Size

1 - 1

Storm Slabs

An icon showing 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: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Loose Dry

An icon showing 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

Possible

Expected Size

1 - 1

Valid until: Dec 24th, 2015 12:06PM