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Avalanche Forecast

Archived

Feb 18th, 2017–Feb 19th, 2017

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Stevens Pass.

Up to several inches of new snow should be seen on Sunday mainly at Mt Rainier and the south volcanoes. Evaluate snow and terrain carefully for possible wind or storm slab on Sunday.

Detailed Forecast

Moderately increasing south to southwest alpine winds, with light east winds in Cascade Passes, and up to several inches of new snow should be seen on Sunday mainly at Mt Rainier and the south volcanoes. Lighter amounts of snow are likely in the Cascade Passes. Temperatures shouldn't change much again on Sunday.

This should bring a slight avalanche danger increase on Sunday.

Watch for firmer wind transported snow on potential lee slopes. Alpine winds at the tail end of the last storm and on Sunday will have been mostly south to southwest so north to southeast aspects will be indicated.

Small areas of shallow storm slab might be possible in any areas that receive a few inches of rapidly accumulating snowfall.

Underlying wet snow especially at lower elevations should continue to drain and refreeze.

Snowpack Discussion

Weather and Snowpack

Yet another atmospheric river arrived Tuesday night, bringing heavy rain to all forecast elevations along the Cascade west slopes. The storm total rain amounts ranged mostly from 1.5 - 4.5 inches of water, with the Mt Baker area receiving nearly 7 inches of rain by Thursday morning. This weather caused some avalanches, lots of consolidation and the formation of surface crusts as temperatures began to cool at the tail end of the storm.

Localized freezing rain also occurred during this event along the lower slopes of the Cascade Passes, where a variable strength, freezing rain crust formed at mid and lower elevations.

A slow cooling trend began overnight Wednesday with light to moderate snow showers depositing 2-6 inches of new snow by Thursday evening at mid and upper elevations.

A fair day was seen on Friday with light winds and no precipitation.

Light south to southwest alpine winds, with light east winds in the Cascade Passes, and light amounts of snow are seen along the Cascade west slopes on Saturday.

Temperatures haven't changed much on Friday and Saturday.

Recent Observations

North

On Thursday 2/16, the Mt Baker pro patrol reported a very large naturally triggered slab at the east end of Shuksan Arm that released sometime during the rain event. The initial estimates of the crown height are 20 feet!

Central

Alpental pro-patrol did not perform avalanche control Wednesday, but reported a 1/2" freezing rain crust up to about mid-mountain with rain falling on the upper mountain.

South

Dallas Glass was observing conditions at Paradise Thursday, 2/16. Dallas reported wet snow conditions in the upper two feet of the snowpack. Though previous rain events were allowing the liquid water to quickly drain, helping the stabilization process.

A couple reports are available for Friday via Turns All Year. At Crystal a report indicated 2 inches on new snow over wet but supportable snow. A report from Paradise indicated that 5 inches of new snow only added to deep wet snow conditions.

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.