Avalanche Forecast

Issued: Mar 16th, 2016 11:27AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

A mixed bag of avalanche problems Thursday will keep heightened avalanche conditions in specific terrain. You can mitigate the loose wet and cornice risk by being aware of the terrain your connected with and by recognizing the rapidly increasing hazard in locations that warm quickly. Also, wind slabs should still be sensitive above treeline along the east slopes. 

Summary

Detailed Forecast

Sunshine and slowly moderating temperatures should be the story on Thursday. 

Mid-March sunshine will affect solar slopes by increasing the likelihood of loose wet avalanches on steeper solar aspects by quickly breaking down sun or melt-freeze crusts. While most loose wet avalanches should be small, they will have the ability to entrain recent moist snow in specific terrain. Recently formed cornices have grown large and should become more sensitive with warming and sunshine. You can mitigate the loose wet and cornice risk by being aware of the terrain your connected with and by recognizing the rapidly increasing hazard in locations that warm quickly.

Northwest winds on Wednesday likely continued to build wind slab on lee aspects in the above treeline band. Wind slab will mostly likely be found on N-SE facing slopes. Firm wind transported snow and snowpack cracking can be good signs of wind slab instabilities. Treat wind-loaded slopes in higher terrain with caution Thursday.

Storm slabs will not be listed several days out from the storm cycle, but be aware that stubborn storm slabs may still be possible in the NE zone. 

The persistent slab avalanche problem involving the 2/27 buried surface hoar will not be listed in the northeast zone due to an abundance of other avalanche problems, high unlikelihood of triggering due to depth, and general strengthening of this interface over the last few weeks.  

Snowpack Discussion

Weather and Snowpack

During a period of fair weather in late February, widespread surface hoar formed in the northeast zone mainly surviving outside of steeper solar aspects and wind affected terrain. A weak front buried the surface hoar layer in the Washington Pass area about 2/27 and to a lesser extent the central-east zone.  

Storms have moved across the Northwest at a nearly daily frequency the past few weeks with fluctuating but generally moderate snow levels. Two deep surface low-pressure systems moved across the Olympic Peninsula last week with the first passing last Thursday, March 10th and the next on Sunday, March 13th. These systems caused very stormy weather.  

Cool and showery weather Monday and Tuesday with continuing moderate westerly winds deposited additional snow at lower snow levels. Recent storm snow amounts along the east slope sites have varied but have been mostly in the 1.5 - 3.5 foot range since Wednesday, March 9th!

Recent storm instabilities have been slow to consistently settle out throughout the east slopes. Specifically in the Washington Pass zone the last week has seen many natural and human triggered storm and wind slab avalanches. These recent storms have also built large cornices along ridges in many areas with several large cornice releases noted over the last few days by the North Cascade Mountain Guides.

We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack due to lack of recent activity at these interfaces and confirming field observations. 

Recent Observations

NWAC observer Tom Curtis tested conditions in the Blewett Pass area Monday, 3/14 following the strong storm Sunday. Sensitive storm and wind slabs of up to 20 inches were found in open terrain, especially wind affected slopes. Easy failure in test pits was noted on a crust-facet layer buried beneath the current storm snow, about 16-20 inches. Failures were found with PST and ECT tests on this interface.

Reports from the Washington Pass area Tuesday 3/15 reported an active avalanche day with several storm slab avalanches triggered on NE aspects from 5600-6600 feet.  One large size 3 avalanche was seen on an NNE aspect about 7200 feet, likely releasing by a cornice failure triggering a storm slab on the slope below. Also on 3/15 there was a partial skier burial in the Hairpin Valley. The avalanche was skier triggered from well above the partially buried skier. More details are being gathered but gladly there were no injuries. The slab reportedly stepped down into deeper recent storm layers, becoming large and involving up to 50 cm of the recent storm 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: North, North East, East, South East.

Elevations: Alpine, Treeline.

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.

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

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 1

Cornices

An icon showing 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: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 17th, 2016 11:27AM