Avalanche Forecast

Issued: Feb 29th, 2016 10:00AM

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

NWAC, Northwest Avalanche Center

Cascades - East.

Very dangerous avalanche conditions are expected to develop late Monday night and last through Tuesday, especially in the near and above treeline elevation bands. Travel in avalanche terrain is not recommended Tuesday. Don't be fooled if your area sees a mid-day break in the storm as the avalanche danger will ramp up again in the afternoon.

Summary

Detailed Forecast

The avalanche danger will increase Monday night as warm frontal precipitation spreads over the area and increases during the early morning hours. Snow levels will slowly rise during the day as warmer air is advected into the region. Winds will also increase during the day with a wind shift in the late afternoon.

Storm slab instabilities should develop during intense precipitation periods and due to the overall warming trend Tuesday. New wind slab should develop mainly near and above treeline on NW through E slopes, but be wary of wind slab on all aspects due to east winds near the Passes and a wind shift with the frontal passage late Tuesday afternoon. Loose wet avalanches will become very likely on steeper slopes in areas that see a switch to rain in the below treeline elevation band. Avalanches that begin in the new snow may become larger as they entrain or step down to snowfall received Sunday and Sunday night, or step down to persistent weak layers.

New wind or storm slab layers could step down to the latest buried surface hoar layers, but persistent slab will not be identified as an avalanche problem until the scope and sensitivity of the PWL is known post-storm.

Snowpack Discussion

Weather and Snowpack

Springlike weather under high pressure Wednesday and Thursday of last week led to abundant sunshine with daytime highs climbing into the 40's and 50's. The fair and mild weather caused surface crusts on solar slopes in most areas by Friday and helped stabilize shallow wind slab formed early last week. However, good powder snow was found on non-solar aspects. Also during this period wide-spread surface hoar formed in the NE Cascades, mainly surviving outside of steeper solar aspects.

A weak front brought light amounts of rain and snow Friday night to Saturday morning, burying the surface hoar layer in the Washington Pass area on 2/26.

A strong Pacific frontal system blew through the Cascades mid-day Sunday. A few inches of snow accumulated through the early afternoon above 3-4000 feet near the Cascade crest with rapid drying and very little accumulation further east. Alpine winds were strong with significant W-SW transport winds. By Monday morning, 2-10 inches of new snow had accumulated along the east slopes accompanied by a cooling trend.

We aren't tracking any layers of concern formed earlier this winter in the mid or lower snowpack.

Recent Observations

Tom Curtis was out at Blewett Pass on Thursday and found cool wind transported snow on lee slopes near tree line that was not cohesive or reactive. Solar and windward slopes are bare or have a shallow snowpack.

NWAC pro-observer Jeff Ward was out near Washington Pass on Thursday and found powder and good ski conditions on north to east slopes. Surface hoar up to 15 mm was also seen on non-solar slopes.

Jeff was out again on Friday and reported numerous loose wet avalanches to size 1.5 on the south slopes of Delancy Ridge. He noted widespread 6-8 mm surface hoar on non-solar slopes. Good cool snow was also still found on non-solar slopes. On Saturday, Jeff noted the surface hoar buried intact in the Washington Pass area, specifically the Cedar Creek drainage.

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.

Problems

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

Very Likely

Expected Size

1 - 1

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

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

Likely

Expected Size

1 - 1

Valid until: Mar 1st, 2016 10:00AM