Avalanche Forecast

Issued: Jan 30th, 2015 10:00AM

The alpine rating is low, the treeline rating is low, and the below treeline rating is low. Known problems include Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

There is still some uncertainty regarding potential persistent slab layers east of the crest, however, recent tests indicate overlying strong snow layers are limiting the potential for human trigger. Snow pits may help determine if this layer is still present or reactive in your area.

Summary

Detailed Forecast

An upper ridge will weaken slightly, but remain over the region Saturday. This will cause light winds, sunny weather and mild temperatures. High clouds should move in during the day a weak frontal system approaches. Persistent cooler temperatures at lower elevations should maintain low clouds or fog, especially in the valleys and elevations below about 3000 feet. 

This weather will cause little change in snow conditions and maintain an overall low danger.

Warmer weather is causing persistent weak layers from January 15th to become less or non-reactive. This problem will be listed as unlikely for the northeast and central east zones. 

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. This caused loose wet avalanches, some possible wet slab avalanches and consolidation.

Mainly mild or sunny weather has been seen so far this week at upper elevations with low clouds or fog mainly below about 3000 feet.

Recent Observations: Observations Friday just east of Stevens Pass indicated extensive surface hoar from about 3000-5000 exists on most shaded terrain. The most extensive surface hoar growth was seen between about 3000-4500 feet where recent low clouds or fog have persisted east of the crest this week. While not a current issue, it bears watching if snowfall expected Sunday buries this layer intact.

NWAC pro-observer Jeff Ward was at Sugarloaf Mountain a little east of Plain on Monday. He found snow cover lacking on most slopes. On a north slope, withisothermal snow, the buried January 15th facet/crust layers were about 12 cm below the surface and produced a SC in a test but stuck to the bed surface. Here is his video:

Several other NWAC pro-obs or North Cascade Guides reports this week in the northeast and central east zones have found the January 15th facet/crust layers at 40 cm-1 m below the surface with the facets starting to turn to rounded grains and not reactive or not releasing in pit tests. So the January 15th facet/crust layers expected in the northeast and central east zones should be strengthening and stabilizing.

NWAC pro-observer Tom Curtis was at 7500 feet on Mt Cashmere in the Wenatchee Mountains on Wednesday and found a local facet/crust combo buried on a lee slope next to a ridge crest under 35 cm of thick, surface crust and 1F-P snow. This sort of slab could be very hard for a skier to trigger, but could be possible, perhaps with a large sudden load, such as snowmobiles.

Problems

Persistent Slabs

An icon showing 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.

Aspects: North, North East, East, North West.

Elevations: Alpine, Treeline.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Jan 31st, 2015 10:00AM