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Avalanche Forecast

Archived

Mar 11th, 2017–Mar 12th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Stevens Pass.

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. Careful snowpack evaluation and cautious routefinding will be essential on Sunday.

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. Expect about 5-10 inches of new snow above the pass levels by Sunday morning.

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

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.

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 drifts 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.

Loose Wet

Loose Wet avalanches are the 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. 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.