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Avalanche Forecast

Archived

Feb 27th, 2016–Feb 28th, 2016

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

Regions

Stevens Pass.

Mostly small, shallow areas of new wind and storm slab will be possible in the near and above treeline on Saturday. These avalanche problems would be greater if there is more new snow than expected. Watch for signs of loose wet avalanches mainly below treeline.

Detailed Forecast

Rain or snow should change to showers Friday night with lower snow levels. A little convergence may be seen in the central Cascades near Stevens and Snoqualmie Friday night to Saturday morning.

Light showers mainly along the west slopes should end by Saturday afternoon.

This weather should bring a couple to a few inches of snow near and above treeline along the west slopes with a cooling trend. The cooling trend with rain changing to snow mainly in the near and above treeline should help bond new snow to previous snow.

New small areas of mostly shallow wind slab will be possible on isolated lee slopes. This should be mainly N to E slopes near and above treeline. Watch for firmer wind transported snow.

Isolated older wind slab from E-SE winds Monday night and Tuesday may also linger on unusual W aspects.

Small areas of shallow storm slab will also be possible if any areas receive a few inches of rapidly accumulating snowfall.

These avalanche problems would be greater if there is more new snow than expected.

Possible, mostly small, loose wet avalanches also will remain a problem in the below treeline on Saturday.

Snowpack Discussion

Weather and Snowpack

The last storm cycle  occurred late last week when about 1-3 feet of snow fell from February 17th-20th. Some cornices and wind slabs formed during this period, with many triggered wind slabs reported last weekend.

Strong E-SE crest level winds in many areas Monday night and Tuesday redistributed surface snow and built new local wind slabs on unusual W facing slopes. Some of these wind slabs were touchy earlier this week but have stabilized quickly under the warm weather.  The image below highlights a 1-2 foot wind slab on W aspect near Pan Face above Paradise that released Wednesday afternoon February 24th.

Wind slab formed from E winds, Tuesday, 2/23/16. Released Wednesday, 2/24/16. W aspect, crown 1-2 feet, near Pan Face, Mt Rainier, ~7000ft elevation. Photo Peter Ellis.

Springlike weather under high pressure Wednesday and Thursday caused abundant sunshine with temperatures climbing into the upper 40's to mid 50's Thursday afternoon. This fair and mild weather will have caused thick melt form surface crusts especially on solar slopes in most areas by Friday.

Increasing clouds Friday should have helped limit solar effects in most areas.

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.

Surface hoar grew on non-solar slopes over the past few days and where not destroyed by sun or expected rain could be a layer to watch the next couple days.

Recent Observations

NWAC pro-observer Dallas Glass was in the Alpental Valley Thursday reporting strong early morning melt-freeze crust that rapidly turned into wet surface snow on solar aspects. No wind slabs were found with mainly an increasing threat of loose-wet avalanches.

NWAC pro-observer Lee Lazzara was near the north side of Mt Baker on Thursday in the 4300-6500 foot elevation range and found local 30 cm wind slab on ridges to be unreactive and melt form crusts on solar slopes. There was some powder surviving on non-solar slopes.

A report on the NWAC Observations page indicated numerous loose wet avalanches on Mt Herman near the Mt Baker ski area on Thursday.

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.

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.