Avalanche Forecast

Issued: Mar 5th, 2016 10:33AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Loose Wet, Storm Slabs and Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Wet snow conditions will prevail below treeline with a transition to shallow storm hazards above treeline. Avoid steeper slopes where even a small loose wet avalanche could entrain significant amounts of recent snow or where you could be flushed into a terrain trap. Be wary of unstable layers created during heavier showers that would make shallow wind or storm slabs found at higher elevations more likely to trigger.

Summary

Detailed Forecast

Light to locally moderate rain Saturday night will transition to showers Sunday morning with a slow cooling trend during the day. Showers may occasionally be intense on Sunday forming unstable storm layers.   

The avalanche danger will temporarily rise Saturday night during periods of peak rainfall. On Sunday, wet snow conditions will prevail below treeline with a transition to shallow storm hazards above treeline. Avoid steeper slopes where even a small loose wet avalanche could entrain significant amounts of recent snow or where you could be flushed into a terrain trap. Be wary of unstable layers created during heavier showers that would make shallow wind or storm slabs found at higher elevations more likely to trigger.

Cornices formed over the last week should still be weakened by the mild temperatures so be aware of the overhead hazard. 

Non-avalanche hazard: Despite our seasonally, healthy snowpack, many creeks are open and difficult to cross due to the periodic warm temperatures and rain events.  

Snowpack Discussion

Weather and Snowpack

The last week has been wet and relatively mild with a series of active fronts transiting the region and causing fluctuating freezing levels and periods of very strong winds. The heaviest snowfall and precipitation occurred late Sunday 2/28 and Tuesday 3/1. The latter half of the week including Saturday has featured more rain than snow at NWAC sites along the west slopes. For perspective, the averaged freezing level measured in March thus far has been 5700 ft at Forks (Washington Coast) and 6200 ft at Spokane. When the average freezing levels are warmer on the east side of the Cascades versus the west side, that bodes poorly for the influence of cool easterly flow and freezing levels along the west slopes including the Passes become more aligned with the average Forks freezing level. 

Wind slabs have formed near ridges most days this week, with daytime warming causing cold surface snow to settle and quickly form storm slabs. Mid and lower elevations have seen natural, loose-wet avalanches most days as radiation increases in early March or during rain events. 

The mid and lower snow pack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.

Recent Observations

Both Wednesday through Friday periods of sunshine along with significant daytime warming caused numerous natural point release, loose wet avalanches on steep aspects, mostly below about 4500-5000 feet, seen in the Alpental Valley and the Mt Baker backcountry.

Locally heavy rain at the Mt. Baker area led to sensitive loose wet ski cuts by the Mt. Baker pro-patrol early Saturday morning. A natural loose wet cycle was observed in the Bagley Lakes area and one glide avalanche in unsupported terrain released nearby. 

The fluctuating freezing levels, daytime melt-freeze, strong winds and periods of rain and snow have left behind a variety of surfaces, ranging from supportive to breakable crusts, settled powder snow in mid and upper elevation shaded terrain and even spring corn snow conditions on some solar aspects.

Here's a nice look at the conditions in the upper Alpental Valley near Chair Peak Friday, March 4th. Rain runnels from mid and lower elevations and loose wet debris below cliffs. Photo: Shane Robinson   

Problems

Loose Wet

An icon showing 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

Possible

Expected Size

1 - 1

Storm Slabs

An icon showing 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, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Wind Slabs

An icon showing 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, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 6th, 2016 10:33AM