Avalanche Forecast

Issued: Mar 11th, 2017 8:36PM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Mother Nature is throwing a lot of avalanche problems at us right now so review the forecast to remind yourself of what to watch for if you are planning a trip into the backcountry on Sunday. Re-evaluate or ratchet back your plans if you encounter significantly more new snow than described at higher elevations.

Summary

Detailed Forecast

A short break between frontal systems should be seen Sunday. Light showers mainly over the Cascade west slopes should taper off in the morning. Renewed light rain or snow should be seen mainly over the Olympics and northwest to central west Cascades as moisture from the warm front of the next system reaches the Northwest. Re-evaulate your plans if the weather is not turning out as expected.

Mother Nature is throwing a lot of avalanche problems at us right now so review the forecast to remind yourself of what to watch for if you are planning a trip into the backcountry.

New or previous wind slab should be suspected mainly on NW-SE aspects above treeline due to recent SW-W winds. But watch for firmer wind transported snow on all aspects especially in areas of complex terrain.

New sensitive storm slab is most likely above treeline in areas that receive more than a few inches of rapidly accumulating snowfall.

Avoid travel on ridges near where cornices will have been loaded and weakened by recent storms and avoid steep slopes below cornices that may fail at any time. Cornices have been reported as large and sensitive in several areas and have been involved in recent accidents and close calls.

Loose wet avalanches are most likely on solar slopes in the near and below treeline especially if you are in an area with sunshine or little cloud cover. Watch for surface wet snow deeper than a few inches, pinwheels and initial small releases that will get bigger the longer the sun heats the snowpack.

Dangerous avalanche conditions are expected Sunday. We only got a taste of the spring stabilizing process the past couple days. Avalanches may step down or entrain deeper layers and be large and dangerous especially if they reach the Valentine's Day crust layer.

Snowpack Discussion

Weather and Snowpack 

The first week or so of March was very cool and snowy. NWAC stations along the Cascade west slopes piled up about 3 to 8 ft of snow with the most at Mt Baker.

A strong frontal system brought increasing precipitation and winds along with a warming trend to the Cascades on Thursday. On Thursday night rain pushed up to about 5000 feet in the northwest Cascades and about 6500 feet in the southwest Cascades. By Friday morning NWAC stations along the Cascade west slopes had about 1-2 inches of WE  with only 0-7 inches of snow with most snow at Stevens and Mt Baker. This gave the snowpack a huge test and caused an avalanche cycle of several types of avalanches along the Cascade west slopes.

Another front crossed the Cascades on Saturday causing more SW-W winds, snow at higher elevations, and rain at low elevations. NWAC stations along the Cascade west slopes have about 1-7 in of new snow on Sunday morning with the most at Mt Baker and the top of Alpental.

Recent Observations

North

The Mt Baker patrol reported natural avalanches on Thursday night. An explosive control avalanche on Shuksan Arm produced a 8'x75-100 yd crown.

 

Shuksan Arm debris on 3/10. Photo by Sam Llobet.

Central

The Stevens Patrol and Stevens and Snoqualmie WSDOT crews reported many wet slab and loose wet avalanches from control and natural cycles Wednesday to Friday with some pass closures. Several large slab avalanches ran on the south side of Granite Mountain near Snoqualmie.

The Alpental patrol reports that a widespread natural cycle of slab avalanches occurred there Thursday night after full control on Thursday.

NWAC observer Tom Curtis make a trip to Rainy Pass just east of Stevens Pass from the Lake Wenatchee side on Saturday. He noted wind effects on nearby ridges and 1-3 days old loose wet and ws releases. The upper snowpack consisted of a surface crust over right side up wet 4F snow that increased to 1F with depth and crust layers with good bonds and no results in tests.

NWAC observer Jeremy Allyn was in the Alpental Valley from 3000-4000 ft on Saturday and reported evidence of the avalanche cycle on Thursday and Friday. A 2 inch thick surface crust was stabilizing underlying moist F snow.

South

NWAC observer Dallas Glass was in the Crystal backcountry on Friday 3/10 and reported a wet slab cycle occurred there on Thursday night with several releases on the ridge running from 3 Way to Crown point on W-NW-N slopes in the 5800-6000 range. Natural loose wet avalanches also ran from steep rock terrain. The snowpack had also consolidated and gained strength by Friday.

The Paradise rangers reported a large natural wet slab or loose wet avalanche covered the road this morning just up the road from the Nisqually Bridge.This was a northwest aspect at about 4500 ft.

Just above the Nisqually Bridge to Paradise on Friday morning. Photo Doug Jones.

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

Likely

Expected Size

1 - 1

Cornices

An icon showing Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

 

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

 

A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

Aspects: North, North East, East, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 2

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

Likely

Expected Size

1 - 1

Valid until: Mar 12th, 2017 9:36PM