Avalanche Forecast

Issued: Apr 5th, 2016 11:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Significant warming and intense solar radiation Wednesday should increase the likelihood of heat related avalanche problems, especially cornice failures and shallow loose-wet avalanches on steep solar slopes.  

Summary

Detailed Forecast

Clearing and warming is expected overnight Tuesday through Wednesday.

Any wind slab deposits caused from the snow received Monday, in combination with moderate westerly winds should have mostly stabilized by Wednesday. However, watch for signs of wind slab on mainly easterly facing near and above treeline where extra caution is warranted.  Firm wind transported snow and snowpack cracking can be good signs of wind slab layers. 

Small loose wet avalanches, mainly involving any recent storm snow from Monday and Monday night will be possible on any steeper solar slopes, especially during the warmest part of the day Wednesday afternoon. 

The likelihood of cornice failure will increase Wednesday from intense solar radiation and warming temperatures. Cornice releases can be unpredictable during the spring so continue to be aware of the overhead hazard and that cornices can break much further back than expected along ridges.

Although not listed as an avalanche problem, large, powerful glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces.

Snowpack Discussion

Weather and Snowpack

 

A large upper ridge dominated the weather last week through early Sunday, resulting in very warm and dry weather. A spring avalanche cycle occurred in most areas over this stretch, likely peaking Thursday or Friday during the warmest temperatures and lightest winds. After a stormy March, the snowpack has undergone significant settlement and multiple melt-freeze cycles over the last week.

A front moved across the Cascades early Monday morning, resulting in a period of mostly light precipitation, cooling and moderate westerly crest level winds.  Precipitation transitioned from rain to snow as cooling occurred, forming a generally good bond to the old melt freeze crust. Most east slope areas received storm snow amounts ranging from 2-6 inches as of Tuesday.

Frequent March storms have built unusually large cornices along many ridges.

No persistent weak layers formed earlier this winter are layers of concern as we transition to a spring snowpack along the east slopes. 

Recent Observations

NWAC professional observer Tom Curtis was out on the Wenatchee Ridge near Lake Wenatchee Wednesday, 3/30 in the 2-4000 foot range and found many slopes melted out. On slopes still holding snow he found natural and easily triggered, loose wet avalanches on all solar slopes in the top 15-20 cm of snow.

Tom was out yet again on Mt Lichtenberg near Stevens Pass on Thursday and found widespread large natural loose wet avalanches on nearly all solar aspects with easy to trigger loose wet avalanches on non-solar slopes.

A report on Turns All Year for the Chiwaukums described big rollerballs and big resulting debris piles on Thursday.

Problems

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: Alpine, Treeline.

Likelihood

Possible

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: South East, South, South West, West.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Apr 6th, 2016 11:00AM