Avalanche Forecast

Issued: Mar 1st, 2016 10:02AM

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

NWAC, Northwest Avalanche Center

Snoqualmie Pass.

With several avalanche problems to deal with Wednesday, conservative terrain selection will be essential to stay safe because human triggered avalanches are still likely. Choose moderately angled terrain and avoid wind loaded slopes, saving those steeper lines for another day.

Summary

Detailed Forecast

Showers should wind down Wednesday morning with the next frontal system already moving in later Wednesday afternoon. Despite a modest cooling trend, storm related danger will continue through Wednesday due to strong southwest to west winds loading lee slopes during moderate showers Tuesday night.

Storm slab instabilities should be healing but may still be sensitive Wednesday. Wind slab will continue to build Tuesday night on lee slopes with moderate to strong west transport winds. Loose wet avalanches will become more likely on steeper slopes in the afternoon as snow levels begin to rise once again. Avalanches that begin in the new snow may become larger lower down the path as they entrain moist or wet snow.

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

Springlike weather under high pressure Feb 23rd and 24th led to abundant sunshine with daytime highs climbing into the 40's and 50's. The fair and mild weather caused thick surface crusts, especially on solar slopes in most areas by Friday.

A Pacific frontal system blew through the Cascades mid-day Sunday. 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. By Monday morning, 4-12 inches of new snow had accumulated along the west slopes accompanied by a cooling trend.

A warm front quickly dropped up to 10 inches of new snow early Tuesday morning at NWAC stations before snow either eased off mid-morning or changed to rain outside the Cascade Passes. Precipitation rates increased again in the afternoon as the cold front approached. In the Mt. Baker area, precipitation rates increased later in the morning with the snow level rising above 5000 ft during the day.

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

Crystal and Stevens Pass pro-patrol reported Monday that while wind loading was evident, wind loaded slopes were not very sensitive with only very small slabs releasing during control work.

NWAC observer Tom Curtis was out at 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 winds 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 storm 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.

Problems

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

Likely

Expected Size

1 - 1

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

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

Likely

Expected Size

1 - 1

Valid until: Mar 2nd, 2016 10:02AM