Avalanche Forecast

Issued: Jan 23rd, 2018 12:35PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The avalanche danger will lower slightly during the day on Wednesday, but the potential for triggering a large avalanche remains near and below treeline. Above treeline terrain, where high winds have created very sensitive and deep wind slabs, backcountry travel is not recommended. Recent storm and wind slabs require time to heal, so be patient and match your terrain selection appropriately and avoid avalanche run and stop zones. 

Summary

Detailed Forecast

Decreasing light to moderate rain and snow will become showery by Wednesday afternoon, however Tuesday and Tuesday's higher density storm snow and wind loading will have elevated the avalanche danger significantly.

A loose wet avalanche problem will develop during the afternoon hours as snowfall changes to rain at lower elevations. Small natural loose wet avalanches are likely on steep, unsupported, or convex terrain up to about 5000 ft, where enough recent snowfall has accumulated. Be aware of the consequences of a small loose avalanche, particularly where cliffs, rocks, gullies, and creeks magnify the consequences.

Storm slabs developing in many areas Tuesday afternoon will build to 1-2' in the Mt Hood area by Wednesday morning. Storm slabs will remain touchy Wednesday, with the potential to entrain the deeper lower density snow from the past week that exists in all areas, allowing these slabs to run farther and become larger than you expect. Stick to lower-angle terrain that is well-supported or heavily terrain anchored. Step back your travel plans and recognize that this new snow needs significant time to settle.

Strong winds are transporting the new snow, particularly near and above treeline. These will continue to build very touchy and large wind slabs near and above treeline on a variety of aspects. New or recent wind slabs may also produce large avalanches, entraining significant amounts of deeper loose dry snow. Above treeline terrain is not recommended on Wednesday. Elsewhere, avoid wind-loaded terrain and consider any aspect to be fair game for wind slabs on Wednesday. Also avoid avalanche paths with higher elevation start zones.

The avalanche danger will lower slightly during the day on Wednesday, but the potential for triggering a large avalanche remains near and below treeline. Above treeline terrain, where high winds have created very sensitive and deep wind slabs, backcountry travel is not recommended. Recent storm and wind slabs require time to heal, so be patient and match your terrain selection appropriately and avoid avalanche run and stop zones. 

Snowpack Discussion

6-12" of new storm snow has fallen on Tuesday at Mt. Hood stations below treeline with 1" of snow water at Mt Hood Meadows and 1.5" of snow water at Timberline. This has created a thick and dense storm slab along with sizeable wind slabs.

An active weather pattern from 1/18 onwards and continuing through Monday has deposited 1.5-2.5' of snow falling at temperatures generally in the upper 20's over the most recent 1/17 crust. Observations indicate the recent snow bonded well to the crust.

Poor visibility and dangerous travel conditions have made observations in above treeline terrain difficult to obtain. A high degree of uncertainty exists regarding the extent and sensitivity of wind slabs above treeline.

Below treeline, lower snowdepths have limited the development of widespread avalanche problems. 

Observations

On Tuesday Mt. Hood Meadows Pro Patrol reported rain to 3800'. Wind slabs were building near and above treeline on top of lower-density snow received over the weekend. The slabs were sensitive to skier triggering down to the crusts from 1/16 or within the storm snow. Slab depth was variable. Winds were moderate near and below treeline and strong above treeline.

On Monday morning, Mt Hood Meadows Pro Patrol reported 6" of new snow at 5300' and 13" at the top of their terrain that fell on Sunday night. Wind slabs up to 1' were found into near treeline terrain, with the larger releases triggered by explosives.

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

Very 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

Loose Wet

An icon showing Loose Wet

Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Elevations: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 24th, 2018 12:35PM