Avalanche Forecast

Issued: Feb 7th, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Expect a decreasing avalanche danger Sunday with a break between systems. Watch for areas where more significant new snow has accumulated, mainly above treeline where new storm and wind slab are more likely. 

Summary

Detailed Forecast

A break between systems is expected during the daylight hours Sunday. This should give time for any recent shallow storm slab or wet new snow to settle and possibly refreeze to form new surface crust layers. This should allow for a slightly decreasing danger at higher elevations where more recent snow may have accumulated. Watch for wind transported new snow on some higher elevation lee slopes, mainly NW-SE facing   

Remember, due to the unseasonably low snowpack especially at lower elevations, numerous terrain hazards such as open creeks, partially covered rocks and vegetation are widespread.  

Snowpack Discussion

Periods of light snowfall were this week east of the crest with more snowfall at higher elevations in the northeast and central east zones. Steady but generally light precipitation was seen during the day Thursday with a gradual warming trend that continued into Friday. The snow level hung around 5500 feet in the northeast Cascades for most of Thursday night with snowfall accumulation likely in the 6-10 inch range in the Washington Pass area before likely mixing with light rain Friday. Much less new snow was reported in the Stuart range and Mission Ridge area above 6500 feet Friday.  Slight cooling with a frontal system Saturday has deposited an additional about 2-4 inches of snow above about 4-5000 feet. 

Observer, Tom Curtis traveled to Dirty Face peak above Lake Wenatchee Saturday. Shallow wet snow was accumulating above about 4800 feet where some storm and wind slabs where developing through the day near and above treeline while wet snow avalanches were possible below treeline.  There were no natural avalanches or instabilities observed with minimal wind slab developing along ridges from 5400 feet and above. The mid January crust layer we have been tracking was now poorly defined with no results from several stability tests performed. The new storm snow was only a few inches an was limiting the avalanche potential. 

NWAC pro-obs and North Cascade Guides, report last week in the northeast and central east zones have the January 15th facet/crust layers at 40 cm-1 m below the surface with the facets starting to turn to rounded grains and not reactive or not releasing in pit tests. It thus appears the January 15th facet/crust layers are strengthening and stabilizing. This problem has been removed from the central east zone forecast, but retained in the northeast zone until more information becomes available following this storm cycle.

Other private groups in the Washington Pass area on February 2 and 3 report some upside down densities in the recent snow from earlier in the week and natural and ski cut loose dry avalanches. 

Jeff Ward was in the Stuart Range Friday and found small but sensitive, loose wet avalanches naturally and skier triggered above about 6500 ft. Below this elevation, there was not enough new snow for wet loose danger and the most recent crust had not significantly softened. In the area traveled, the rest of the pack was consolidated and the Jan 15th layer was non-reactive in snowpit tests.

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, North West.

Elevations: Alpine.

Likelihood

Possible

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: Feb 8th, 2015 10:00AM