Avalanche Forecast
Regions: Olympics.
A natural avalanche cycle should occur late Monday night through early Tuesday morning during peak warming and precipitation. Expect wind loading further downslope than usual due to strong westerly winds on a variety of aspects. Generally shallow slabs may fail on weak persistent grain types formed prior to this storm cycle. Travel in avalanche terrain is not recommended Tuesday, especially near and above treeline!
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 snow along the Olympics and 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 mid Tuesday morning with light 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 on lee aspects in the Olympics 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.Â
New and shallow storm slab is also very likely 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.
A natural avalanche cycle should occur late Monday night through early Tuesday morning during peak warming and precipitation. Expect wind loading further downslope than usual due to strong westerly winds on a variety of aspects. Generally shallow slabs may fail on weak persistent grain types formed prior to this storm cycle. Travel in avalanche terrain is not recommended Tuesday, especially near and above treeline!
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 3 feet of snowfall was seen at the NWAC station at Hurricane.
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 Olympics and 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.
Recent Observations
NWAC pro-observer Matt Schonwald was at Hurricane on Friday and found a mostly right side up snowpack. The December 8th layer was found at a depth of 30 cm but gave moderate to hard compression tests and did not show signs of propagation. Isolated small wind slab was found on NE slopes above 5000 feet. He noted widespread large surface hoar and near surface faceted snow.
Surface hoar at Hurricane Ridge on December 16th. Photo by Matt Schonwald.
An observation for Hurricane for Saturday via the NWAC Observations page indicated surface hoar and near surface faceted snow in sheltered a spot sheltered from the sun. But perhaps not as widespread as seen by Matt Schonwald on Friday.
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: North, North East, East, South East, South.
Elevations: Alpine, Treeline.
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: 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: Likely
Expected Size: 1 - 1