Avalanche Forecast

Issued: Jan 10th, 2018 11:05AM

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

NWAC, Northwest Avalanche Center

Mt Hood.

Large or very large avalanches are very likely on Thursday. Travel in avalanche terrain is not recommended!

Summary

Detailed Forecast

Travel in avalanche terrain is not recommended on Thursday!

On Thursday, a strong storm arrives in the early morning hours with warm front bringing rising snow levels and strong to very strong crest level winds with heavy rain and snow.

Storm snowfall will be quite intense over the Mt Hood area with greater than 1 foot of snow expected to accumulate during daylight hours at most locations above 5,000 ft. This snow will fall at increasing density, creating an upside down storm layers on top of the multitude of crusts and recent lower density snow already existing in the snowpack. This storm is very likely to produce a widespread natural avalanche cycle, with large and very large avalanches in all elevations bands.

Wind slabs forming Tuesday through Wednesday were already showing signs of significant reactivity in many areas on the crust surfaces present. On Thursday, Strong to very strong SSW crest level winds (turning westerly in the late afternoon) will quickly form very touch large to very large wind slabs on a variety of aspects, but most notably NW though SE.

The current snowpack structure should not support the incoming strong storm Wednesday night and Thursday. Avalanches releasing in weak storm layers may break down to buried crusts and run long distances.

Snowpack Discussion

About 4-8 inches of new snow has accumulated in the Mt Hood area since Tuesday morning. The new snow overlies a strong melt freeze crust to most elevations into the above treeline band.

Strong winds since Tuesday have continued to actively build wind slabs near and above treeline.

Several rain and freezing rain crusts are sandwiched in the upper snowpack depending on elevation. Current observations do not suggest these layers to be reactive.

Observations

Pro patrol at Mt Hood Meadows Tuesday and again Wednesday, reported increasing fresh wind slabs forming in exposed terrain near treeline. White out conditions prevented observations above treeline.

On Sunday, the Mt. Hood Meadows patrol reported that the surface crust was softening at all elevations in their area.

NWAC pro-observer Laura Green was in the Bennett Pass area of the White River drainage on Saturday up to 6100 ft. Laura found challenging travel conditions with a surface crust of variable strength and no notable layers of concern in the upper snowpack. No new avalanche activity was observed.

Problems

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

Very Likely

Expected Size

1 - 2

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.