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Stevens Pass

Issued: 6:00 PM PST Friday, April 20, 2018
by Dallas Glass

See the NWAC Spring Schedule 2018  for details on avalanche forecast products for the remainder of the season.

NWAC avalanche forecasts apply to backcountry avalanche terrain in the Olympics, Washington Cascades and Mt Hood area. These forecasts do not apply to developed ski areas, avalanche terrain affecting highways and higher terrain on the volcanic peaks above the Cascade crest level.

A change in the weather will bring a change in avalanche conditions Saturday. Use visual clues such as blowing snow, fresh cornices, and snow drifts to identify and avoid steep wind loaded slopes.  As the sun comes out in the afternoon expect snow surface conditions to change quickly. New rollerballs and fresh fan-shaped avalanche debris indicate that you may trigger a loose wet avalanche on similar slopes.

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Avalanche Problems for Saturday

Wind Slabi

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.

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Loose Weti

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.

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Forecast for Saturday:

Changing weather will bring changing snow and avalanche conditions to the area Saturday. New snow Friday night and Saturday morning will combine with moderate to strong winds to form fresh wind slabs on lee slopes. Use visual clues such as blowing snow, fresh cornices, and drifts to identify and avoid steep wind loaded slopes. You will be most likely to trigger a wind slab below a fresh cornice, near a convex rollover, or on an unsupported slope.

When the sun comes out, expect conditions to change quickly. The new snow will rapidly become wet and begin to move downhill on steep slopes. You are likely to see new rollerballs and small loose wet avalanches initiating from steep rocky slopes. This is an indication that you may be able to trigger loose wet avalanches on sunny slopes greater than 35 degrees. While most of these slides should be small, recent loose wet avalanches this past week have been able to gouge down into older wet snow layers, entraining significant snow, growing large, and traveling long distances.

We have received reports over the last week of isolated avalanche events including cornice failures, large deep wet slabs, and glide avalanches. Our slow transition to spring and a complex and diverse snowpack makes it very difficult to predict when and where these events will take place. Do not linger below slopes overhung by large cornices, glide cracks, or where larger deeper avalanches may occur.

Other springtime hazards exist in the mountains. Creeks have begun to open, running high and fast with snowmelt. Use caution and choose your creek crossings carefully if you are traveling near or over large creeks.

Avalanche Summary:

Several inches of new snow are expected in most locations by the morning hours. Winds during the storm should form new and growing wind slabs on lee slopes. In many areas this new snow will be falling on a new melt-freeze crust. A slow cooling trend will likely leave this crust weak and thin. Above 7000’ colder drier snow may still be found.

The upper and mid snowpack is quite variable depending on aspect and elevations. In general, the upper snowpack consists of frozen melt-freeze crusts, weak large wet grains, and strong rounded grains. The 4/1 melt-freeze crust which may be the culprit in some of these recent very large wet slab avalanches is generally found 4-6 feet (120-200cm) below the snow surface.

Very large slab and glide avalanches have been reported over the last several days. Mt Baker, Stevens Pass, Alpental Valley, Chinook Pass, and the Mt Rainier area have all reported very large deep avalanches. They are a reminder that very large triggers, such as cornice falls or other avalanches, may trigger older deep layers that have become wet with recent rain and warm temperatures.

Observations

Baker

On Thursday NWAC professional observer Lee Lazzara traveled near Heliotrope Ridge. Lee found lingering wind slabs 12-16 inches (30-40cm) near and above treeline. Below 5000 feet, all aspects had formed moist to wet snow on the surface. Lee reported one very large avalanche in the Sisters Range SW of Mt Baker.

Stevens Pass

NWAC professional observer Matt Primomo traveled in the Stevens backcountry Friday. Matt found a poor overnight refreeze below 4500’. A layer of weak large grained wet snow was observed near treeline (5800’). This weak layer was 14-30” (35-75cm) below the snow surface. Numerous loose wet avalanches were observed in the area from the last several days.

Snoqualmie Pass

NWAC avalanche forecaster Dallas Glass traveled in the Alpental Valley Friday. He found weak large grains of wet snow in the upper 30” (75cm) of the snowpack. He was able to trigger loose wet avalanches on small test slopes near and below treeline. This corresponded with numerous recent natural loose wet avalanches on all aspects.

Tuesday and Wednesday Alpental ski patrol reported glide avalanches in the Alpental Valley that entrained significant snow and traveled farther than normal.

South

We received several reports of a very large avalanche in the Frying Pan Creek drainage near Mt Rainier Thursday. The avalanche appears to be on a NE aspect around 7500’.

NWAC staff visited the Crystal backcountry Wednesday and Thursday. They reported many large overhanging cornices. Wind slabs in this area had bonded and gained strength. Surface snow on nearly all slopes showed signs of warm air temperatures.

No Corresponding Mountain Weather Forecast Available

USE AT YOUR OWN RISK

This Backcountry Avalanche Forecast is provided in conjunction with the US Forest Service, and is intended for personal and recreational purposes only. Safe backcountry travel requires preparation and planning, and this information may be used for planning purposes but does not provide all the information necessary for backcountry travel. Advanced avalanche education is strongly encouraged.

The user acknowledges that it is impossible to accurately predict natural events such as avalanches in every instance, and the accuracy or reliability of the data provided here is not guaranteed in any way. This forecast describes general avalanche conditions and local variations will always occur. This forecast expires 24 hours after the posted time unless noted otherwise.