Avalanche Forecast

Issued: Feb 26th, 2017 11:15AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs and Loose Dry.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Watch for generally shallow new wind slab on lee easterly aspects near and above treeline. Fast moving loose dry avalanches are possible on steep slopes in non-wind affected terrain.

Summary

Detailed Forecast

A weak low pressure system moving south along the Washington Coast on Monday should produce light snow showers, mainly in the afternoon, and mainly for the central-west and southwest Cascades.  

Watch for generally shallow new wind slab formed Saturday afternoon through early Sunday morning on lee easterly aspects. Fresh wind slab should be found near and above treeline. Be aware that lower density snowfall received as the winds eased during the day on Sunday may obscure recently wind loaded slopes.  

Watch for loose dry avalanches on steeper slopes in non-wind affected terrain. Be especially wary of fast running loose dry avalanches near terrain traps. 

Storm slabs will not be listed as an avalanche problem but watch if recent storm storm forms a cohesive slab above weaker snow in specific areas on Monday. 

Give cornices a wide berth when traveling along ridgelines and avoid lingering on slopes below cornices as they may fail at any time.

Snowpack Discussion

Weather and Snowpack

The most recent atmospheric river arrived on Valentines Day 2/14 and formed the uppermost significant rain crust in our snowpack. 

A pair of storms deposited about a foot of snow in most areas by Monday 2/20. This was combined with periods of moderate to strong W-SW winds, forming wind slabs at the time. A slightly unstable weather pattern last week caused a mix of sun and light snow showers with sun crusts forming on solar aspects.

Saturday was mostly cloudy with light snow showers seen mainly in the north and central Cascades. Westerly ridgetop winds began to increase Saturday afternoon and night ahead of a frontal system. A low pressure system situated off the Washington Coast brought highly variable amounts of low density snowfall Saturday night and Sunday along the west slopes of the Cascades. 24 hr snowfall totals through 5 pm Sunday: Mt. Baker 5 inches, Stevens and White Pass 6 inches, Snoqualmie Pass 8-10 inches, Paradise about 12 inches, and Crystal 3-4 inches.  

Recent Observations

North

NWAC observer Lee Lazarra was out in the Mt. Baker backcountry on Saturday and found lingering wind slab on lee slopes near treeline but that the older wind slab was unreactive in snowpit tests and unlikely to trigger. Loose dry avalanches were a concern in steep terrain.   

Central

NWAC observer Ian Nicholson was in the Stevens Pass area on Saturday and found sun crusts affecting the snow surface on an increasing range of solar aspects with shallow amounts of new snow below treeline above the stout 2/14 crust. Weak surface snow near treeline was beginning to be transported by westerly winds beginning around mid-day. Ian was in the Yodelin area Sunday and found 6 inches of low density snow had refreshed the snow surface. Small loose dry avalanches were easily ski triggered on steep test slopes. Shallow new wind slabs were found on lee aspects near ridgetop. 

On Sunday, Alpental pro-patrol reported widespread and easily ski triggered loose dry avalanches running fast and entraining the surface snow on steep slopes. 

South

NPS rangers at Mt. Rainier National Park on Saturday initially relayed a report that five people were hit by a large loose dry avalanche that naturally released in the Fly Couloir on Lane Peak in the Tatoosh Range. Two or three individuals were partially buried, able to self-excavate but suffered some minor injuries. 

Backcountry observations from the Crystal area on Sunday reported small and predictable but fast moving loose dry avalanches on steep N-NE slopes near treeline. 

Problems

Wind Slabs

An icon showing Wind Slabs

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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Loose Dry

An icon showing Loose Dry

Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.

Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.

 

Loose Dry avalanche with the characteristic point initiation and fan shape.

Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Feb 27th, 2017 11:15AM