Avalanche Forecast

Issued: Mar 3rd, 2016 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Conservative terrain selection will be essential near and above treeline as human triggered avalanches are still likely. Choose moderately angled terrain and avoid wind loaded slopes. Avoid travel on or below overhanging cornices. Watch for wet snow conditions below treeline. 

Summary

Detailed Forecast

A mostly dry but cloudy day is in store Friday with warming and increasing winds. The exception is the Mt Baker area where precipitation should increase from late morning or midday. A period of light to moderate rain and snow at rising freezing levels is expected again Friday along with strong S-SW  ridgetop winds. New storm related danger will continue Friday, especially near and above treeline as strong southwest wind loads lee slopes building new wind slab layers. Wind slabs will build on lee slopes, facing generally NW-NE.  Earlier formed wind slabs may still be reactive Friday and may add to the increasingly complex snow structure, by Thursday. Large cornices should form near ridges as well, requiring travelers to avoid overhead hazard.

Conservative terrain selection will be essential near and above treeline to avoid wind loaded terrain and features. Choose moderately angled terrain and avoid wind loaded slopes, choose the terrain void of overhead hazard such as wind slabs or cornices.

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

Very mild weather a week ago allowed for the formation of a strong melt-freeze crust, especially thick on solar slopes in most areas by Friday.

A series of active fronts have transited the region since Sunday, with fluctuating freezing levels and periods of very strong winds. The heaviest snowfall and precipitation occurred late Sunday and again late Tuesday. Total snowfall accumulations since late Sunday through Thursday afternoon have been about 2-3 feet at most sites along the west slopes. Some periods of rain have occurred below treeline at times as well, producing loose-wet avalanches.

The most significant transport winds were from the W-SW and seen early Sunday afternoon with gusts in the 40s or 50s below or near treeline with much stronger winds above treeline.

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. 

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

NWAC observer Tom Curtis was out on Jove Peak Monday. Jove Peak is NE of Stevens Pass and often overlaps snowpack characteristics of both the east and west slopes of the Cascades. Tom noted few instabilities in the upper snowpack until he observed a large natural wind slab avalanche (R2-D2) on the north side of Jove Peak that probably released Sunday night. The wind slab was 40 cm in depth and stepped down to a 2 mm buried surface hoar layer at 50 cm. The buried surface hoar was rounding, but clearly still sensitive to new loading. 

A stormy day on Tuesday limited observations, but one observation submitted via the NWAC observation page from behind Windy Ridge near Stevens Pass reported increasing instability and sensitivity to triggering with a large soft slab avalanche on a WNW aspect running well and entraining moist snow lower in the path. 

Both Wednesday and Thursday sunshine or daytime warming caused numerous natural point release, Loose-Wet avalanches on steep aspects, mostly below about 4500 feet, seen in the Alpental Valley and the Mt Baker backcountry.  

Problems

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, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Likely

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

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: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 4th, 2016 10:00AM