Avalanche Forecast

Issued: Dec 19th, 2016 10:31AM

The alpine rating is high, the treeline rating is high, and the below treeline rating is high. Known problems include Wind Slabs, Storm Slabs and Loose Wet.

NWAC, Northwest Avalanche Center

Stevens Pass.

A natural avalanche cycle should occur Monday night through Tuesday morning during peak warming and precipitation. Expect wind loading further downslope than usual due to strong westerly winds on a variety of aspects. Slabs may fail on weak persistent grain types formed prior to this storm cycle. Travel in avalanche terrain is not recommended Tuesday!

Summary

Detailed Forecast

Strong westerly flow will carry another strong frontal system across the Northwest Monday night and Tuesday morning. This will cause strong winds with moderate to heavy snow along the west slopes and a warming trend overnight. The warming trend should contribute to the formation of both new wind slab and new storm slab layers. A sharp cooling trend will take place beginning late Tuesday morning with light to moderate snow showers continuing through the day in continued strong westerly flow. See the NWAC mountain weather forecast for more details on precipitation intensity, precipitation types and temperature trends.

New wind slab is very likely along the west slopes on Tuesday. Continued wind transport will deepen these layers mainly on lee north to southeast slopes but wind slab is possible on other aspects due to strong gusts. Also, wind slab may develop below treeline due to the persistence of strong westerly winds.

New storm slab is also very likely along the west slopes on Tuesday on any slopes that rapidly accumulate new snow. A change from snow to rain below treeline will also make storm slabs more likely to initiate or cause natural loose wet avalanches in areas lacking a slab structure. Loose wet should only be likely in the morning due to a rapid cool down beginning later Tuesday morning.

The December 8th layer continues to round and gain strength and appears to have become non-reactive along the west slopes, thus the persistent slab problem has been removed from the forecast for the remainder of this storm cycle. However, the persistent slab avalanche problem may return once we see how the latest round of buried persistent grain types survived heavy loading Monday and Tuesday.

Snowpack Discussion

Weather and Snowpack

The last storm cycle to affect the Olympics and most of the Washington Cascades was about Thursday, December 8th to Monday morning, December 12th. About 2-4 feet of snowfall was seen at NWAC stations along long the west slopes.

There has been a lot of snowpack settlement since the last storm cycle ended, allowing this underlying snow to mostly stabilize.

Clear or fair and cold weather has been seen about Wednesday, December 14th to Sunday, December 18th . This caused widespread surface hoar and near surface faceted snow to develop in the Cascades. Thin sun crusts have also formed on steeper solar slopes. These layers are expected to act as widespread weak layers or sliding surfaces for the upcoming moderate to heavy snowfall.

Strong westerly winds affected most locations in the Olympics and Cascades Monday morning before easing off Monday afternoon. Generally 3-8" of new snow fell across the west slopes with a warming trend. A freezing rain crust developed at Snoqualmie Pass mid-morning before switching to plain rain at Pass level. Paradise on Mt. Rainier picked up the most snow with 13" through early Monday afternoon.

Recent Observations

NWAC pro-observer Ian Nicholson was at Paradise on Saturday and found a mix of surface hoar and faceted surface snow that was reactive in shovel tilt tests. Past wind loading was noted on some west aspects. The December 8th persistent weak layer crystals seen at 52 cm were becoming rounded and were not reactive.

NWAC pro-observer Simon Trautman was on Mt Herman on Sunday and found a widespread weak surface snow sandwich that included faceted snow and a crust. These layers were giving highly reactive cracking and natural and triggered slab avalanches on steep slopes.

Mt. Baker pro-patrol noted sensitive 12" storm slabs Monday AM during control work. West winds were strong enough to affect the below treeline band, including the usually wind protected 24 hour snowboard in Heather Meadows.

Snoqualmie Pass DOT noted shallow new storm slabs failing on isolation at the new snow/old snow interface during compression tests Monday morning (before freezing rain/rain occurred later that morning).

Paradise NPS rangers observed evidence of a natural avalanche cycle from earlier Monday morning on steep slopes near and below treeline. Storm slabs released during ski cuts on steep test slopes but were not propagating wide as of mid-morning. A thin rain crust was forming as of 2 pm Monday in the Paradise area.

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: North, North East, East, South East, South.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1 - 1

Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

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

Very Likely

Expected Size

1 - 1

Valid until: Dec 20th, 2016 10:31AM