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Avalanche Forecast

Archived

Feb 28th, 2016–Feb 29th, 2016

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

Regions

Snoqualmie Pass.

Play it conservative on Monday and give new wind slabs time to settle by avoiding freshly loaded slopes. Wind slab may extend into the below treeline zone as strong winds Sunday and Sunday night transported snow further downslope than usual.

Detailed Forecast

Showers and at times blustery west winds Sunday night should taper down Monday morning. High clouds from an approaching warm front will spread over the area in the afternoon. Snow levels will remain relatively cool. 

New wind slab should be sensitive on lee aspects Monday, with significant amounts of new snow transported Sunday and Sunday night. Play it conservative on Monday and give new wind slabs time to settle by avoiding freshly loaded slopes. Wind slab may extend into the below treeline zone as strong winds Sunday and Sunday night transported snow further downslope than usual. Watch for firmer wind transported snow and surface snow cracking as you travel throughout the terrain.

The storm system came in right side up with a cooling trend, so storm slabs will be a lower concern and possible in non-wind affected terrain. 

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

A rain crust was buried on 2/17 when a storm cycle from February 17th-20th dropped 1-3 feet of snow along the west slopes. 

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 thick surface crusts, especially on solar slopes in most areas by Friday and helped stabilize wind slab formed early last week. 

A strong Pacific frontal system blew through the Cascades mid-day Sunday. A few inches of wet snow accumulated through the early afternoon above 4000 feet, but the strong winds were the main story. The most significant transport winds were from the W-SW and seen in the early afternoon post-frontal passage with gusts in the 40s or 50s below or near treeline with much stronger winds above treeline. 

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

Storm snow received Sunday and forecast for Sunday night will be the focus for avalanche problems moving forward along the west slopes including the Cascade Passes. NWAC observer Dallas Glass was at Kendall Peak Sunday morning but new wind and storm slab had not yet become an issue in his area. 

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.