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Olympics

Issued: 6:05 PM PST Thursday, March 8, 2018
by Robert Hahn

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.

The avalanche danger will increase during the day on Thursday as a storm system impacts the Olympics. Look for developing Wind Slabs on lee slopes near and above treeline that you can easily trigger. There is uncertainty around the potential for large Persistent Slab avalanches. Until we receive more information, reduce your risk by avoiding open slopes 35 degrees and steeper. Rainfall will increase the likelihood for Loose Wet avalanches below treeline. 

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Avalanche Problems for Thursday

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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Persistent Slabi

Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.

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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 Thursday:

The avalanche danger will increase during the day on Thursday as a storm system brings light to moderate rain and snow to the Olympics along with windy conditions. Look for developing Wind Slabs on lee slopes near and above treeline that you can easily trigger. Watch for cracking, wind stiffened snow, and freshly formed drifts. Steer around fresh wind features, convex rolls, and slopes holding a foot or more of new, cohesive snow that are 35 degrees and steeper. Shallow Storm Slabs may develop by the afternoon in less wind affected terrain.

Although it is becoming unlikely, you may still be able to trigger Persistent Slab avalanches on slopes 35 degrees and steeper. We think this problem is decreasing in the Olympics because we have no reports of recent avalanche activity. Facets (weak snow) have not been found to be widespread. However, variability exists across the terrain and persistent slabs have proven to be quite dangerous in the nearby Cascades so reduce your risk by avoiding steep, open slopes and large avalanche path and dig down and check for weak sugar-like facets or buried surface hoar above the most recent firm crust layer formed in early February on all but south aspects before engaging with avalanche terrain. This crust layer is down about 2-4 feet. 

The rain/snow line is forecast to rise to 4000-5000' Thursday. Rainfall will raise the likelihood of small Loose Wet avalanches on steep slopes below treeline, especially those that have received fresh snowfall. Avoid steep slopes below treeline connected to terrain traps, where even a small Loose Wet avalanche could have unintended consequences. 

Avalanche Summary:

Temperatures at Hurricane Ridge pushed above 40F Wednesday with high overcast skies. This likely moistened the snow surface on all aspects in the Hurricane Ridge area.

On shaded aspects, about 3 ft of settled snow sits a strong crust formed in February. In some areas, weak faceted snow was found above this crust. We do not have recent observations on the distribution or likelihood of triggering an avalanche on this interface. There are no other significant layers of concern in the mid and lower snowpack.

A very different and very shallow snowpack exists on south-facing aspects where warm temperatures and sunshine prior to mid-February melted the majority of the snowpack. Much of the snow on south-facing was deposited from mid-February onwards. Basal facets were present on the ground on southerly aspects and this will be something to watch if warm temperatures haven't healed this interface prior to the incoming storm.

Observations

On Saturday March 3rd, NPS rangers found 2-3 mm buried surface hoar from mid February intact on a NNW aspect at 4990’. The surface hoar was about 4" (10 cm) above a crust 32" (80 cm) below the surface. No new or recent avalanche activity was observed on this layer. The snow depth was much shallower on due south slopes (as little as 1.5 ft deep, recent snow only) as compared to northerly slopes (120" deep). On south-facing aspects, the total snow was 1.5' deep and 1-2 mm basal facets were present on the ground.

On Friday, March 2nd, NPS rangers reported a large avalanche on a recently wind loaded slope. Elsewhere, profiles showed a faceted weak layer buried in mid- February was found 2-4 feet below the surface and was showing some signs of rounding.

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.