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Archived

Avalanche Forecast

Nov 26th, 2016–Nov 27th, 2016
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Mt Hood.

Watch mainly for previously formed wind slab layers on Sunday. Be prepared to ratchet back your plans if conditions get stormy again before the end of the daylight hours on Sunday.

Detailed Forecast

A relative short break in the weather should be seen much of Sunday with some light snow showers so conditions shouldn't change too much on Sunday.

There is still a lot of variation in snow cover depending on elevation since it is early in the year. Expect winter conditions at the higher elevations and too little snow for avalanches at the low elevations.

Note that the next system is due to begin to arrive late in the day on Sunday. Be prepared to ratchet back your plans if conditions get stormy again before the end of the daylight hours on Sunday.

 

Snowpack Discussion

There  has been a lot of variation depending on location in weather and snow conditions in the Olympics and Cascades the past couple days.

Snowfall has been moderate at Mt Hood with about 12-18 inches the past 4 days at the NWAC Timberline and Mt Hood Meadows stations. There has been enough southwest winds for some transport to mainly lee north to east slopes.

We have very limited information and no recent reports yet for the season for Mt Hood.

Decreasing south winds and a cooling trend should be seen on Saturday as a weakening front shifts to the east. A few more inches of snow is possible at Mt Hood.

Avalanche Problems

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: Possible

Expected Size: 1 - 1

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: Alpine, Treeline.

Likelihood: Unlikely

Expected Size: 1 - 1