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West Slopes South - South of I-90 to Columbia River

Issued: 6:00 PM PST Saturday, April 22, 2017
by Kenny Kramer

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.

NWAC Spring Forecast Schedule

The NWAC issued daily mountain weather and avalanche forecasts through Saturday, April 15th. Mountain weather and avalanche forecasts will be issued during the spring transition April 20-22nd and April 27-29th. Weekend outlooks will be issued Thursdays, May 4th, 11th, 18th and 25th.

Special advisories, watches and warnings will be issued throughout the spring for unusual or dangerous avalanche conditions. You can find out what constitutes a special advisory, watch or warning here.

New wind slabs may build above treeline, mainly below ridges. Loose wet avalanches are less likely Sunday, except at the lowest elevations below treeline. Cornices are large, so give them a wide safety margin.   

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Avalanche Problems for Sunday

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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Cornicesi

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.

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Snowpack Analysis:

The active weather pattern continues into late April in the Pacific Northwest despite what the calendar says. Between Monday and Thursday night, Mt. Baker picked up about 3 inches of water equivalent (WE) while other areas along the west slopes picked up 1 - 1.5 inches of WE. Much or all of this fell as snow above 5000 feet in the north and 5500-6000 feet in the central and south Washington Cascades. Natural loose wet avalanches likely occurred throughout this period as the snow-line oscillated or on solar aspects at lower elevations during sunbreaks. 

Friday was a warm day with most NWAC stations in the Olympics and Cascades reaching into the 40s and 50s. Breezy E-SE winds were seen in the Passes and in the southwest Cascades including Mt. Rainier and Crystal.

A renewed active weather pattern Saturday brought periods of light rain and snow showers through the day, with a slow cooling trend. These were mainly rain showers below about 5500 feet.   

Recent observations

The Chinook Pass DOT on Thursday reported loose wet avalanches becoming increasingly sensitive during sunbreaks below 5000 feet with ski cuts and explosives, with larger slides gouging down to older wet snow. A 12-16 inch wind slab was triggered below the ridge crest with explosives.

NWAC pro-observer Lee Lazzara was in the Mt. Baker backcountry on Wednesday. Lee found recent wind slabs on lee slopes near and just above 5000 ft up to 12 inches thick but unreactive in ski or snowpack tests. Lee observed a large recent glide avalanche on Mt. Herman (see Lee's Instagram post) as well as evidence of a recent loose wet cycle from earlier in the week.

Lee was back out on Friday in the Baker backcountry on Ptarmigan Ridge. Lee observed numerous loose wet slides on all aspects near and below treeline. Natural cornice fall along Ptarmigan Ridge ripped out a D2.5 wind slab on a north aspect (see photo below, thanks Lee!), and other natural cornice fall was noted as well. Note the massive cornices in the photo! 

Photo by Lee Lazzara, 4-21-17

Detailed Forecast for Sunday:

Cooling with diminishing showers and some partial clearing overnight Saturday and early Sunday should allow for surface snow to partially or completely refreeze and strengthen. This should allow for a brief decrease in avalanche danger through early Sunday.

Another frontal system will lift over the area from south to north Sunday, followed by post-frontal showers late Sunday. Rain should generally be seen below 4500 feet. Light to moderate amounts of new snow should accumulate above 4500 feet Sunday. Above treeline, areas of shallow new wind slabs may develop on lee aspects.

Due to cooling by Sunday, loose wet avalanches should be less likely to initiate, except in the lower elevations below treeline where shallow wet snow is expected. 

Cornices are still large, so give them a wide safety margin. Natural cornice releases and resulting slab avalanches are dangerous and unpredictable. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.

Avoid unsupported slopes with overhanging blocks of snow and smooth rock underneath. Glide avalanches can release at any time, not just during the heat of the day, and are by definition difficult to predict and manage.