Avalanche Forecast

Issued: Mar 1st, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Winds slabs should be isolated, but may have formed on infrequently loaded S-SW facing slopes, especially below ridges. Careful route finding and attention to local loading patterns will be necessary to have a safe and enjoyable day in avalanche terrain.

Summary

Detailed Forecast

Clouds should clear rapidly from the north Monday with cool temperatures. Increasing northerly winds are expected later Monday, especially above treeline and near crest level. Previously formed wind slab should have mainly stabilized. Watch for newly transported surface snow to mostly southerly facing slopes. Winds slabs will remain the primary concern Monday, so pay attention to the recent local loading pattern in your area, but generally expect non-traditional S-SW aspects to be the most suspect. 

Due to the low snowpack, especially below treeline, watch for terrain hazards such as open creeks, partially covered rocks and vegetation. Many areas below treeline do not have enough snow (new or existing) to pose an avalanche hazard. 

Snowpack Discussion

A warm and wet start to February was followed by mostly fair weather through mid-February, leading to generally thick surface crusts, as well as further deterioration of the snow cover below treeline. 

The last precipitation occurred Thursday night and Friday when NWAC and Snotel snowdepth sensor readings increased about 3-7 inches from the new snow around Washington and Harts Pass in the NE zone as well as at Mission Ridge. These new snowfall amounts were likely representative of most of east slope locations above 4000 ft.

From early Friday through Saturday morning, moderate to strong east winds were beginning significant snow transport near treeline and fresh wind slabs were likely developing, especially on westerly aspects. Watch for evidence of recent wind loading, such as exposed windward slopes and windward sides of trees stripped of recent snow.

While the January 15th facet/crust layers can still be identified in parts of the NE Cascades, this persistent weak layer has been stabilizing and become unlikely to trigger. As a result the Persistent Slab problem has been removed from the northeast 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: South, South West, West, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 2nd, 2015 10:00AM