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Avalanche Forecast

Archived

Mar 18th, 2016–Mar 19th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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Check the forecast for a mixed bag of spring avalanche problems and evaluate snow and terrain carefully on Saturday.

Detailed Forecast

Increasing mid and high clouds should be seen on Saturday east of the crest.

The avalanche forecast won't change much from Friday. East winds should decrease over the Cascade crest and in the Cascade passes on Saturday. East winds the past couple days should mainly have affected areas along the crest and along the west slopes.

Loose wet avalanches may still be possible on steeper solar aspects on Saturday. Loose wet avalanches may start small but could have the ability to entrain recent moist snow in mainly on steep solar terrain. High clouds Friday night may limit a refreeze of surface snow by Saturday morning.

Storm winds 3/13-15 likely built wind slab on lee aspects in the upper elevations of the above treeline band. This wind slab will mostly likely be found on NW-SE facing slopes. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.

Recently formed cornices have grown large and can become sensitive when it is warm and sunny. Avoid areas on ridges or summits where there may be a cornice and avoid slopes below cornices in the spring.

The persistent slab avalanche problem involving the 2/27 buried surface hoar will not be listed in the northeast zone due to an abundance of other avalanche problems, high unlikelihood of triggering due to depth, and general strengthening of this interface over the last few weeks.  

Snowpack Discussion

Weather and Snowpack

During a period of fair weather in late February, widespread surface hoar formed in the northeast zone mainly surviving outside of steeper solar aspects and wind affected terrain. A weak front buried the surface hoar layer in the Washington Pass area about 2/27 and to a lesser extent the central-east zone.  

Storms moved across the Northwest at a nearly daily frequency the past few weeks with fluctuating but generally moderate snow levels. Two deep surface low-pressure systems moved across the Olympic Peninsula last week with the first passing last Thursday 3/10, and the next on Sunday, 3/13. These systems caused very stormy weather.  

Cool and showery weather Monday and Tuesday with prolonged  westerly winds deposited additional snow at lower snow levels. Storm snow amounts along the east slopes were in the 1.5 - 3.5 foot range for Wednesday 3/9 to Tuesday 3/15.

Recent storm instabilities have been slow to consistently settle out throughout the east slopes. Specifically in the Washington Pass zone the last week has seen many natural and human triggered storm and wind slab avalanches. The recent storms have also built large cornices along ridges in many areas with several large cornice releases noted over the last few days.

We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack due to lack of recent activity at these interfaces and confirming field observations. 

Recent Observations

NWAC pro-observer Tom Curtis tested conditions in the Blewett Pass area Monday 3/14 following the strong storm Sunday. Sensitive storm and wind slabs of up to 20 inches were found in open terrain, especially wind affected slopes. Easy failure in test pits was noted on a crust-facet layer buried beneath the current storm snow, about 16-20 inches. Failures were found with PST and ECT tests on this interface.

A report via the NWAC Observations page for the east central zone on Monday 3/14 indicated that most of the area visited had wind affected snow.

Recent guide reports from the Washington Pass area Tuesday, 3/15 indicated an active avalanche day.  There was a partial skier burial due to a ski triggered 45 cm storm slab on a north slope at about 6800 feet in the Hairpin Valley. More details are being gathered but gladly there were no injuries. Ski cuts were giving better propagation than expected also in the Hairpin Valley. AT least 2 cornice releases in NE facing bowls up the Valley caused a large size 3 storm slab and car sized blocks running to the valley floor.

Recent storm slab in the Washington Pass area was reported to be more sensitive due to surface hoar that may have been buried there about during a couple day period March 10-12th.

Tom Curtis was out again on Mt Cashmere on Thursday 3/17 and found 35 cm of storm snow well bonded to an underlying crust from last week. An extended column test did not indicate propagation and only small loose wet avalanches were seen on steep solar slopes. Temperatures were not greatly affecting the snowpack.

The Mission Ridge pro-patrol reported limited wind transport on Friday 3/18 from east winds and surface crusts below about 6000 feet from sun and warmth.

 

Problems

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.

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.

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.