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Archived

Avalanche Forecast

Mar 8th, 2018–Mar 9th, 2018
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Olympics.

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. 

Detailed Forecast

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. 

Snowpack Discussion

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.

Avalanche Problems

Wind Slabs

Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

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.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1

Persistent Slabs

Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.

 

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

 

This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.

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.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Unlikely

Expected Size: 1 - 1

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.

Elevations: Below Treeline.

Likelihood: Possible

Expected Size: 1 - 1