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Avalanche Forecast

Archived

Apr 16th, 2014–Apr 17th, 2014

Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.
Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.

Regions

Snoqualmie Pass.

A mix of increasing winter and spring avalanche conditions should be seen in the above treeline zone especially in the Washington Cascades near and west of the crest and at Mt Hood on Thursday.

Detailed Forecast

The cold front will cross the Northwest on Thursday with southwest winds and snow levels continuing at about 5-6000 feet during the day then lowering Thursday afternoon and night. This should produce about 5-10 inches of further new snowfall in the above treeline zone in the Washington Cascades near and west of the crest and at Mt Hood.

New or further building wind slab is likely on lee slopes mainly in that area. This should be north to southeast slopes or on cross loaded aspects or due to varied terrain may be seen on other aspects. Watch for signs of firmer wind transported snow or cracking snow.

New or further building storm slab is also likely on sheltered slopes mainly in the same area. This should be in areas that receive at least several inches of snowfall. Variations in winds and temperatures promote weak temporary storm layers that help cause storm slab.

The sun continues to get stronger as we get further into spring. Even with all the clouds on Thursday the sun could do its work mainly on solar slopes in areas where there is more than a few inches of snowfall. So wet loose avalanches are possible on such slopes. This will also be mainly in the above treeline zone in the Washington Cascades near and west of the crest and at Mt Hood but may extend down into the near treeline zone as well. Watch for increasing pinwheels and natural or triggered wet loose avalanches that increase in size or extent during the daylight hours.

Rain or much less new snow should continue in the near and below treeline areas on Thursday especially east of the crest. Less change in snow and avalanche conditions should be seen in those areas.

Note that the NWAC will issue a complete mountain weather forecast during the day and a complete avalanche forecast at 6 pm on Friday and Saturday.

Snowpack Discussion

A warm front is crossing the Northwest today with west-southwest winds and snow levels at about 5-6000 feet. This should produce about 4-8 inches of new snow in the above treeline zone in the Washington Cascades near and west of the crest.

So new wind slab on lee slopes and new storm slab on sheltered slopes should be starting to build in that area today.

Rain or much less new snow is expected in other areas today such as Hurricane, near and below treeline in the Washington Cascades near and west of the crest, east of the crest and at Mt Hood.

Problems

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.

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.

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.