Avalanche Forecast

Archived

Mar 7th, 2016–Mar 8th, 2016

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

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You will need to watch for a combination of winter and spring conditions if you venture out on Tuesday. The persistent slab avalanche problem will only be listed in the northeast zone.

Detailed Forecast

A weakening front should move over a flat ridge and over the Northwest Tuesday afternoon and night. Snow from this front should mainly reach the Olympics, the southwest Cascades and Mt Hood by the end of the daylight hours on Tuesday.

Older or previous wind slab may also still be present near and above treeline along the east slopes. Watch for firmer wind transported snow.

Loose wet avalanche conditions may still be possible mainly on solar slopes below tree line. Watch for wet snow deeper than a few inches, pinwheels and natural loose wet avalanches.

The persistent slab avalanche problem will only be listed in the northeast zone, most likely found on non-solar aspects in the above and near treeline band and stretching into the upper portion of the below treeline band. This interface is likely getting harder for a human to trigger but if triggered is capable of producing large avalanches. Be aware that at depths approaching the 1 m mark, the extended column test becomes a less reliable indicator of propagation across a column. You can attempt deeper column tests like the propagation saw test or deep tap test but layer identification and terrain selection are your best friends for managing the persistent slab danger.

Snowpack Discussion

Weather and Snowpack

During a period of fair weather in late February, widespread surface hoar formed in the northeast zone mainly surviving outside of steeper solar aspects and wind affected terrain above treeline.

A weak front buried the surface hoar layer in the Washington Pass area about 2/27 and to a lesser extent the central-east zone.

The recent active weather pattern continued over the weekend with a system on Saturday night producing 0.50 to 1 inch of rain along the east slopes, except in the Washington Pass zone where rain likely stayed below 5500 feet and a few inches of snow accumulated above 6000 feet.

We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack due to lack of recent activity and confirming field observations.

Recent Observations

An observation via our NWAC observation page came in March 2nd from the Pine Creek drainage in the Washington Pass area. A skier triggered and was caught and buried in a persistent slab avalanche on a N-NE aspect at 6600 feet releasing on buried surface hoar about 70 cm down. The full observation with photos can be found here. No one was injured.

Strong winds on Thursday March 3rd caused widespread natural wind slab avalanches in the NE zone. While many were contained to the recent storm snow, one larger slide on a N-NE aspect at Windy Pass likely released down to the February 27th PWL.

Jeff Ward made observations near Wedge Mountain in the Central-East zone Friday, March 4th. Mild daytime temperatures and sunshine Friday allowed for the crust to soften on solar aspects, even providing spring corn conditions in some areas. Shaded terrain was still holding the recent colder powder. The interface from February 27th buried a little over 2 feet did not support propagation in a test pit at 6100 feet on NNE slope.

Observations from Mission Ridge pro-patrol Sunday indicated a firm upper snowpack after rainfall received Saturday night began to refreeze.

Problems

Wind Slabs

Wind Slab avalanches are the 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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.

Loose Wet

Loose Wet avalanches are the 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. 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.