Avalanche Forecast

Issued: Mar 18th, 2016 11:15AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Check the forecast for a mixed bag of spring avalanche problems and evaluate snow and terrain carefully on Saturday.

Summary

Detailed Forecast

Increasing mid and high clouds should be seen on Saturday east of the crest.

The avalanche forecast won't change much from Friday. East winds should decrease over the Cascade crest and in the Cascade passes on Saturday. East winds the past couple days should mainly have affected areas along the crest and along the west slopes.

Loose wet avalanches may still be possible on steeper solar aspects on Saturday. Loose wet avalanches may start small but could have the ability to entrain recent moist snow in mainly on steep solar terrain. High clouds Friday night may limit a refreeze of surface snow by Saturday morning.

Storm winds 3/13-15 likely built wind slab on lee aspects in the upper elevations of the above treeline band. This wind slab will mostly likely be found on NW-SE facing slopes. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.

Recently formed cornices have grown large and can become sensitive when it is warm and sunny. Avoid areas on ridges or summits where there may be a cornice and avoid slopes below cornices in the spring.

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 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 3/10, and the next on Sunday, 3/13. These systems caused very stormy weather.  

Cool and showery weather Monday and Tuesday with prolonged  westerly winds deposited additional snow at lower snow levels. Storm snow amounts along the east slopes were in the 1.5 - 3.5 foot range for Wednesday 3/9 to Tuesday 3/15.

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. The recent storms have also built large cornices along ridges in many areas with several large cornice releases noted over the last few days.

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 pro-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.

A report via the NWAC Observations page for the east central zone on Monday 3/14 indicated that most of the area visited had wind affected snow.

Recent guide reports from the Washington Pass area Tuesday, 3/15 indicated an active avalanche day.  There was a partial skier burial due to a ski triggered 45 cm storm slab on a north slope at about 6800 feet in the Hairpin Valley. More details are being gathered but gladly there were no injuries. Ski cuts were giving better propagation than expected also in the Hairpin Valley. AT least 2 cornice releases in NE facing bowls up the Valley caused a large size 3 storm slab and car sized blocks running to the valley floor.

Recent storm slab in the Washington Pass area was reported to be more sensitive due to surface hoar that may have been buried there about during a couple day period March 10-12th.

Tom Curtis was out again on Mt Cashmere on Thursday 3/17 and found 35 cm of storm snow well bonded to an underlying crust from last week. An extended column test did not indicate propagation and only small loose wet avalanches were seen on steep solar slopes. Temperatures were not greatly affecting the snowpack.

The Mission Ridge pro-patrol reported limited wind transport on Friday 3/18 from east winds and surface crusts below about 6000 feet from sun and warmth.

 

Problems

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

Possible

Expected Size

1 - 1

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, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

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 19th, 2016 11:15AM