Avalanche Forecast

Issued: Jan 18th, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The avalanche danger east of the crest should gradually decrease on Monday. Watch for lingering wind and storm slab especially above treeline.

Summary

Detailed Forecast

Light west flow, light snow showers mainly west of the crest and slightly lower snow levels should be seen on Monday. East of the crest this should not build significant new layers. The avalanche danger should gradually decrease on Monday as new layers from Sunday partly stabilize.

Wind slab from Sunday should linger east of the crest mainly above treeline on Monday. Watch for firmer wind transported snow on previous lee slopes.

Storm slab from Sunday may linger to some extent east of the crest on Monday but will be stabilizing. Watch for cracking and releases on steep sheltered slopes where snowfall exceeded an inch an hour for more than a few hours on Sunday.

The cooling should cause some good right side up snow conditions on other slopes on Monday.

Mid pack crust and facet layers east of the crest from earlier in the winter are not currently expected to be reactive.

Snowpack Discussion

Strong southwest flow carried a wet front across the Olympics and Cascades Saturday night. This front tapped subtropical moisture so snow levels got pretty high with .5-1.5 inches of WE at sites of the crest. Snow was most significant in the northeast and central east with about 12 inches at Holden and areas east of Stevens Pass. There may have been some avalanches in this area though we do not have any reports yet. About 4 inches of snow fell at Harts Pass with less elsewhere.

West southwest flow and mostly moderate to heavy orographic showers are occurring Sunday with showers probably spilling to east of the crest with slightly lowering snow levels.

Several reports are on hand for the past couple days.

NWAC observer Tom Curtis was north of  Blewett Pass on Thursday and found widespread 5mm+ surface hoar up to 5400 feet. This layer may not have survived through the weekend. The non-reactive persistent facet layer found on Jove Peak on Wednesday was also found at 85 cm Thursday.

The North Cascade Mountain Guides were at Washington Pass on Saturday and reported about 10-20 cm of snow from the previous storm and some wind stiffened snow but no real slab layers.

An avalanche class near Mission Ridge on Saturday reported via the NWAC Observations that facets between crust layers in the mid pack gave a Q1 shear but did not show a tendency to propagate via an ECT.

The Mission Ridge ski patrol Sunday reported a surface rain crust with no natural or ski cut avalanches.

The overall snowpack remains well below normal for this time of year and some windward or southerly aspects have little if any snow cover.

 

 

 

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: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 19th, 2015 10:00AM