Avalanche Forecast

Issued: Feb 15th, 2018 10:35AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Avoid steep open slopes at all elevations where you will be likely to trigger an avalanche Friday. Snow and wind throughout the day will cause avalanches to become larger and easier to trigger. Conservative decision making will be essential for safe travel.

Summary

Detailed Forecast

Additional snow and wind Friday will cause the avalanche danger to increase throughout the day. Expect changing conditions as avalanche problems become easier to trigger.

Identify and avoid steep wind loaded terrain and mid-slope crossloaded features. Visual clues such as blowing snow, plumes, and fresh cornices signal wind slabs exist on slopes below. Some of these avalanches will be large and may propagate widely. Give wind loaded terrain a wide berth Friday.

As the storm totals grow, you will be more likely to trigger storm slab avalanches. Storm slabs exist on all aspects. Avoid open slopes greater than 35 degrees where you will be likely to trigger a storm slab.

Snowpack Discussion

Winds at Mt Hood have significantly redistributed the recent storm snow forming a variety of surfaces near and above treeline. Scoured surfaces can be found on windward slopes, while lee sloped hold 2-4 feet of wind deposited snow. In sheltered areas 10-12 inches storm snow fell Wednesday onto a strong firm crust layer. Soft subtle storm slabs have formed with settlement of the new snow.

The recent snow sits on top of a firm crust layer formed on Feb 8th. The warm wet weather from the beginning of February created a strong consolidated snowpack. There are no significant layers of concern in the mid and lower snowpack.

Observations

NWAC professional observer Laura Green and the Mt Hood Meadows ski patrol Thursday reported wind transported snow near and above treeline. Wind scoured slopes were reported on windward aspects. Deep wind slabs up to 6 feet were observed during avalanche mitigation work Thursday morning.

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, 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: All elevations.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Feb 16th, 2018 10:35AM