Avalanche Forecast
Issued: Jan 28th, 2015 8:51AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Persistent Slabs.
NWAC,
Northwest Avalanche Center
There is still some uncertainty regarding potential persistent slab east of the crest. Snow pits may help determine if this layer is still present or reactive in your area.
Summary
Detailed Forecast
Partly cloudy weather with a slight chance of a few light rain showers should be seen this evening to Thursday morning. Most areas will see no precipitation. Decreasing clouds should be seen Thursday afternoon and night. Winds will be light with a slow warming trend.
This weather will cause little change in snow conditions and an overall low danger.
The main limited avalanche problem to watch for should be loose wet avalanches. Watch for wet snow deeper than a few inches on steep solar slopes. But the likelihood and size will be listed as unlikely and small respectively.
Warm weather and recent rain should be causing persistent weak layers from January 15th to become less reactive. This problem will be listed for the northeast and central east zones as unlikely and near and below treeline. Snow pits may help determine if this layer is still present or reactive in your area. A persistent layer is not expected in the southeast zone.
Due to the low snowpack at lower elevations watch for terrain hazards such as open creeks, partially covered rocks and vegetation.
Snowpack Discussion
Last weekend a warm front caused high snow levels and rain east of the crest though less rain than west of the crest. NWAC sites east of the crest had up to an 1 inch of rain. Generally mild or warm weather and some sun has been Sunday to today.
Recent Observations: The Mission Ridge ski patrol on Sunday reported that depth hoar at 45 cm below the surface was becoming rounded but still failed on isolation.
NWAC pro-observer Jeff Ward was at Sugarloaf Mountain a little east of Plain on Monday. He found snow cover lacking on most slopes. The buried January 15th facet/crust layer was about 12 cm below the surface and produced a SC in a test but stuck to the bed surface. Here is his video:
NWAC pro-observer Tom Curtis was at Dirty Face Peak north of Lake Wenatchee on Tuesday and found the January 15th buried facet/crust layer about 50 cm below the surface, It was also not reactive to tests. He also ski triggered some small loose wet avalanches. Here is his video:
The North Cascade Mountain Guides were in the Silver Star area on Tuesday and found the January 15th facet/crust layers at 40 cm below the surface which produced moderate SC tests but blocks also stuck the bed surface so propagation seemed unlikely.
Problems
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.
Aspects: South East, South, South West.
Elevations: All elevations.
Likelihood
Expected Size
Persistent Slabs
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Jan 29th, 2015 8:51AM
