Avalanche Forecast

Issued: Jan 10th, 2016 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

New small shallow storm and wind slab seems possible by the end of the day on Monday. Evaluate snow and terrain carefully on Monday. Adjust your plans if winds are stronger or new snowfall is heavier than expected.

Summary

Detailed Forecast

A front should weaken as it moves over the Northwest on Monday. Over the west slopes this should bring moderately increasing southwest alpine winds and mostly light occasional moderate rain or snow Monday morning changing to showers Monday afternoon with lowering snow levels.

Wet surface snow from Sunday should refreeze on Monday. 

Light to moderate new snow amounts should be favored on the volcanoes and very limited in the Cascade passes by the end of the day. Up to a few inches of snow may be seen mainly on the volcanoes by the end of the day.

New small areas of shallow storm slab seem possible mainly on the volcanoes. The limited amounts of new snow and the cooling trend should limit the extent and depth of new storm slab by the end of the day.

New small areas of shallow wind slab also seem possible mainly on lee slopes on the volcanoes by the end of the day. The limited amounts of new snow and the cooling trend should also limit the extent and depth of new wind slab.

 

Snowpack Discussion

Deep storm snow from late December is now well settled, homogeneous and stabilized in the numerous recent snowpits dug throughout the region.

Fair weather for about a week over the New Year caused extensive surface hoar and near surface faceted snow. About 4-15 inches of snowfall over the west slopes last week buried the surface hoar and near surface faceted snow along with some wind loading and wind slab formation. Several avalanches occurred on the New Year layer last week.

Mild weather, sunshine and light winds over the past several days with above freezing temperatures in most areas have caused the New Year layer to bond and strengthen.

NWAC pro-observer Ian Nicholson visited Chair Peak in the Alpental Valley on Friday. On a northeast slopes at 5500 feet he found the New Year layer to be gaining significant strength from earlier in the week. Tests gave hard results and did not indicate propagation with the layer difficult to distinguish about 6 inches below the surface.  

NWAC pro-observer Simon Trautman was also out on Friday near the Mt Baker ski area and found no signs of instability and no evidence of the layer from the New Year. Ski tests on steep slopes in all the elevation bands did not give results.

Ian was out again at Paradise on Saturday and generally found 2-3 cm melt form crusts over about 5 inches of snow from last week over the New Year layer. Snow pit tests gave moderate to hard results without propagation at the New Year layer.

 

Problems

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: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

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

Possible

Expected Size

1 - 1

Valid until: Jan 11th, 2016 10:00AM