Avalanche Forecast
Issued: Feb 4th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
A mixed bag of ugly snow conditions and a considerable avalanche danger in all the elevation bands is expected Friday. Careful snowpack evaluation will be needed along the east slopes on Friday.
Summary
Detailed Forecast
The next weather system will move strongly to the Olympics and north Cascades Friday late morning and spread to the rest of the Cascades by Friday evening with fairly high snow levels.
A fairly broad brush will be applied to the avalanche forecast for Friday. Expect a mixed bag of ugly snow conditions and a considerable avalanche danger in all the elevation band along the east slopes.
Wet snow and rain or warm temperatures should further build or activate wind and storm slab mainly in the above tree line band.
Recent snow will be available for activation by rain and warm temperatures elsewhere for loose wet avalanches. Watch for surface wet snow deeper than a few inches and pinwheels or small natural loose wet avalanches that usually precede large loose wet avalanches.
But new snow and cooler temperatures should be seen by Saturday.
Snowpack Discussion
Weather and Snowpack
Two fair weather periods in January allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th throughout the Cascades.
A warm front about January 21st caused up to about a foot of snow in the northeast zone. Freezing rain or rain created a crust in the central east and southeast zones.
Another wet and warm front was seen Jan 27-28th. This brought up to another 1.5 ft of snow in the northeast zone and likely formed another crust in the central and southeast zones.
A strong occluded front with strong winds crossed the Northwest last Friday and Saturday. NWAC stations along the east slopes had about 4-14 inches of new snow by Saturday morning.
Cool, benign weather followed Sunday through Tuesday. A sun crust formed on many solar slopes and surface hoar was seen on many non-solar slopes.
A cold front crossed the Northwest on Wednesday. A warm front is causing more snow mainly in the south Cascades on today. NWAC stations along the east slopes for the 2 days ending Friday morning should have about 2-7 inches of increasing damp or wet upside down snow.
Recent Observations
The January 3rd and 11th layers were reactive with widespread triggering occurring during the late January storm cycles. The latest observations indicate that these layers have gradually become non-reactive and the persistent slab problem has been removed from the east slope zones. You can still check for these layers in snow pits in your specific area of travel.
NWAC pro-observer Jeff Ward was in the Chumstick Mountain area Monday. The January 3rd and 11th layers were found at about 75-85 cm and 70-80 cm and gave hard or no results in tests. Jeff noted isolated, stubborn to release small wind slab near ridges and new widespread surface hoar.
Jeff was in the Stevens Pass area Tuesday. He generally found a stable upper snowpack but observed new surface hoar growth most prominent in Highland Bowl near ridgecrest.
Tom Curtis observed healthy surface hoar growth in the Blewett Pass area on all aspects Monday and in the Icicle Creek drainage near and above 4500 feet Tuesday. Increased cloud cover in the Washington Pass zone may have limited surface hoar development in this area.
The surface hoar from early this week will need to be watched in case it makes new snow Wednesday and Thursday more reactive.
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.
Elevations: Alpine.
Likelihood
Expected Size
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: Alpine.
Likelihood
Expected Size
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: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Feb 5th, 2016 10:00AM
