Avalanche Forecast
Regions: Olympics.
You may trigger a small wind slab at higher elevations on Thursday. Avoid large pillows of snow on steeper slopes. 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.
Detailed Forecast
Moderate to occasionally strong winds have transported the shallow new snow to produce small wind slabs by Friday morning at higher elevations. 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.
Light rain approached ridge-crest on Thursday with precipitation trailing off. Temperatures late Thursday night will drop significantly and freeze the snow more solidly, reducing the avalanche danger in many locations by Friday morning.
Although Thursday's rain may have impacted the persistent weak layers in some locations, you may still be able to trigger Persistent Slab avalanches on slopes 35 degrees and steeper. Facets (weak snow) have been rounding in some locations and still present in shallower locations. With some uncertainty remaining regarding this layer, reduce your risk by avoiding steep, open slopes and large avalanche paths; 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. Facet crust combinations may also be present on south-facing aspects, formed more recently. These layers have not been well tested.
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Snowpack Discussion
On Thursday, Hurricane ridge saw 3-4" of snow with a rain/snow mix at ridge crest during the late afternoon, moistening the snow surface.
On Wednesday, temperatures at Hurricane Ridge pushed above 40F with high overcast skies, likely creating a thin crust upon cooling Wednesday night.
On shaded aspects, about 3-4 ft of settled snow sits a strong crust formed in February. In some areas, weak faceted snow was found above this crust. Recent observations indicate large facets where this layer is found in shallow locations near ridgelines, but the facet layer tends to be gaining strength where the snowpack is deeper. 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.
Rain up to 5,000 ft on Thursday may have helped to reduce some of the shallower facets in the snowpack, but until we have direct observations, nothing more can be ascertained.
Observations
On Wednesday March 7th, a NPS ranger traveled in the Victor Pass area and found many crusts with facets forming on south aspect; moderately reactive in compression tests. Near ridge-tops, the 2/13 the PWL was 8-12"Â down and 3 mm facets were observed. Mid-slope on N-NW aspects and at the bottom of Victor Bowl the facets had transformed to rounded grains under 3-4 feet of new snow.
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: Possible
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