Avalanche Forecast

Issued: Mar 4th, 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 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

Showers early Saturday should end with mild and cloudy conditions much of the day Saturday before yet another front makes its way inland late Saturday and Saturday night. Mild temperatures with a lull in winds through midday should help recent wind slabs to settle.  Earlier formed wind slabs may still be reactive Saturday and large cornices may become sensitive along 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 melt-freeze crusts, especially on solar slopes in most areas by Friday. Also during this period, wide-spread surface hoar formed in the NE Cascades, mainly surviving outside of steeper solar aspects and wind affected terrain above treeline. 

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 by 2/27, and to a lesser extent the central-east zone.  

A Pacific frontal system passed the Cascades midday 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. 

Another strong frontal system brought 8-20 inches of snow along the east slopes Tuesday through early Wednesday along with a slow warming trend. Rain may have pushed as high as 4500-5000 feet along the central and northeast Cascades Tuesday afternoon before cooling Wednesday morning.  

The active weather continued Wednesday depositing another 3-6 inches of snow by Thursday. 

In the NE zone near treeline there are about 2 feet of snow now over the 2/27 interface as of Thursday afternoon, providing some fantastic ski and riding conditions in areas away from wind effects.

  

We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack, due to lack of recent activity and confirming field observations. 

Recent Observations

NWAC pro-observer Jeff Ward was out near Washington Pass a week ago Thursday and found surface hoar up to 15 mm on non-solar slopes.  Jeff was out again last Friday and noted widespread 6-8 mm surface hoar on non-solar slopes. By Saturday, 2/27 the surface hoar was noted as buried intact in the Washington Pass area, specifically the Cedar Creek drainage. There has been daily travel in this zone this week with a few slides noted, involving this 2/27 layer, however, it remains difficult to tell how active or widespread this layer is at present, but important to be thinking about and looking for.

Observations from Thursday 3/3 in the NE zone indicated very strong winds near and above treeline were rapidly building wind slab with abundant cold snow available for transport. Several natural wind slab releases were observed Thursday on N-E aspects, producing impressive powder clouds, one being triggered by a cornice failure. It was not felt they stepped to a deeper layer, namely the 2/27 layer, as the largest crown was about 30 cm or 1 ft. The slides ran good distances and indicate how quickly conditions can change! 

Jeff Ward, made observations near Wedge Mountain in the Central-East zone Friday, March4th. Mild daytime temperatures and sunshine Friday, allowed for the crust to soften on solar aspects, even providing spring corn conditions in some areas. Shaded terrain was still holding the recent colder powder. The interface at 2/27, buried a little over 2 ft,  did not support propagation in a test pit at 6100 ft on NNE slope. Much of the loose surface snow has now been stripped and redistributed along exposed ridges, forming wind slab deposits on many lee northerly facing slopes below ridges. These wind slabs have been settling quickly, however, remaining the primary concern for east slope backcountry travelers.

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, North West.

Elevations: Alpine, Treeline.

Likelihood

Likely

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: Treeline, Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Mar 5th, 2016 10:00AM