Avalanche Forecast

Issued: Apr 18th, 2015 11:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The snowpack is in transition from winter to spring conditions in the Olympics and Cascades. This should keep the focus on likely loose wet avalanches and possible cornice failures especially during the midday and afternoon hours on Sunday.

Summary

Detailed Forecast

Sunny warm weather will continue on Sunday. This should keep the focus on likely loose wet avalanches and possible cornice failures especially during the midday and afternoon hours. Avoid areas below rocks or cliffs where loose wet avalanches can suddenly release.

Morning crusts and midday to afternoon wet snow especially on solar slopes should be common again on Sunday. There is still some uncertainty to the danger levels on Sunday since the snowpack in transition and  there was already some avalanches and consolidation late in the week but it will be warm through the weekend. It will be beautiful to be in the Olympics and Cascades Sunday. Remember to make careful snowpack evaluations and cautious route finding decisions.

Wind slab will not be listed as an avalanche problem but watch for signs of lingering firmer wind transported snow if you visit any high, cool, lee slopes.

There was a widespread weak layer of small reactive faceted crystals found at Washington Pass a week ago about 5-8 inches below the surface. Skiers have not reported any triggered storm or wind slab avalanches due to this layer and it is unknown if the layer might still be reactive. It should be a good idea to check for this layer around Washington Pass in quick snow pits on cool lee slopes where it might still be preserved.

Remember that this forecast applies to elevations up to the Cascade crest and not to the high parts of the volcanoes where conditions at this time of year are often more dangerous.

The work of the 3 NWAC forecasters is transitioning to other essential parts of the program before the NWAC closes for the summer. Going Deep session #6 is next Thursday and the topic is Making the Transition From Winter to Spring, when daily NWAC forecasts are not available. The session is at the Seattle REI from 6-9 pm.

 

Snowpack Discussion

Cold fronts crossed the Northwest last weekend and early in the week. Snowfall totals for these storms ranged from about 14 inches at Hurricane, to anywhere from about 4-28 inches west of the crest and Mt Hood, to about 3-12 inches east of the crest. Avalanche conditions were touchiest in the north Cascades with NWAC observers and staff reporting storm and wind slab and skier triggered storm slab. Skiers also found great late season powder conditions during this time.

The weather started to become sunny and much warmer late in the week and Saturday. The sun is getting high and strong and is causing a transition to spring loose wet avalanche conditions and consolidation.

NWAC observers began to report loose wet avalanches in the Mt Baker back country on Tuesday and Wednesday. A private party reported impressive pinwheels there on Thursday.

The DOT crew at Washington Pass triggered small to large loose wet slides during control work that reached the highway Thursday morning. A natural cornice release late Wednesday in Spire Gulch triggered a very large loose wet avalanche.

 A private party reported a natural storm or wind slab with a 4-5 foot crown crossed skier tracks at 7000 feet above Paradise on Thursday. The Paradise ranger also reported extensive pinwheels and small loose wet avalanches around Paradise on Wednesday and Thursday.

The Mt Baker patrol reported one large loose wet avalanche that began from rocks with debris to 6 feet deep on the north side of Shuksan Arm on Friday.

The Meadows patrol had only small loose wet avalanches to report so far on Friday and Saturday with knee deep boot penetration in wet snow on solar slopes by Saturday midday.

The mid and lower snowpack is most areas should consist of stable rounded grains and crust from warm periods this winter. Many areas at low elevations especially in the Olympics and east of the crest do not have enough snow to cause an avalanche danger.

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: All aspects.

Elevations: Alpine, Treeline.

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.

Elevations: Alpine.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Apr 19th, 2015 11:00AM