Avalanche Forecast

Issued: Feb 21st, 2015 11:23AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Watch for shallow new wind slab on non-traditional westerly aspects especially as you increase in elevation and reach into the alpine where there has been more recent snow available for transport.

Summary

Detailed Forecast

Moderate easterly winds at crest level and near the Passes should lessen Sunday afternoon. Temperatures will warm a few degrees over Saturday, especially in the alpine with high pressure moving into the region.

Watch for shallow new wind slab on non-traditional westerly aspects especially as you increase in elevation and reach into the alpine where there has been more recent snow available for transport. E-NE winds should continue to build new wind slab Saturday night and Sunday morning. There may also be pockets of shallow lingering wind slab on other aspects as well.  

Small loose wet avalanches should be very localized and will not be listed as an avalanche problem. 

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

Since the mild and wet start to February, fair weather through mid-February has lead to generally thick surface crusts and further consolidation of the snowpack, as well as further deterioration of the snow cover below treeline especially further from the crest.

A few weather disturbances passed through in NW flow aloft the latter half of the week, bringing an inch or two of new snow near and above treeline and near the Cascade crest.

Winds veered to the east or northeast on Saturday, likely causing some shallow new wind slab formation on non-traditional westerly aspects mainly above treeline.  

No new observations have been received from the east slopes over the last few days. 

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: Feb 22nd, 2015 11:23AM