Register
Get forecast notifications
Create an account to receive email notifications when forecasts are published.
Login
Archived

Avalanche Forecast

Mar 24th, 2017–Mar 25th, 2017
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
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Mt Hood.

We continue to load and stress our snowpack. Don't push it in consequential avalanche terrain - keep it simple and conservative and wait for this pattern to break. Recent storm/wind slab concerns up high and loose wet potential on steeper solar aspects/lower elevations will challenge us to manage terrain safely. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches. 

Detailed Forecast

Showers should decrease late Saturday morning through mid-day and become light and scattered in the afternoon. Sunbreaks are possible in the afternoon. The air mass will be cooler, but it's spring-time and solar effects should push temperatures near to above freezing at lower and mid-elevations in the afternoon.  

We continue to load and stress our snowpack. Don't push it in consequential avalanche terrain - keep it simple and conservative and wait for this pattern to break. Recent storm/wind slab concerns up high and loose wet potential on steeper solar aspects/lower elevations will challenge us to manage terrain safely. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.   

Recent winds have been mostly S-SW, so firmer wind slab should be found mainly on NW-SE slopes near and above treeline. Avoid recently wind loaded slopes as human triggered avalanches are considered likely. There is the potential for very large wind slab avalanches on lee slopes above treeline due to the persistent loading pattern. 

New storm snow is expected to bond well due to a cooling trend Friday night but storm slab may still become locally sensitive in areas that receive rapid accumulations Friday night or Saturday morning. 

Subtle daytime warming and/or potential sunbreaks Saturday afternoon will increase the potential for loose wet avalanches on steeper solar slopes and at lower elevations. Loose wet avalanches won't be listed as an avalanche problem, but watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases. Avoid steeper slopes with terrain traps where even small loose wet avalanches could have unintended consequences. 

You need to avoid areas on ridges where there may be a cornice and slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.

Snowpack Discussion

Weather and Snowpack

The first week or so of March was very cool and snowy.  NWAC stations at Mt Hood piled up about 6-7 ft of snow.

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

This past week has also been active weather-wise, but water amounts/snowfall totals have been slightly lower relative to the extreme wetness of the past few weeks. In the last 24 hours ending at 5 PM Friday, Mt. Hood stations have picked up 7-9" of snow above 5000'. A slight warming trend occurred during the day on Friday. Winds above treeline have been moderate out of the S-SW.  

Recent Observations

On Thursday, Mt. Hood Meadows reported widespread but stubborn shallow hard wind slab, 1-2 ft deep, on NE slopes between 6000 and 7300 ft. Small loose wet slides remained possible on steeper solar slopes. By Friday, 6-12" storm slabs were sensitive to ski cuts specifically on N-E aspects above treeline. Storm slabs were relatively less reactive near treeline. 

Laura Green was out in the Timberline area on Friday. Moderate winds, wet snow and poor visibility prevented her from accessing terrain above 6500 feet, but she could see active wind transport occurring in the elevations she traveled. 1 natural wet slab, 20 cm deep, was observed below treeline but in general the rain moistened snow below treeline was not very sensitive to human triggering and no other natural wet snow activity was observed. 

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: Likely

Expected Size: 1 - 2

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: Possible

Expected Size: 1 - 1

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

 

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

 

A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine.

Likelihood: Unlikely

Expected Size: 1 - 2