Avalanche Forecast

Issued: Feb 26th, 2018 11:58AM

The alpine rating is moderate, 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

Intense storms over the weekend created dangerous avalanche conditions and very large Wind Slabs avalanches on Mt Hood. Three feet of snow has accumulated since Friday. Avoid obvious wind drifts and deep pillows of snow on leeward and crossloaded slopes 35 degrees and steeper. Conditions have improved but continue to chose terrain carefully as any avalanche release could be deadly.

Summary

Detailed Forecast

The avalanche danger Tuesday will continue to gradually decrease with cool temperatures and generally light snow showers expected.

You could still trigger an avalanche Tuesday, but avalanches are becoming more difficult to trigger as the extensive storm and wind slabs deposited last weekend gradually settle and stabilize. Continue to watch for areas that received significant wind loaded snow, such as steep slopes below ridges, mainly facing North to Southeast. Any triggered avalanche could become very large as there is a significant amount of storm snow available to become involved in an avalanche.

The strong storm Sunday produced widespread wind and storm slabs in the Mt Hood area. These large wind and storm slabs will take some extra time to heal and strengthen.

Back country travelers should continue to travel with extra caution and best to travel in shallower angled terrain well away from avalanche paths or runout zones where avalanches release from above. 

Snowpack Discussion

While avalanche conditions are improving, triggering avalanches remains possible Tuesday.

Older low density snow from last week has been loaded by back to back strong storms over the weekend with about 3 ft of new storm snow accumulating by Sunday evening! 

Very strong winds and heavy snowfall intensities since late Friday have built large to very large wind slabs near and above treeline. Storm slabs have built over weaker lower density snow deposited late last week. 

Avalanches releasing in the most recent storm snow may step down to older storm layers creating large avalanches.

The deepening storm snow now sits over a strong crust layer formed and buried on Saturday 2/17. This crust has been reported up to 6600 feet by professionals in the region. 

The avalanche danger has improved markedly as we move beyond the dangerous avalanche conditions that developed Sunday and unfortunately claimed 3 lives in two separate accidents near Snoqualmie Pass. NWAC field staff traveled to both accident sites to investigate the avalanches in preparation for forthcoming accident reports. Our condolences go to the friends and families of the victims. 

Observations

On Monday, evidence of the Sunday storm was in plain view with several very large crowns from natural avalanche releases were visible above Mt Hood Meadows above treeline. Storm slabs were still being triggered near treeline Monday. 

Huge natural slab avalanche in Newton Canyon above Mt Hood Meadows releasing Sunday, 2/25. Image, Laura Green

By Sunday morning, MHM professional patrol reported "monumental" snow drifts! These conditions have only become larger by Monday.

MHM professional patrol on Saturday reported white out conditions above treeline with significant wind transport occurring. Below treeline the surface snow was forming unstable storm slabs. Still relatively shallow storm slab avalanches were releasing with explosives Saturday morning. 

NWAC pro-observer Laura Green traveled in the Mt Hood backcountry Friday. Laura reported stable conditions prior to Friday’s incoming storm. She found a right-side-up snowpack with loose unconsolidated surface snow.

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

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.

Elevations: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Feb 27th, 2018 11:58AM