Avalanche Forecast
Issued: Feb 13th, 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
Moderate W-SW winds will transport new snow onto lee slopes near and above treeline Sunday, building unstable wind slab on lee aspects. The avalanche danger will increase late Saturday night and on Sunday as snow or rain increases along with a warming trend.
Summary
Detailed Forecast
Increasing moderate rain and snow is expected with a warming trend Saturday night through Sunday. With the snow level expected to rise to the near tree line elevations band by early Sunday, a significant increase in avalanche danger is expected overnight with natural avalanches becoming increasingly likely. If a natural avalanche cycle occurs overnight, this should locally lower the danger during the day Sunday, at least in those areas that experienced avalanches. The main question is about the rate and timing of warming. If the warming is not as great as expected or delayed, a very heightened avalanche danger should persist into Sunday.
On slopes that have not released due to warming and additional load, natural or triggered avalanches should be increasingly likely Sunday.
Strong W-SW winds will transport new snow onto lee slopes near and above treeline where wind-driven snow may build unstable wind slab on lee aspects.
Wet loose avalanches will be a concern on steeper slopes at lower elevations.
Avoid cornices along ridges and slopes below cornices since cornices may still be weaker due to the warm weather.
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, but are no longer considered a persistent slab threat after several warm and wet systems tested this layer and it has been unreactive where it can still be identified in recent snowpit tests. A warm and wet system late last month caused a rain crust to form in most areas and elevations along the east slopes on Jan 28th.
The warmest weather of the winter occurred this past Sunday through Wednesday with extended temperatures climbing into the 50's in most areas east of the crest. The warm temperatures and solar effects earlier in the week caused loose wet avalanches and overall snowpack consolidation. A warm front brought periods of very light rain and snow to the east slopes Thursday night through Friday, with snow levels oscillating around 5000-6000 ft Friday. Cool snow showers Friday night and Saturday combined to deposit 2-6 inches of storm snow as of Saturday evening.
Recent Observations
Extensive observations in the Washington Pass area over the past few days indicated an active loose-wet avalanche cycle occurred mainly Monday afternoon. NWAC pro-observer Tom Curtis in the Blewett Pass area in the 4000-5900 foot range Tuesday and also saw several small to large wet loose avalanches on solar slopes near and above tree line.
A professional observation from the central-east zone Thursday indicated that a facet/Jan 28th rain crust combo was reactive in snowpit tests, but no avalanches were observed down to this interface. However, a skier triggered avalanche down to the faceted 1/28 crust was observed in the Washington Pass area on a NE aspect around 7000 ft Thursday. While we believe the upper snowpack to generally be well bonded to the 1/28 crust, this interface should get a decent test over the next few days and bears watching.
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, Treeline.
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: All elevations.
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: Below Treeline.
Likelihood
Expected Size
Valid until: Feb 14th, 2016 10:00AM
